Dissertations / Theses on the topic 'Monsoons - India'

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012.
Cataloged from PDF version of thesis.
Includes bibliographical references.
The Indian monsoon affects the livelihood of over one billion people. Despite the importance of climate to society, knowledge of long-term monsoon variability is limited. This thesis provides Holocene records of monsoon variability, using sediment cores from river-dominated margins of the Bay of Bengal (off the Godavari River) and the Arabian Sea (off the Indus River). Carbon isotopes of terrestrial plant leaf waxes ([delta]¹³Cwax) preserved in sediment provide integrated and regionally extensive records of flora for both sites. For the Godavari River basin the ([delta]¹³Cwax record shows a gradual increase in aridity-adapted vegetation from ~4,000 until 1,700 years ago followed by the persistence of aridity-adapted plants to the present. The oxygen isotopic composition of planktonic foraminifera from this site indicates drought-prone conditions began as early as -3,000 years BP. The aridity record also allowed examination of relationships between hydroclimate and terrestrial carbon discharge to the ocean. Comparison of radiocarbon measurements of sedimentary plant waxes with planktonic foraminifera reveal increasing age offsets starting -4,000 yrs BP, suggesting that increased aridity slows carbon cycling and/or transport rates. At the second site, a seismic survey of the Indus River subaqueous delta describes the morphology and Holocene sedimentation of the Pakistani shelf and identified suitable coring locations for paleoclimate reconstructions. The ([delta]¹³Cwax record shows a stable arid climate over the dry regions of the Indus plain and a terrestrial biome dominated by C₄ vegetation for the last 6,000 years. As the climate became more arid ~4,000 years, sedentary agriculture took hold in central and south India while the urban Harappan civilization collapsed in the already arid Indus basin. This thesis integrates marine and continental records to create regionally extensive paleoenvironmental reconstructions that have implications for landscape evolution, sedimentation, the terrestrial organic carbon cycle, and prehistoric human civilizations in the Indian subcontinent.
by Camilo Ponton.
Ph.D.

Das Ziel dieser Arbeit ist es Geheimnisse des Indischen Monsuns aufzudecken-ein groß-skaliges Klimaphänomen,das mehr als 1,7 Milliarden Menschen stark beeinflußt.Folglich ist das Verständnis der Mechanismen des Indischen Monsuns und seine erfolgreiche Prognose nicht nur eine Frage von größtem Interesse,sondern auch eine bedeutende wissenschaftliche Herausforderung.Der erste Teil dieser Arbeit ist den extremen Niederschlagsereignissen über dem Indischen Subkontinent gewidmet.In dieser Arbeit wurde gezeigt,dass eine Synchronizität zwischen extremen Niederschlagsereignissen in den Eastern Ghats und Nord Pakistan Regionen durch das Zusammenspiel zwischen dem indischen Monsun und einem nicht-Monsun-Niederschlagsmuster verursacht wird.Dieses Ergebnis unterstreicht die Bedeutung der Region Nord-Pakistan zur Ableitung der Wechselwirkung zwischen dem indischen Monsun-System und den West-Störungen,und verbessert daher das Verständnis der Kopplung des indischen Monsuns mit den Extratropen.Der zweite Teil der Arbeit befasst sich mit dem Problem der räumlichen und zeitlichen Organisation des abrupten Übergangs auf den indischen Monsun.Hier wird ein neuartiger Mechanismus des räumlich-zeitlichen Übergangs zur Regenperiode vorgeschlagen.Er hat mehrere Vorteile gegenüber bestehenden Erklärungen der Natur des indischen Monsuns:Es beschreibt den abrupten Übergang in einer gewählten Region des indischen Subkontinents sowie die räumliche Ausbreitung und Variabilität des indischen Monsuns beim Einsetzen entlang der Achse des Monsuns.Der dritte Teil dieser Arbeit konzentriert sich auf das Problem der Vorhersagbarkeit des indischen Monsuns.Das vorgeschlagene Verfahren ermöglicht die Vorhersage des Einsetzens und Endens über einen mehr als zwei Wochen bzw.einen Monat früheren Zeitraum im Vergleich zu bisher bekannten Methoden.Schließlich kann die vorgeschlagene Instrumentarium direkt in das bestehende lang-reichweitige Vorhersagesystem für den Monsuns implementiert werden.
The aim of this thesis is to uncover some of the mysteries surrounding the Indian Monsoon - a large-scale climatic phenomenon affecting more than 1.7 billion people. Consequently, understanding the mechanisms of the Indian monsoon and its successful forecasting is not only a question of great interest, but also a significant scientific challenge. The first part of this thesis is devoted to extreme rainfall events over the Indian subcontinent. In this thesis, I have shown that a synchronicity between extreme rainfall events in the Eastern Ghats and North Pakistan regions is caused by the interplay between the Indian Monsoon and a non-monsoonal precipitation pattern driven by the Westerlies - Western Disturbances. This result highlights the importance of the North Pakistan region for inferring the interaction between the Indian Monsoon system and Western Disturbances, and, therefore, improves the understanding of the Indian Monsoon coupling with the extratropics. The second part of this dissertation is concerned with the problem of the spatial and temporal organization of the abrupt transition to the Indian monsoon. Here, I have proposed a novel mechanism of a spatio-temporal transition to monsoon. It has several advantages in comparison to existing explanations of the Indian Monsoon nature: it describes the abrupt transition to monsoon in a chosen region of the Indian subcontinent, as well as the spatial propagation and variability of the Indian Monsoon onset along the axis of advance of monsoon. The third part of this thesis focuses on the problem of predictability of the Indian Monsoon. I have developed a novel method that predicts the onset and withdrawal dates more than two weeks and a month earlier than existing methods, respectively. Finally, the proposed scheme can be directly implemented into the existing long-range forecasting system of the monsoon''s timing.

The Indian Monsoon is an extremely important and large-scale meteorological phenomenon that occurs with monotonous regularity every year in the northern hemisphere summer. India and the surrounding regions depend on the monsoon's rains which, although averaged over a season are relatively constant, can vary on timescales of days to weeks with devastating economic and social impact. This study was performed in an effort to gain a little more insight in to the intraseasonal variability of the Indian Monsoon. This was done with the help of an extensive observational study using ECMWF reanalysis data and also some modelling with a simple atmospheric model. In order to interpret results, degrees of simplification will be introduced by compositing and averaging processes that betray the complexities of the monsoon system. However, it will be shown that this allows well-defined atmospheric structures to be obtained for what are known as active and break states of the monsoon. Some observational aspects of the active and break monsoon states will be shown, highlighting the major differences between the two states. This will be followed by a series of modelling experiments with increasing sophistication, although remaining very simple. The results of these will show enough consistency with observations to allow an investigation in to the effects of ENSO on the intraseasonal variability of the Indian monsoon.

Observational evidence is presented to show the existence of a boreal summer teleconnection between the Indian summer monsoon (ISM) and East Asian summer monsoon (EASM). Based on station rainfall data, the dominant patterns of variability in monthly and seasonal rainfall over India-East Asia region are investigated mainly through composite analysis. The association between the midlatitude circulation and Indian monsoon rainfall on interannual time scale has also been examined by using 54-year NCEP/NCAR reanalysis data. The major results are as follows: (1) Associated with the year-to-year fluctuation of Indian summer rainfall, a well-organized upper level teleconnection pattern is obvious over the Eurasia continent with two anomalous anticyclonic (cyclonic) circulations in the strong (weak) monsoon year. One center is located over West Tibet, and the other with an equivalent barotropic structure resides in northeast Asia. This teleconnection establishes a linkage between two monsoon systems. (2) Within the summer season from May to September, above mentioned midlatitude teleconnection pattern undergoes a different structure with the strongest intensity in June and August. Hence, depending on the condition of Indian monsoon, the 'window of linkage' between Indian subcontinent rainfall and East Asian monsoon seems to open only in June or August. (3) On the other hand, a global anomalous wavetrain with favored longitudinal phase has been found in the upper and middle troposphere in each summer month. During June and August, this geographically fixed wavetrain characteristic of a circumglobal feature coincides with the ISM-EASM teleconnection. Based on all these observational results, three possible scenarios are proposed to explain the establishment of the teleconnection. And it seems that the upper level climatological westerly jet, Indian monsoon heating and midlatitude stationary wave activity are important factors that control the establishment of the teleconnection. On intraseasonal time scale, simultaneous and lagged correlation statistics have been calculated between height in northern hemisphere and convection over north India. It is found that prior to the breakout of the convection in north India two anomalous ridges have existed over north of Pakistan and northeast Asia, respectively. A plausible mechanism of this interaction between the westerly flow in the midlatitude and Indian monsoon is also briefly discussed.
ix, 89 leaves

Thesis (Ph.D.)--George Mason University, 2009.
Vita: p. 179. Thesis director: V. Krishnamurthy. Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Climate Dynamics. Title from PDF t.p. (viewed June 10, 2009). Includes bibliographical references (p. 171-178). Also issued in print.

The Indian Summer Monsoon is a major component of the global climatic system. Stalagmites have a proven ability to provide information of such monsoon systems. In this thesis, examination of cave records and field work provides a framework of the spatial and temporal distribution of stalagmites in peninsular India. Stable oxygen isotope records from stalagmites are supported by trace element records. An aragonite stalagmite from the west coast of India suggests that changes in growth surface can effect precipitation through time available for dissolved inorganic carbon removal. Calculation of empirical partition coefficients from the aragonite stalagmite indicates that U/Ca, Sr/Ca and P/Ca ratios may be paleo-aridity indicators through the process of Prior Aragonite Precipitation. There may also be source and/or temperature control on the partitioning of Sr/Ca into the aragonite stalagmite. These are the first trace element measurements for stalagmites from peninsular India and some of the few available from aragonite stalagmites. The δ¹⁸O composition of a calcite stalagmite from central India that grew from 3130 to 2100 years BP is consistent with the hypothesis that δ18O is controlled by air parcel trajectory and amount of rainout between source and cave site. P/Ca and U/Ca records from this stalagmite provide information on past rainfall conditions. Correlation analysis of δ¹⁸O, P/Ca and U/Ca indicates that rainfall amount was not the dominant control on δ¹⁸O composition at this cave site. Examination of a stalagmite that has diagenetically altered from aragonite to calcite shows that the δ¹⁸O system is extremely susceptible to diagenesis. Sr/Ca and U/Ca of the primary aragonite are retained in secondary calcite. Higher Mg/Ca ratios in secondary calcite compared to primary aragonite indicates that diagenetic fluid adds material to the primary carbonate. The variation in U-Th ages are caused by differential addition and losses of U and Th isotopes.

Weather – cold, wet, hot and windy – pervades life, material and social. So present and obvious as to provide a challenge for research, material though ephemeral too, weather breaks boundaries and refuses categorisation. While night becomes day, the cold season warms up over weeks and annual patterns are changing on a scale of years, practices in the face of weather transitions are themselves shifting. Based on ten months of fieldwork in the small village of Gau in the Pahari Indian Himalayas this thesis interrogates the saliencies and permeations of weather in people’s lives. It investigates how people intra-act (Barad 2007) with the weather, though practices, infrastructures and relationships with others. My approach argues for the validity of weather as a means by which to learn about socio-material lives. Pahari villagers live and act within the weather that moves around them. They are subject to, but also modify, their thermal environment. Through housing, clothing and tools such as the fire and the fan they affect the impact of the weather as it meets their bodies, but also daily patterns of movement are coloured by weather considerations. This work views weather in relation to health practices (such as refraining from working during the rain so as not to fall ill), for care of others (such as domestic bovines), for house-building and hospitable relationship-building among neighbours, for negotiation of landslide-fraught access roads to elsewhere and for understandings of pollution in the air. This focus on weather is intended to connect dots for people working on climate change, both within and beyond anthropology, and to contribute to discussions in areas including human-animal relations, health and illness and housing.

Millennial-scale variability in the Indian monsoon was temporally linked to changes in global ocean circulation during the last glacial period, as evidenced by planktic-benthic foraminiferal stable isotope and trace element results from an intermediate depth sediment core from the northwestern Bay of Bengal (Core NGHP-01-19B; 18°58'N, 85°39'E; 1,422 m). Paired planktic foraminiferal Mg/Ca and δ¹⁸O_c of G. ruber constrain sea surface temperatures and isolate millennial-scale variations in the δ¹⁸O of surface waters (δ ¹⁸O_sw) which resulted from changes in river runoff in the northwestern Bay. Concurrently with low δ¹⁸O_sw events, benthic foraminiferal δ¹³C of Cibicidoides spp. decreased, suggesting an increased influence of an aged water mass at this intermediate depth site during the low salinity events. Benthic foraminiferal Cd/Ca of H. elegans supports the identification of this water mass as aged Glacial Antarctic Intermediate Water (GAAIW). Lagged correlation analysis (r= 0.41) indicates that changes in subsurface properties led changes in surface properties by an average of 380 years. The implication is that Southern Hemisphere climate exerted a controlling influence on the Indian monsoon during the last glacial period.

The Indian summer monsoon (ISM) is one of the largest climate systems on earth and impacts the livelihood of nearly 40% of the world’s population. Despite dedicated efforts, a comprehensive picture of monsoon variability has proved elusive largely due to the absence of long term high resolution records, spatial inhomogeneity of the monsoon precipitation, and the complex forcing mechanisms (solar insolation, internal teleconnections for e.g., El Niño-Southern Oscillation, tropical-midlatitude interactions). My work aims to improve the understanding of monsoon variability through generation of long term high resolution palaeoclimate data from climatically sensitive regions in the ISM and westerlies domain. To achieve this aim I have (i) identified proxies (sedimentological, geochemical, isotopic, and mineralogical) that are sensitive to environmental changes; (ii) used the identified proxies to generate long term palaeoclimate data from two climatically sensitive regions, one in NW Himalayas (transitional westerlies and ISM domain in the Spiti valley and one in the core monsoon zone (Lonar lake) in central India); (iii) undertaken a regional overview to generate “snapshots” of selected time slices; and (iv) interpreted the spatial precipitation anomalies in terms of those caused by modern teleconnections. This approach must be considered only as the first step towards identifying the past teleconnections as the boundary conditions in the past were significantly different from today and would have impacted the precipitation anomalies. As the Spiti valley is located in the in the active tectonic orogen of Himalayas, it was essential to understand the role of regional tectonics to make valid interpretations of catchment erosion and detrital influx into the lake. My approach of using integrated structural/morphometric and geomorphic signatures provided clear evidence for active tectonics in this area and demonstrated the suitability of these lacustrine sediments as palaleoseismic archives. The investigations on the lacustrine outcrops in Spiti valley also provided information on changes in seasonality of precipitation and occurrence of frequent and intense periods (ca. 6.8-6.1 cal ka BP) of detrital influx indicating extreme hydrological events in the past. Regional comparison for this time slice indicates a possible extended “break-monsoon like” mode for the monsoon that favors enhanced precipitation over the Tibetan plateau, Himalayas and their foothills. My studies on surface sediments from Lonar lake helped to identify environmentally sensitive proxies which could also be used to interpret palaeodata obtained from a ca. 10m long core raised from the lake in 2008. The core encompasses the entire Holocene and is the first well dated (by 14C) archive from the core monsoon zone of central India. My identification of authigenic evaporite gaylussite crystals within the core sediments provided evidence of exceptionally drier conditions during 4.7-3.9 and 2.0-0.5 cal ka BP. Additionally, isotopic investigations on these crystals provided information on eutrophication, stratification, and carbon cycling processes in the lake.
Der Indische Sommer Monsun (ISM) ist eines der bedeutendsten Klimaphänomene auf der Erde und hat großen Einfluss auf die Lebensbedingungen und -grundlagen von nahezu 40% der Weltbevölkerung. Trotz großer Bemühungen ist es bisher nicht gelungen ein genaues und umfassendes Verständnis der Monsun-Variabilität zu gewinnen. Hauptgründe dafür sind das Fehlen von langjährigen und hochaufgelösten Klimazeitreihen, räumlichen Inhomogenitäten in den Niederschlagsverteilungen und die Komplexität der treibenden klimatischen Mechanismen (Sonneneinstrahlung, interne Wechselwirkungen des Klimasystems, wie z.B. zwischen Tropen und mittleren Breiten oder die Auswirkungen der El Niño Oszillation). Die Zielsetzung der hier vorgestellten Arbeit ist ein verbessertes Verständnis der Monsun-Variabilität zu entwickeln, auf Basis von hochaufgelösten und weit reichenden Paläoklimazeitreihen aus klimasensitiven Regionen des ISM und der Westwindzone. Um die Zielsetzung umzusetzen habe ich: (i) Proxys identifiziert (sedimentologische, geochemische, isotopische, und mineralogische), die empfindlich auf Umweltveränderungen reagieren; (ii) die identifizierten Proxys zur Erzeugung von langjährigen Paläoklima-Daten für zwei klimasensible Regionen verwendet, eine im NW des Himalaja (Übergangs-Westwindzone und ISM Gebiet von Spity Valley) und eine in der Kernzone des Monsun (Lonar-See) in Zentralindien; (iii) Übersichts-"Momentaufnahmen" der regionalen klimatischen Bedingungen für ausgewählte Zeitpunkte der Vergangenheit erzeugt; und (iv) räumliche Niederschlagsanomalien in Hinblick auf heutige Wechselbeziehungen im Klimasystem interpretiert. Dieser Ansatz stellt allerdings nur einen ersten Schritt zur Identifizierung von paläoklimatischen Wechselbeziehungen im Monsunsystem dar, da sich die Randbedingungen in der Vergangenheit deutlich von den heutigen unterscheiden und diese einen signifikanten Einfluss auf die Niederschlagsanomalien haben. Da das Spity Valley im tektonisch aktiven Himalaja-Orogen lokalisiert ist, ist es von entscheidender Bedeutung die regionalen tektonischen Prozesse zu verstehen, um Erosionsvorgänge des Einzugsgebiets und die Einfuhr von Detritus in den See korrekt interpretieren zu können. Mein Ansatz der Nutzung kombinierter strukturell/morphometrischer und geomorphologischer Charakteristiken lieferte klare Beweise für aktive Tektonik im untersuchten Gebiet und demonstrierte damit die Eignung dieser lakustrinen Sedimente als paläoseismisches Archiv. Die Untersuchung lakustriner Aufschlüsse in Spity Valley lieferte auch Informationen saisonale Änderung der Niederschlagsverteilung sowie das Auftreten von häufigen und intensiven Perioden (ca. 6,8-6,1 cal ka BP) detritischer Einfuhr, welche auf extreme hydrologische Ereignisse in der Vergangenheit schließen lässt. Ein regionaler Vergleich dieser Periode deutet auf einen möglicherweise erweiterten „break-monsoon-like“ Modus für den Monsun hin, welcher hohe Niederschläge über dem Tibetischen Plateau, dem Himalaja und seinen Gebirgsausläufern begünstigt. Meine Studien an den Oberflächensedimenten des Lonar-Sees haben dazu beigetragen umweltsensitive Proxys zu identifizieren, die auch zur Interpretation von Paläodaten von einem ca. 10 m langen Sedimentkern genutzt wurden, der 2008 erbohrt wurde. Der Kern umfasst das gesamte Holozän und stellt das erste gut 14C-datierte Archiv aus der Kernmonsunzone Zentralindiens dar. Die Identifizierung von authigenen Evaporit-Kristallen (Gaylussite) innerhalb der Sedimente liefert einen Beweis für ungewöhnlich trockene Bedingungen in den Perioden zwischen 4,7-3,9 und 2,0-0,5 cal ka BP. Darüber hinaus lieferten Isotopen-Untersuchungen dieser Kristalle Informationen zur Eutrophierung, Stratifikation und zum Kohlenstoff-Kreislauf des Sees.

Climatological winds over the equatorial Indian Ocean (EqlO) are westerly most of the year. Twice a year, in April-May ("spring") and October-December ("fall"), strong, sustained westerly winds generate eastward equatorial jets in the ocean. There are several unresolved issues related to the equatorial jets. They accelerate rapidly to speeds over lms"1 when westerly wind stress increases to about 0.7 dyne cm"2 in spring and fall, but decelerate while the wind stress continues to be westerly; each jet is followed by westward flow in the upper ocean lasting a month or longer. In addition to the semi-annual cycle, the equatorial winds and currents have strong in-traseasonal fluctuations. Observations show strong 30-60 day variability of zonal flow, and suggest that there might be variability with periods shorter than 20 days in the central EqlO. Observations from moored current meter arrays along 80.5°E south of Sri Lanka showed a distinct 15 day oscillation of equatorial meridional velocity (v) and off-equatorial zonal velocity (u). Recent observations from current meter moorings at the equator in the eastern EqlO show continuous 10-20 day, or biweekly, oscillations of v. The main motivation for the present study is to understand the dynamics of intraseasonal variability in the Indian Ocean that has been documented in the observational literature. What physical processes are responsible for the peculiar behavior of the equatorial jets? What are the relative roles of wind stress and large scale ocean dynamics? Does intraseasonal variability of wind stress force intraseasonal jets? What is the structure and origin of the biweekly variability? The intraseasonal and longer timescale variability of the equatorial Indian Ocean circulation is studied using an ocean general circulation model (OGCM) and recent in Abstract ii situ observations. The OGCM simulations are validated against other available observations. In this thesis, we document the space-time structure of the variability of equatorial Indian Ocean circulation, and attempt to find answers to some of the questions raised above. The main results are based on OGCM simulations forced by high frequency reanalysis and satellite scatterometer (QuikSCAT) winds. Several model experiments with idealized winds are used to interpret the results of the simulations. In addition to the OGCM simulations, the origin of observed intraseasonal anomalies of sea surface temperature (SST) in the eastern EqlO and Bay of Bengal, and related air-sea interaction, are investigated using validated satellite data. The main findings of the thesis can be summarized as: • High frequency accurate winds are required for accurate simulation of equatorial Indian Ocean currents, which have strong variability on intraseasonal to interannual time scales. • The variability in the equatorial waveguide is mainly driven by variability of the winds; there is some intraseasonal variability near the western boundary and in the equatorial waveguide due to dynamic instability of seasonal "mean" flows. • The fall equatorial jet is generally stronger and longer lived than the spring jet; the fall jet is modulated on intraseasonal time scales. Westerly wind bursts can drive strong intraseasonal equatorial jets in the eastern EqlO during the summer monsoon. • Eastward equatorial jets create a westward zonal pressure gradient force by raising sea level, and deepening the thermocline, in the east relative to the west. The zonal pressure force relaxes via Rossby wave radiation from the eastern boundary. • The zonal pressure force exerts strong control on the evolution of zonal flow; the decel eration of the eastward jets, and the subsequent westward flow in the upper ocean in the presence of westerly wind stress, is due to the zonal pressure force. • Neither westward currents in the upper ocean nor subsurface eastward flow (the ob served spring and summer "undercurrent") requires easterly winds; they can be gener ated by equatorial adjustment due to Kelvin (Rossby) waves generated at the western (eastern) boundary. • The biweekly variability in the EqlO is associated with forced mixed Rossby-gravity (MRG) waves generated by intraseasonal variability of winds. The biweekly MRG wave in has westward and upward phase propagation, zonal wavelength of 3000-4500 km and phase speed of 4 m s"1; it is associated with deep off equatorial upwelling/downwelling. Intraseasonal SST anomalies are forced mainly by net heat flux anomalies in the central and eastern EqlO; the large northward propagating SST anomalies in summer in the Bay of Bengal are due to net heat flux anomalies associated with the monsoon active-break cycle. Coherent variability in the atmosphere and ocean suggests air-sea interaction.

La Mousson Indienne d’été (ISM) et l’Oscillation Australe El Niño (ENSO) sont parmi les phénomènes climatiques les plus énergétiques et importants de la planète. Bien qu’ils soient localisés dans la région tropicale Indo-Pacifique, ces deux phénomènes peuvent avoir des répercussions climatiques à l'échelle globale via les téléconnections atmosphériques. Cette thèse vise à améliorer notre compréhension et la prévisibilité interannuelle d’ENSO et de l’ISM, en analysant leurs téléconnections grandes échelles, et en déterminant le rôle, dans cette prévisibilité, des modes majeurs de variabilité interannuelle observés dans les régions tropicales et extratropicales de l’Indo-Pacifique. A partir de diagnostiques statistiques d’observations sur les périodes 1950-1976 et 1979-2007, et d’expériences de sensibilité réalisées à l’aide du modèle couplé SINTEX-F, nous montrons l'importance des latitudes tempérées dans la prévisibilité du système ENSO-mousson. Des précurseurs robustes des évènements ENSO et ISM sont identifiés dans les régions du Pacific Nord et de l’Océan Indien Sud pendant l’hiver boréal précédent, apportant ainsi de la prévisibilité plus tôt que leurs traditionnels précurseurs tropicaux. De plus, cette prévisibilité de la mousson d’été est accrue pour les pluies en fin de saison (Août-Septembre). Cette deuxième partie de la saison de mousson se caractérise également par l’émergence de processus couplés océan-atmosphère dans l’Océan Indien, pouvant contrecarrer l’effet d’ENSO sur la mousson. Une amplification de ces rétroactions locales pourrait expliquer l’affaiblissement de la relation ENSO-mousson observé lors des dernières décennies
El Niño-Southern Oscillation (ENSO) and the Indian Summer Monsoon (ISM) are two of the most energetic and influential climatic phenomena on the planet. Although they originate in the tropical Indo-Pacific region, they can extend their reach well beyond, through atmospheric teleconnnections that can affect patterns of climate variability worldwide. This thesis takes part in a global effort to improve our understanding of the potential predictability of ISM rainfall and ENSO, by exploring the large-scale teleconnections associated with the whole monsoon-ENSO system on interannual timescales, as well as the role played by leading modes of coupled variability, particularly in tropical and extratropical parts of the Indo-Pacific region. Based on statistical diagnoses of observations before and after the 1976-77 climate shift, and numerical experiments with the SINTEX-F coupled model, our results highlight the importance of mid-latitudes in the predictability of the ISM-ENSO system. Indeed, significant and robust precursors are identified in the North Pacific and South Indian Oceans during the previous boreal winter/early spring, and have the potential to predict ISM and ENSO events with longer lead-times than their traditional tropical predictors. Besides, this predictability is stronger for the ‘late’ ISM rainfall during August-September. This second part of the ISM season is also characterized by the occurrence of ocean-atmospheric processes in the Indian Ocean, which compete with the effect of ENSO on the monsoon. The enhancement of these local feedbacks could explain the apparent weakening of the seasonal ENSO-monsoon relationship observed in recent decades

A monção indiana está diretamente relacionada a inversão sazonal dos ventos controlada pela migração latitudinal da Zona de Convergência Intertropical (ITCZ). Em longo período os sistemas de monções são fortalecidos em resposta a períodos interglaciais (quentes e húmidos) e enfraquecidos em resposta a períodos glaciais (frios e secos). Durante os últimos 500 mil anos o clima global tem oscilado com uma periodicidade de ∼100 mil anos sendo controlado principalmente pela forçante orbital. Dados de testemunhos do Mar da Arábia, mostram que o aporte de sedimentos fluviais ocorre principalmente na parte noroeste com valores reduzidos à sudeste onde está localizada a República das Maldivas, que compreendem uma região distante o bastante das fontes fluviais de sedimento, sendo um local ideal para a obtenção de registros de sedimentos eólicos. Dados de satélite da pluma de poeira nas quatro diferentes estações do ano mostram que as Maldivas estão fortemente sob a influência da pluma de poeira relacionada a monção de inverno com ventos vindos de nordeste, tornando esta, uma região ideal parra o estudo das condições de aridez na área fonte na massa de terra indiano-asiática. Aqui nós apresentamos dados não destrutivos em alta resolução da Fluorescência do Raio X (XRF) e de magnetismos ambiental e de rocha dos vinte metros superiores do testemunho U1471, IODP EXP. 359, compreendendo um registro continuo de 500 mil anos. As boas correlações entre os diferentes elementos associados aos sedimentos terrígenos indicam que as variações nos dados de Fe, K, Al e Ti são robustas, sendo destes, o Fe, o elemento que menos apresentou ruído. Com a premissa de que, o aumento na extensão de regiões áridas na área fonte durante períodos frios está relacionada com o aumento na quantidade de elementos terrígenos que fluíram para as Maldivas, construímos um modelo de idade correlacionando os dados de Fe com os dados de δ18O, a alta resolução dos nossos dados e a boa correlação com os dados de δ18O permitiram identificar os Marine Isotope Stages (MIS) 1 ao 14. Dados de próxies paleoclimáticos como as razões, Al/Si, Fe/K e Sr/Ca indicam eventos abruptos extremamente húmidos em especial durante a transição MIS 5-6 sucedendo em poucas centenas de anos o evento Heinrich 11 (H11). Os dados de magnetismos ambiental e de rocha apontam para forte diagênese redutora de magnetita, formando minerais compostos de sulfeto de ferro, associada com a transição sulfato metano (SMT) na coluna sedimentar, abaixo desta transição a resposta magnética fica extremamente reduzida, mas ainda conserva as variações do sinal anterior a diagênese. Dados de First Order Reverse Curve (FORC) e de X Ray Absorption Near the Edge Structure (XANES) apontam a presença de magnetita bacteriana no topo do registro, mas devido ao caráter redutor do sedimento coluna abaixo e ao tamanho extremamente reduzido, Single Domain (SD), da magnetita bacteriana, este sinal está restrito ao topo do registro.
The Indian monsoon is directly related to the seasonal inversion of the winds driven by the latitudinal migration of the Intertropical Convergence Zone (ITCZ). Over a long term, monsoon systems are strengthened in response to interglacial periods (hot and humid) and weakened in response to glacial periods (cold and dry). Over the last 500 thousand years the global climate has oscillated with a periodicity of ∼100 thousand years being controlled mainly by the orbital forcing. Data from the Arabian Sea cores show that the contribution of fluvial sediments occurs mainly in the northwestern part with reduced values to the southeast where the Republic of the Maldives is located, which comprise a region far enough from the fluvial sediment sources, being an excellent place for the obtaining records of dust. Satellite data of the dust in the four different seasons of the year show that the Maldives are strongly influenced by the winter monsoon with north-easterly winds, making this an ideal region for the study of arid conditions in the source area in the Indian-Asian landmass. Here we present non-destructive high-resolution X-ray Fluorescence (XRF) and, environmental and rock magnetisms data from the upper twenty meters of the U1471, IODP EXP. 359, comprising a continuous record of 500 thousand years. The good correlations between the different elements associated to the terrigenous sediments indicate that the variations in the Fe, K, Al and Ti data are robust, of which Fe is the least noise element. With the premise that the increase in the extent of arid regions in the source area during cold periods is related to the increase in the amount of terrigenous elements that flowed into the Maldives, we constructed an age model by correlating the Fe data with the data of δ18O, the high resolution of our data and the good correlation with the data of δ18O allowed to identify the Marine Isotope Stages (MIS) 1 to 14. Paleoclimatic proxy data as the reasons, Al / Si, Fe / K and Sr / Ca indicate extremely humid abrupt events, especially during the MIS 5-6 transition succeeding in a few hundred years the event Heinrich 11 (H11). Environmental and rock magnetism data point to strong magnetite reduction diagenesis, forming minerals composed of iron sulphide, associated with the methane sulphate transition (SMT) in the sedimentary column, below this transition the magnetic response is extremely reduced, but still conserved the variations of the pre-diagenesis signal. First Order Reverse Curve (FORC) and X Ray Absorption Near the Edge (XANES) data indicate the presence of bacterial magnetite at the top of the core, but due to the reduced character of the sediment column below and the extremely small size, Single Domain (SD), of bacterial magnetite, this signal is restricted to the top.

Climatological winds over the equatorial Indian Ocean (EqlO) are westerly most of the year. Twice a year, in April-May ("spring") and October-December ("fall"), strong, sustained westerly winds generate eastward equatorial jets in the ocean. There are several unresolved issues related to the equatorial jets. They accelerate rapidly to speeds over lms"1 when westerly wind stress increases to about 0.7 dyne cm"2 in spring and fall, but decelerate while the wind stress continues to be westerly; each jet is followed by westward flow in the upper ocean lasting a month or longer. In addition to the semi-annual cycle, the equatorial winds and currents have strong in-traseasonal fluctuations. Observations show strong 30-60 day variability of zonal flow, and suggest that there might be variability with periods shorter than 20 days in the central EqlO. Observations from moored current meter arrays along 80.5°E south of Sri Lanka showed a distinct 15 day oscillation of equatorial meridional velocity (v) and off-equatorial zonal velocity (u). Recent observations from current meter moorings at the equator in the eastern EqlO show continuous 10-20 day, or biweekly, oscillations of v. The main motivation for the present study is to understand the dynamics of intraseasonal variability in the Indian Ocean that has been documented in the observational literature. What physical processes are responsible for the peculiar behavior of the equatorial jets? What are the relative roles of wind stress and large scale ocean dynamics? Does intraseasonal variability of wind stress force intraseasonal jets? What is the structure and origin of the biweekly variability? The intraseasonal and longer timescale variability of the equatorial Indian Ocean circulation is studied using an ocean general circulation model (OGCM) and recent in Abstract ii situ observations. The OGCM simulations are validated against other available observations. In this thesis, we document the space-time structure of the variability of equatorial Indian Ocean circulation, and attempt to find answers to some of the questions raised above. The main results are based on OGCM simulations forced by high frequency reanalysis and satellite scatterometer (QuikSCAT) winds. Several model experiments with idealized winds are used to interpret the results of the simulations. In addition to the OGCM simulations, the origin of observed intraseasonal anomalies of sea surface temperature (SST) in the eastern EqlO and Bay of Bengal, and related air-sea interaction, are investigated using validated satellite data. The main findings of the thesis can be summarized as: • High frequency accurate winds are required for accurate simulation of equatorial Indian Ocean currents, which have strong variability on intraseasonal to interannual time scales. • The variability in the equatorial waveguide is mainly driven by variability of the winds; there is some intraseasonal variability near the western boundary and in the equatorial waveguide due to dynamic instability of seasonal "mean" flows. • The fall equatorial jet is generally stronger and longer lived than the spring jet; the fall jet is modulated on intraseasonal time scales. Westerly wind bursts can drive strong intraseasonal equatorial jets in the eastern EqlO during the summer monsoon. • Eastward equatorial jets create a westward zonal pressure gradient force by raising sea level, and deepening the thermocline, in the east relative to the west. The zonal pressure force relaxes via Rossby wave radiation from the eastern boundary. • The zonal pressure force exerts strong control on the evolution of zonal flow; the decel eration of the eastward jets, and the subsequent westward flow in the upper ocean in the presence of westerly wind stress, is due to the zonal pressure force. • Neither westward currents in the upper ocean nor subsurface eastward flow (the ob served spring and summer "undercurrent") requires easterly winds; they can be gener ated by equatorial adjustment due to Kelvin (Rossby) waves generated at the western (eastern) boundary. • The biweekly variability in the EqlO is associated with forced mixed Rossby-gravity (MRG) waves generated by intraseasonal variability of winds. The biweekly MRG wave in has westward and upward phase propagation, zonal wavelength of 3000-4500 km and phase speed of 4 m s"1; it is associated with deep off equatorial upwelling/downwelling. Intraseasonal SST anomalies are forced mainly by net heat flux anomalies in the central and eastern EqlO; the large northward propagating SST anomalies in summer in the Bay of Bengal are due to net heat flux anomalies associated with the monsoon active-break cycle. Coherent variability in the atmosphere and ocean suggests air-sea interaction.

This thesis examines the behaviour of Indian monsoon depressions (MDs), synopticscale systems that frequent the monsoon trough throughout the boreal summer. The original contribution to knowledge is a substantial advancement of our understanding on the structure, variability, and dynamics of MDs. A feature-tracking algorithm is developed to emulate the subjective methods used by the India Meteorological Department, based on surface wind speed and surface pressure. This is applied to reanalysis data spanning from 1979 to the present day (2016), extracting 106 events. These are used to build a statistical composite, with which we interrogate the structure and variability subject to external forcings. It is found, for example, that MDs significantly intensify during La Ni˜na and active monsoon spells, and have different spatial structure over ocean than land. These events are also examined in satellite data, where we discover and characterise a bimodal, diurnal cycle in surface precipitation; the structure of cloud type with dominating deep convection south of the centre; and present the first composite picture of vertical hydrometeor structure in MDs. A propagation mechanism whereby the MD behaves as a vortex in the presence of a wall (i.e. the Himalayan massif) is presented and shown to perform better than competing theories at predicting the velocity and heading of MDs. It is also shown, using a case study in the Met Office Unified Model (MetUM), that varying antecedent soil moisture conditions can significantly change the length of a MD track but not its direction. The sensitivity of MDs to changes in horizontal resolution in the MetUM are also explored across seven case studies at eight resolutions. It is found that the intensity is typically slightly overpredicted, correlating with too great a latent heat release in the mid-troposphere. Spatial structure is shown to improve with resolution but improvement saturates beyond N512 (40 km) resolution.

Recent years witnessed a vast advent of stalagmites as palaeoclimate archives. The multitude of geochemical and physical proxies and a promise of a precise and accurate age model greatly appeal to palaeoclimatologists. Although substantial progress was made in speleothem-based palaeoclimate research and despite high-resolution records from low-latitudinal regions, proving that palaeo-environmental changes can be archived on sub-annual to millennial time scales our comprehension of climate dynamics is still fragmentary. This is in particular true for the summer monsoon system on the Indian subcontinent. The Indian summer monsoon (ISM) is an integral part of the intertropical convergence zone (ITCZ). As this rainfall belt migrates northward during boreal summer, it brings monsoonal rainfall. ISM strength depends however on a variety of factors, including snow cover in Central Asia and oceanic conditions in the Indic and Pacific. Presently, many of the factors influencing the ISM are known, though their exact forcing mechanism and mutual relations remain ambiguous. Attempts to make an accurate prediction of rainfall intensity and frequency and drought recurrence, which is extremely important for South Asian countries, resemble a puzzle game; all interaction need to fall into the right place to obtain a complete picture. My thesis aims to create a faithful picture of climate change in India, covering the last 11,000 ka. NE India represents a key region for the Bay of Bengal (BoB) branch of the ISM, as it is here where the monsoon splits into a northwestward and a northeastward directed arm. The Meghalaya Plateau is the first barrier for northward moving air masses and receives excessive summer rainfall, while the winter season is very dry. The proximity of Meghalaya to the Tibetan Plateau on the one hand and the BoB on the other hand make the study area a key location for investigating the interaction between different forcings that governs the ISM. A basis for the interpretation of palaeoclimate records, and a first important outcome of my thesis is a conceptual model which explains the observed pattern of seasonal changes in stable isotopes (d18O and d2H) in rainfall. I show that although in tropical and subtropical regions the amount effect is commonly called to explain strongly depleted isotope values during enhanced rainfall, alone it cannot account for observed rainwater isotope variability in Meghalaya. Monitoring of rainwater isotopes shows no expected negative correlation between precipitation amount and d18O of rainfall. In turn I find evidence that the runoff from high elevations carries an inherited isotopic signature into the BoB, where during the ISM season the freshwater builds a strongly depleted plume on top of the marine water. The vapor originating from this plume is likely to memorize' and transmit further very negative d18O values. The lack of data does not allow for quantication of this plume effect' on isotopes in rainfall over Meghalaya but I suggest that it varies on seasonal to millennial timescales, depending on the runoff amount and source characteristics. The focal point of my thesis is the extraction of climatic signals archived in stalagmites from NE India. High uranium concentration in the stalagmites ensured excellent age control required for successful high-resolution climate reconstructions. Stable isotope (d18O and d13C) and grey-scale data allow unprecedented insights into millennial to seasonal dynamics of the summer and winter monsoon in NE India. ISM strength (i. e. rainfall amount) is recorded in changes in d18Ostalagmites. The d13C signal, reflecting drip rate changes, renders a powerful proxy for dry season conditions, and shows similarities to temperature-related changes on the Tibetan Plateau. A sub-annual grey-scale profile supports a concept of lower drip rate and slower stalagmite growth during dry conditions. During the Holocene, ISM followed a millennial-scale decrease of insolation, with decadal to centennial failures resulting from atmospheric changes. The period of maximum rainfall and enhanced seasonality corresponds to the Holocene Thermal Optimum observed in Europe. After a phase of rather stable conditions, 4.5 kyr ago, the strengthening ENSO system dominated the ISM. Strong El Nino events weakened the ISM, especially when in concert with positive Indian Ocean dipole events. The strongest droughts of the last 11 kyr are recorded during the past 2 kyr. Using the advantage of a well-dated stalagmite record at hand I tested the application of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to detect sub-annual to sub-decadal changes in element concentrations in stalagmites. The development of a large ablation cell allows for ablating sample slabs of up to 22 cm total length. Each analyzed element is a potential proxy for different climatic parameters. Combining my previous results with the LAICP- MS-generated data shows that element concentration depends not only on rainfall amount and associated leaching from the soil. Additional factors, like biological activity and hydrogeochemical conditions in the soil and vadose zone can eventually affect the element content in drip water and in stalagmites. I present a theoretical conceptual model for my study site to explain how climatic signals can be transmitted and archived in stalagmite carbonate. Further, I establish a first 1500 year long element record, reconstructing rainfall variability. Additionally, I hypothesize that volcanic eruptions, producing large amounts of sulfuric acid, can influence soil acidity and hence element mobilization.
Stalagmiten erfuhren in den letzten Jahren vermehrt Aufmerksamkeit als bedeutende Paläoklima- Archive. Paläoklimatologen sind beeindruckt von der grossen Zahl geochemischer und physikalischer Indikatoren (Proxies) und der Möglichkeit, präzise absolute Altersmodelle zu erstellen. Doch obwohl substantielle Fortschritte in der speleothem-basierten Klimaforschung gemacht wurden, und trotz hochaufgelöster Archive aus niederen Breiten, welche zeigen, das Umweltveränderungen auf Zeitskalen von Jahren bis Jahrtausenden archiviert und rekonstruiert werden können, bleibt unser Verständnis der Klimadynamik fragmentarisch. Ganz besonders gilt dies für den Indischen Sommermonsun (ISM) auf dem Indischen Subkontinent. Der ISM ist heute als ein integraler Bestandteil der intertropischen Konvergenzzone verstanden. Sobald dieser Regengürtel während des borealen Sommer nordwärts migriert kann der ISM seine feuchten Luftmassen auf dem Asiatischen Festland entladen. Dabei hängt die Stärke des ISM von einer Vielzahl von Faktoren ab. Zu diesen gehören die Schneedicke in Zentralasien im vorhergehenden Winter und ozeanische Bedingungen im Indischen und Pazifschen Ozean. Heute sind viele dieser Faktoren bekannt. Trotzdem bleiben deren Mechanismen und internen Verbindungen weiterhin mysteriös. Versuche, korrekte Vorhersagen zu Niederschlagsintensität und Häufigkeit oder zu Dürreereignissen zu erstellen ähneln einem Puzzle. All die verschiedenen Interaktionen müssen an die richtige Stelle gelegt werden, um ein sinnvolles Bild entstehen zu lassen. Meine Dissertation versucht, ein vertrauenswürdiges Bild des sich wandelnden Holozänen Klimas in Indien zu erstellen. NE Indien ist eine Schlüsselregion für den östlichen Arm des ISM, da sich hier der ISM in zwei Arme aufteilt, einen nordwestwärts und einen nordostwärts gerichteten. Das Meghalaya Plateau ist das erste Hindernis für die sich nordwärts bewegenden Luftmassen und erhält entsprechend exzessive Niederschläge während des Sommers. Die winterliche Jahreszeit dagegen ist sehr trocken. Die Nähe zum Tibetplateau einerseits und der Bucht von Bengalen andererseits determinieren die Schlüsselposition dieser Region für das Studium der Interaktionen der den ISM beeinflussenden Kräfte. Ein Fundament für die Interpretation der Paläoklimarecords und ein erstes wichtiges Ergebnis meiner Arbeit ist ein konzeptuelles Modell, welches die beobachteten saisonalen Veränderungen stabiler Isotope (d18O und d2H) im Niederschlag erklärt. Ich zeige, das obwohl in tropischen und subtropischen Regionen meist der amount effect zur Erklärung stark negativer Isotopenwerte während starker Niederschläge herangezogen wird, dieser allein nicht ausreicht, um die Isotopenvariabilität im Niederschlag Meghalaya's zu erklären. Die Langzeitbeobachtung der Regenwasserisotopie zeigt keine negative Korrelation zwischen Niederschlagsmenge und d18O. Es finden sich Hinweise, das der Abfluss aus den Hochgebirgsregionen Tibets und des Himalaya eine Isotopensignatur an das Oberflächenwasser der Bucht von Bengalen vererbt. Dort bildet sich aus isotopisch stark abgereicherten Wässern während des ISM eine Süsswasserlinse aus. Es ist wahrscheinlich, das Wasserdampf, der aus dieser Linse stammt, ein Isotopensignal aufgeprägt bekommt, welches abgereichertes d18O weitertransportiert. Der Mangel an Daten lässt es bisher leider nicht zu, quantitative Aussagen über den Einfluss dieses plume effect' auf Niederschläge in Meghalaya zu treffen. Es lässt sich allerdings vermuten, das dieser Einfluss auf saisonalen wie auch auf langen Zeitskalen variabel ist, abhängig vom Abfluss und der Quellencharacteristik. Der Fokus meiner Arbeit liegt in der Herauslösung klimatischer Signale aus nordostindischen Stalagmiten. Hohe Urankonzentrationen in diesen Stalagmiten erlaubt eine exzellente Alterskontrolle, die für hochauflösende Klimarekonstruktionen unerlässlich ist. Die stabilen Isotope (d18O und d13C), sowie Grauwertdaten, erlauben einmalige Einblicke in die Dynamik des Sommer und auch des Wintermonsun in NE Indien. Die ISM Stärke (d. h. Niederschlagsmenge) wird in Veränderungen in den d18Ostalagmites reflektiert. Das d13C Signal, welches Tropfratenänderungen speichert, dient als potenter Indikator für winterliche Trockenheitsbedingungen. Es zeigt Ähnlichkeit zu temperaturabhängigen Veränderungen auf dem Tibetplateau. Das sub-annuell aufgelöste Grauwertprofil stärkt das Konzept, das verminderte Tropfraten und langsameres Stalagmitenwachstum eine Folge von Trockenheit sind. Während des Holozäns folgte der ISM der jahrtausendelangen Verringerung der Insolation. Es finden sich aber ebenso rapide Anomalien, die aus atmosphärischen Veränderungen resultieren. Die Phase des höchsten Niederschlages und erhöhter Saisonalität korrespondiert mit dem Holozänen Thermalen Maximum. Nach einer Phase einigermassen stabilen Bedingungen begann vor ca. 4500 Jahren ENSO einen zunehmenden Einfluss auf den ISM auszuüben. Starke El Nino Ereignisse schwächen den ISM, besonders wenn diese zeitgleich mit positiven Indian Ocean Dipole Ereignissen auftreten. Die stärksten Dürren des gesamten Holozäns traten in den letzten 2000 Jahren auf. Um zusätzliche Informationen aus den hervorragenden Proben zu gewinnen nutzte ich die Vorteile der laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Diese erlaubt die Detektion sub-annueller bis sub-dekadischer Elementkonzentrationsveränderungen in Stalagmiten. Mittels einer neu entwickelten Ablationszelle konnten Proben von maximal 22 cm Länge untersucht werden. Jedes analysierte Element ist ein potentieller Träger einer Klimainformation. Die Kombination der früheren Ergebnisse mit denen der LA-IPC-MS zeigt, das die Elementkonzentrationen nicht nur von Niederschlagsveränderungen und assoziiertem Auswaschen aus dem Boden abhängen. Zusätzlich können auch die biologische Aktivität und hydrogeochemische Bedingungen in der vadosen Zone Einfluss auf die Elementzusammensetzung im Tropfwasser und in den Stalagmiten haben. Darum entwickelte ich ein theoretisches Modell für meinen Standort, um zu klären, wie Klimasignale von der Atmosphäre in die Höhle transportiert werden können. Ein anschliessend rekonstruierter 1500 Jahre langer Proxyrecord zeigt Niederschlagsvariabilität an. Zudem besteht die Möglichkeit, das Vulkaneruptionen, welche grosse Mengen an Schwefelsäure produzieren, eine Bodenversauerung verursachen und damit die Elementmobilisierung verstärken können.

Summer monsoon rainfall accounts for at least 80% of the annual-total precipitation in many Indian states. Intraseasonal variations (ISV) in rainfall produce floods and droughts that can devastate agriculture. ISVs are dominated by a 30-50 day northward-propagating oscillation (NPISO) between the eastern equatorial Indian Ocean (EEqIO) and India. This thesis evaluates the hypothesis that atmosphere-ocean interactions are critical to the NPISO's period, intensity, and propagation. Two simple NPISO indices are created from lag correlations in outgoing longwave radiation between the oscillation's two poles. The index in which India leads the EEqIO better captures the NPISO, implying that convection in the EEqIO cannot predict Indian rainfall. An idealized NPISO lifecycle suggests that air-sea interactions occur via atmospheric thermodynamic forcing and a feedback from SSTs on low-level atmospheric stability.

A high-resolution multi-proxy reconstruction of the Arabian Sea Summer Monsoon (ASSM) intensity over the past 90,000 years has been determined using two marine sediment cores: one from the Somali margin and one from the Indian margin. This reconstruction indicates that changes in monsoon- induced upwelling, primary productivity and denitrification have varied in synchrony with Dansgaard-Oeschger (D-O) cycles. Increased monsoon intensity correlates with warm climate events (interstadials) and decreased monsoon intensity, which coincides with stadials and Heinrich Events, is confirmed by elevated dust concentrations in the marine cores. A comparison of the Somali and Indian margin cores with previously reported studies from the Northern and Western Basin allows the identification of discrete sediment signals from the Indus River, the Arabian Peninsula and from local riverine runoff. Sedimentary deposition on the Indian margin during interglacials is dominated by local terrestrial runoff, whereas during glacial periods increased dust input from the Arabian Peninsula is evident. Both signals are related to changes in the intensity of the ASSM. Monsoon intensity has decreased during the Holocene as the Intertropical Convergence Zone (ITCZ) has moved to a more southerly position. The ASSM-ITCZ relationship (increased ASSM intensity and a northern ITCZ, decreased ASSM intensity and a southern ITCZ) has remained consistent over the last glacial cycle suggesting that global millennial scale climatic variability is in part driven by modulations in tropical hydrological cycle. This ASSM reconstruction provides evidence that rearrangements in the tropical convection system affected atmospheric dust concentrations as well as the concentration and location of atmospheric water vapour. In addition to modulating terrestrial and marine emissions of greenhouse gases, variation in the tropical hydrological cycle provides a mechanism of amplifying and perpetuating millennial-scale climatic changes.

The Indian Summer Monsoon (ISM) plays a vital role in the livelihoods and economy of those living on the Indian subcontinent, including the small, mountainous country of Bhutan. The ISM fluctuates over varying temporal scales and its variability is related to many internal and external factors including the El Niño Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). In 2015, a Super El Niño occurred in the tropical Pacific alongside a positive IOD in the Indian Ocean and was followed in 2016 by a simultaneous La Niña and negative IOD. These events had worldwide repercussions. However, it is unclear how the ISM was affected during this time, both at a regional scale over the whole ISM area and at a local scale over Bhutan. First, an evaluation of data products comparing ERA5 reanalysis, TRMM and GPM satellite, and GPCC precipitation products against weather station measurements from Bhutan, showed that ERA5 reanalysis was the most suitable product to investigate ISM change in these two years. Using the reanalysis datasets, it was shown that there was disruption to the ISM during this period, with a late onset of the monsoon in 2015, a shifted monsoon flow in July 2015 and in August 2016 and a late withdrawal in 2016. However, this resulted in neither a monsoon surplus nor deficit across both years but instead large spatial-temporal variability. It is possible to attribute some of the regional scale changes to the ENSO and IOD events, but the expected impact of a simultaneous ENSO and IOD events are not recognisable. This may be due to a supposed weakening of the ENSO/ISM relationship and it is likely that 2015/16 monsoon disruption was driven by a combination of factors alongside ENSO and the IOD, including varying boundary conditions, the Pacific Decadal Oscillation, the Atlantic Multi-decadal Oscillation, and more. At a local scale, the intricate topography and orographic processes ongoing within Bhutan further amplified or dampened the already altered ISM. Whilst ENSO and IOD driven monsoon variability can be recognised at a regional scale, a direct link between ENSO and IOD activity and changes to the monsoon at a local scale over Bhutan is hard to distinguish. It is unknown how the ISM, ENSO, and the IOD will evolve under a future changing climate and therefore this presents a concern to Bhutan with its inherent vulnerability to monsoon variability.

Le silicium est le second élément le plus abondant de la croûte terrestre et un nutriment clefs des écosystèmes aquatiques. Il existe de fortes interactions entre Si, le cycle du carbone et les processus biogéochimiques. Cette étude porte sur la variabilité de Si (amorphe-ASi, lithogène-LSi et dissous-DSi) et les isotopes de Si le long du continuum continent-océan. Nous avons étudié la variabilité saisonnière et spatiale de ASi, LSi, DSi et des isotopes dans ~20 estuaires indiens. Nous avons catégorisé les estuaires selon une analyse statistique (PCA et regroupement). Le prélèvement par les diatomées semble être le principal processus contrôlant ASi en saison sèche, surtout au Sud. L’altération et l’érosion contrôlent LSi dans les autres estuaires. En saison humide, l’impact des diatomées n’est pas observé à cause d’une trop forte charge sédimentaire et tous les estuaires sont dominés par les apports lithogéniques. Les compositions isotopiques de Si peuvent tracer les sources de Si et les interactions biogéochimiques. Les résultats isotopiques montrent une différence saisonnière claire avec un impact fort de l’altération aux deux saisons. Les bassins versants du sud-ouest sont très différents des autres bassins du fait de leur topographie et climat. L’impact de l’agriculture et de la couverture forestière est aussi clairement présent dans tous les bassins tandis que la composition isotopique de Si des eaux souterraines résulte d’une combinaison de production et dissolution de minéraux. Ainsi, cette étude montre le rôle prépondérant de l’altération et du type d’argiles formées sur les isotopes de Si, indépendamment des saisons, plutôt que des processus biologiques ou de mélange tels que rapportés précédemment
Silicon is the second most abundant element in Earth’s crust and one of the key nutrient in aquatic ecosystems. There are strong interactions of Si with carbon cycle and biogeochemical processes. The present thesis focused on variability of silicon (amorphous-ASi, lithogenic-LSi and dissolved-DSi) and Si isotopes along the land to ocean continuum. We investigated the seasonal and spatial variability of ASi, LSi & DSi and Si isotopes in ~20 Indian estuaries. We categorize the estuaries using statistical analysis (PCA and cluster analysis). Diatom uptake seems to be the main process controlling ASi during dry period, especially in the South. Weathering and erosion control the variability of LSi in the remaining estuaries. Similarly lithogenic supply controls Si during wet period in all estuaries and no impact of diatoms was seen because of high suspended load. Si isotopic compositions trace the Si sources and biogeochemical pathways. The isotopic results exhibit clear seasonal difference with high impact of type of weathering during both seasons. They show that southwest watersheds are very special in terms of weathering regime compared to the other watersheds because of topography and climate. The impact of agriculture and forest cover on Si cycle is also clearly evidenced in all the basins during wet period. We show that groundwater Si isotopic variability results from a combination of dissolution and production of minerals. Overall, this study shows the preponderant influence of weathering and type of secondary clays on Si isotopes irrespective to the seasons, rather than the biological uptake or mixing as reported elsewhere

L'objectif principal de cette thèse est de restituer l'évolution passée des moussons asiatiques au cours du Quaternaire et d’en évaluer leurs impacts sur l’érosion continentale et les transferts sédimentaires terre-mer, à partir de l’étude de carottes marines collectées dans la Baie du Bengale, l'ouest de la mer des Philippines et la mer d'Arabie. La stratégie scientifique mise en œuvre implique des analyses minéralogiques (argiles), sédimentologiques (granulométrie laser) et géochimiques (⁸⁷Sr/⁸⁶Sr et εNd) afin de restituer les zones sources sédimentaires, les conditions d’érosion et de transfert sédimentaires à l’océan. Les analyses de la concentration en élément des terres rares et des valeurs de l’εNd ont également été faites sur des échantillons d’eau de mer et de foraminifères collectés dans la Baie du Bengale afin de contraindre l’utilisation de ce traceur dans un contexte de très forts changements saisonniers de débit des fleuves Himalayens. Cette stratégie nous a permis, entre autre, de restituer les précipitations de mousson du domaine ouest tropical Pacific au cours du Quaternaire et d’établir un lien avec l’évolution à long terme de la dynamique de circulation méridienne de type ENSO. Nous avons également apporté de nouvelles contraintes sur l’utilisation du traceur εNd dans les foraminifères de la Baie du Bengale en vue d’en restituer la dynamique passée de l’érosion himalayenne
The main objective of this PhD study is to reconstruct the evolution of the Asian monsoons during the Quaternary and their impacts on the continental erosion and sedimentary transfers from land to sea by the investigation of sediments cores collected in the Northern Bay of Bengal, the western Philippines Sea and the Arabian Sea. The implemented scientific strategy involves mineralogical (clay size fraction), sedimentological (grain-size laser) and geochemical (⁸⁷Sr/⁸⁶Sr and εNd) analyses in order to establish sedimentary sources, conditions of erosion and transfer of sediments to the Ocean. The analyses of the concentration of Rare Earth Elements (REE) and εNd were also made on seawater and foraminifera samples to better constrain the εNd as a proxy of weathering in a context of strong seasonal variations of sediment discharges by Himalayan rivers. Clay mineralogy and laser grain-size analyses have been conducted on sediments from core MD06-3050 collected on the Benham Rise (Philippines Sea). Siliciclastic grain-size results indicate variations of the relative proportion of three grain-size sub-populations corresponding to eolian dusts (EM2 about 9-11 μm) and Luzon rivers inputs (EM1 about 2-5 μm and EM3 about 19-25 μm). The long-term evolutions of the EM1/EM2 and smectite/(illite+chlorite) ratios permit to reconstruct variations of the contribution of detrital material deriving from the volcanic arc of Luzon and rainfall intensity of this tropical region. At long time scale, periods of intensification of monsoon rainfall on Luzon are associated to a reduction of precipitation on central China. These periods are also associated to an increase of the zonal gradient of sea surface temperatures on the equatorial Pacific Ocean suggesting a strengthening of El Niña conditions. These results highlight for the first time a strong role of the dynamics of the meridian circulation of ENSO on the long-term changes of rainfall of the tropical western Pacific during the Quaternary. In the Arabian Sea, clay mineralogy, siliciclastic grain-size, ⁸⁷Sr/⁸⁶Sr ratio and εNd were analysed on Quaternary sediments of the IODP site U1457. Our results suggest a change in the relative proportions of sediments from the Deccan Trapps (smectite) and the Indus river (mainly illite and chlorite). Variability of sedimentary sources and sediment transport (turbidites activity) to the Indus Fan have been reconstructed and attributed to monsoon rainfall and the sea level variations. The concentrations of REE combined with εNd were analysed on seawater samples collected in June 2012 along a North-South cross section in the Bay of Bengal. We highlighted from normalized REE patterns that the contributions of dissolved REE from the Ganges-Brahmaputra river system was the main source of the dissolved REE of surface waters of the Bay of Bengal, whereas the desorption of lithogenic particles dominate the dissolved REE of the intermediate and deep waters masses. We then revalued the residence time of the dissolved REE in the Bay of Bengal. A comparison of εNd, obtained just before the increase of the Ganges-Brahmaputra river discharge inferred by Indian monsoon rainfall, with the results obtained by Singh and al. (2012) for seawater samples collected after the peak of river discharge, allowed us to highlight for the first time a seasonal variability of seawater εNd of the Bay of Bengal. εNd have been analysed on planktonic foraminiferas of core MD77-176 located at 1375 m water depth to reconstruct for the first time the seawater εNd record of the intermediate waters masses of northern Bay of Bengal for the last 27 kyr. This new seawater εNd record of the Northern Bay of Bengal give us new constrain for this proxy already used to reconstruct past changes of the Himalayan weathering

The observed summertime drying over Northern Central India (NCI) during the latter half of the twentieth century is not reproduced by the Coupled Model Intercomparison Project Phase 5 (CMIP5) model ensemble average. At the same time, the spread between precipitation trends from individual model realizations is large, indicating that internal variability potentially plays an important role in explaining the observed trend. Here we show that the drying is indeed related to the observed 1950–1999 positive trend of the Pacific Decadal Oscillation (PDO) index and that the relationship is even stronger for a simpler index (S1). Adjusting the CMIP5-simulated precipitation trends to account for the difference between the observed and simulated S1 trend increases the original multimodel average NCI drying trend from −0.09 ± 0.31 mm d−1 (50 years)−1 to −0.54 ± 0.40 mm d−1 (50 years)−1. Thus, our estimate of the 1950–1999 NCI drying associated with Pacific decadal variability is of similar magnitude as our previous CMIP5-based estimate of the drying due to anthropogenic aerosol. The drying (moistening) associated with increasing (decreasing) S1 can partially be attributed to a southeastward (northwestward) shift of the boundary between ascent and descent affecting NCI. This shift of the ascent region strongly affects NCI but not Southeast Asia and south China. The average spread between individual model realizations is only slightly reduced when adjusting for S1 as smaller-scale variability also plays an important role.

The observed summertime drying over Northern Central India (NCI) during the latter half of the twentieth century is not reproduced by the Coupled Model Intercomparison Project Phase 5 (CMIP5) model ensemble average. At the same time, the spread between precipitation trends from individual model realizations is large, indicating that internal variability potentially plays an important role in explaining the observed trend. Here we show that the drying is indeed related to the observed 1950–1999 positive trend of the Pacific Decadal Oscillation (PDO) index and that the relationship is even stronger for a simpler index (S1). Adjusting the CMIP5-simulated precipitation trends to account for the difference between the observed and simulated S1 trend increases the original multimodel average NCI drying trend from −0.09 ± 0.31 mm d−1 (50 years)−1 to −0.54 ± 0.40 mm d−1 (50 years)−1. Thus, our estimate of the 1950–1999 NCI drying associated with Pacific decadal variability is of similar magnitude as our previous CMIP5-based estimate of the drying due to anthropogenic aerosol. The drying (moistening) associated with increasing (decreasing) S1 can partially be attributed to a southeastward (northwestward) shift of the boundary between ascent and descent affecting NCI. This shift of the ascent region strongly affects NCI but not Southeast Asia and south China. The average spread between individual model realizations is only slightly reduced when adjusting for S1 as smaller-scale variability also plays an important role.

In dieser Arbeit wird das regionale Klimamodell HIRHAM mit einer horizontalen Auflösung von 50 km und 19 vertikalen Schichten erstmals auf den asiatischen Kontinent angewendet, um die indische Monsunzirkulation unter rezenten und paläoklimatischen Bedingungen zu simulieren. Das Integrationsgebiet des Modells erstreckt sich von etwa 0ºN - 50ºN und 42ºE - 110ºE und bedeckt dabei sowohl die hohe Topographie des Himalajas und Tibet Plateaus als auch den nördlichen Indischen Ozean. Das Ziel besteht in der Beschreibung der regionalen Kopplung zwischen der Monsunzirkulation und den orographischen sowie diabatischen Antriebsmechanismen. Eine 44-jährige Modellsimulation von 1958-2001, die am seitlichen und unteren Rand von ECMWF Reanalysen (ERA40) angetrieben wird, bildet die Grundlage für die Validierung der Modellergebnisse mit Beobachtungen auf der Basis von Stations- und Gitterdatensätzen. Der Fokus liegt dabei auf der atmosphärischen Zirkulation, der Temperatur und dem Niederschlag im Sommer- und Wintermonsun, wobei die Qualität des Modells sowohl in Bezug zur langfristigen und dekadischen Klimatologie als auch zur interannuellen Variabilität evaluiert wird. Im Zusammenhang mit einer realistischen Reproduktion der Modelltopographie kann für die Muster der Zirkulation und Temperatur eine gute Übereinstimmung zwischen Modell und Daten nachgewiesen werden. Der simulierte Niederschlag zeigt eine bessere Übereinstimmung mit einem hoch aufgelösten Gitterdatensatz über der Landoberfläche Zentralindiens und in den Hochgebirgsregionen, der den Vorteil des Regionalmodells gegenüber der antreibenden Reanalyse hervorhebt. In verschiedenen Fall- und Sensitivitätsstudien werden die wesentlichen Antriebsfaktoren des indischen Monsuns (Meeresoberflächentemperaturen, Stärke des winterlichen Sibirischen Hochs und Anomalien der Bodenfeuchte) untersucht. Die Ergebnisse machen deutlich, dass die Simulation dieser Mechanismen auch mit einem Regionalmodell sehr schwierig ist, da die Komplexität des Monsunsystems hochgradig nichtlinear ist und die vor allem subgridskalig wirkenden Prozesse im Modell noch nicht ausreichend parametrisiert und verstanden sind. Ein paläoklimatisches Experiment für eine 44-jährige Zeitscheibe im mittleren Holozän (etwa 6000 Jahre vor heute), die am Rand von einer globalen ECHAM5 Simulation angetrieben wird, zeigt markante Veränderungen in der Intensität des Monsuns durch die unterschiedliche solare Einstrahlung, die wiederum Einflüsse auf die SST, die Zirkulation und damit auf die Niederschlagsmuster hat.
In this study the regional climate model HIRHAM with a horizontal resolution of 50 km and 19 vertical levels is applied over the Asian continent to simulate the Indian monsoon circulation under present-day and past conditions. The integration domain extends from 0ºN - 50ºN and 42ºE - 110ºE and covers the high topography of Himalayas and Tibetan Plateau as well as the northern Indian Ocean. The main objective is the description of the regional coupling between monsoon circulation and orographic as well as thermal driving mechanisms of monsoon. A 44-years long simulation from 1958-2001, driven at the lateral and lower boundaries by European reanalysis (ERA40), is the basis for the validation of model results with observations based on station and gridded data sets. The focus is on the the long-term and decadal summer and winter monsoon climatology and its variability concerning atmospheric circulation, temperature and precipitation. The results successfully reproduce the observations due to a realistic simulation of topographic features. The simulated precipitation shows a better agreement with a high-resolution gridded data set over the central land areas of India and in the higher elevated Tibetan and Himalayan regions than ERA40. In different case and sensitivity studies the main driving mechanisms of the Indian monsoon (Sea Surface Temperatures, strength of the Siberian High in winter and soil moisture anomalies) are investigated. The results show, that the simulation of these mechanisms with a regional climate model is also difficult related to the complex non linear monsoon system and the small-scale processes, which are not just sufficiently parameterized and understood in the model. A paleoclimatic experiment for a 44-years long time slice in mid-holocene (6000 years before present), which is driven by a global ECHAM5 simulation, shows significant changes in the monsoon intensity due to the different solar forcing, which influences the SST, the circulation and the precipitation.

The understanding and prediction of the monsoon variability over South Asia region is one of the biggest challenges for climatologist and meteorologist today. The Indian Summer Monsoon (ISM) has different temporal and spatial scales of variability and it is mainly driven by strong air sea interactions. In this thesis we evaluate the performance of the new regional Earth System Model (ESM) RegCM-ES in reproducing the main characteristics of the ISM rainfall (ISMR). We performed two sets of simulations, one with the new RegCM-ES and another with the stand-alone version of the atmospheric component i.e. the regional climate model (RCM) RegCM4. RegCM-ES is composed mainly by three components, the RegCM4, the ocean model MITgcm and the hydrological model HD. Another experiment, performed using RegCM-ES with a more high-resolution hydrological model implemented ad hoc for this study, has been added to this two set of experiments. The climatological mean state of the monsoon is well represented by mostly all the experiments although not with the same skills. The most interesting results are observed in simulating the variability of the monsoon and here we highlight some of them. The intraseasonal northward(eastward) propagation of the convection has been analysed using lag/lead map of the regressed anomalies as a function of latitudes(longitudes). The two propagations are better reproduced by the set of ESM simulations thanks to the role of the air-sea coupling. For what concerns the interannual variability (IAV) of the ISMR the air-sea coupling plays an important role. The time series of simulated anomalies by RCM exhibit no correlation with the observed anomalies obtained from the dataset of the Indian Meteorological Department (IMD). On the other hand, the corresponding ESM simulations exhibits good skills in reproducing the IAV of ISMR and good correlation coefficients are observed with IMD. One new finding of this study is a new source of predictability with one-year lag for the ISMR. It is well known that El Niño Southern Oscillation (ENSO) plays a quite important role in modulating the precipitation over most of the intertropical belt and over the South Asia region. As seen in previous findings, the response of the ISMR to ENSO can be delayed of two seasons through the contribution of different air-sea coupled mechanisms as the decrease of Western Arabian Sea Upwellingthe Indo-western Pacific ocean capacitor and the Indo-Tropical northwest Pacific ocean-atmosphere interaction. Our findings extend and confirm the possibility that this response may have a longer feedback time, a year or so. The coupled simulations are used to explain the mechanism and investigate the models response. They appear quite similar to those proposed in the previous studies but further investigations are needed to understand more in deep the phenomena involved. The role of the new hydrological model (CHyM) implemented inside RegCM-ES on the ISMR is also investigated. Due to the main role that the freshwater discharge plays on the formation of a shallow mixed layer depth on the Bay of Bengal that influences the air-sea coupling and the formation of deep convection over this area, the correct estimation of the freshwater discharge is quite important. Although the new hydrological model produces a more realistic annual cycle of the discharge (verified only for a limited set of data due to the lack of observations over this region), this doesn't seem to have a relevant effect on both the mean climatological state of the monsoon and in its variability. A possible explanation for this comes from a missing representation of the barrier layer (BL) in the Bay of Bengal (BoB) due to many different reasons. Two of them are a not enough resoluted ocean model as well as a too strong wind stress forcing that doesn’t allow the formation of the BL.

To identify extreme changes in the dynamics of the Indian Summer Monsoon (ISM) in the past, I propose a new approach based on the quantification of fluctuations of a nonlinear similarity measure, to identify regimes of distinct dynamical complexity in short time series. I provide an analytical derivation for the relationship of the new measure with the dynamical invariants such as dimension and Lyapunov exponents of the underlying system. A statistical test is also developed to estimate the significance of the identified transitions. Our method is justified by uncovering bifurcation structures in several paradigmatic models, providing more complex transitions compared with traditional Lyapunov exponents. In a real world situation, we apply the method to identify millennial-scale dynamical transitions in Pleistocene proxy records of the south Asian summer monsoon system. We infer that many of these transitions are induced by the external forcing of solar insolation and are also affected by internal forcing on Monsoonal dynamics, i.e., the glaciation cycles of the Northern Hemisphere and the onset of the tropical Walker circulation. Although this new method has general applicability, it is particularly useful in analysing short palaeo-climate records. Rainfall during the ISM over the Indian subcontinent occurs in form of enormously complex spatiotemporal patterns due to the underlying dynamics of atmospheric circulation and varying topography. I present a detailed analysis of summer monsoon rainfall over the Indian peninsular using Event Synchronization (ES), a measure of nonlinear correlation for point processes such as rainfall. First, using hierarchical clustering I identify principle regions where the dynamics of monsoonal rainfall is more coherent or homogenous. I also provide a method to reconstruct the time delay patterns of rain events. Moreover, further analysis is carried out employing the tools of complex network theory. This study provides valuable insights into the spatial organization, scales, and structure of the 90th and 94th percentile rainfall events during the ISM (June to September). I furthermore analyse the influence of different critical synoptic atmospheric systems and the impact of the steep Himalayan topography on rainfall patterns. The presented method not only helps in visualising the structure of the extremeevent rainfall fields, but also identifies the water vapor pathways and decadal-scale moisture sinks over the region. Furthermore a simple scheme based on complex networks is presented to decipher the spatial intricacies and temporal evolution of monsoonal rainfall patterns over the last six decades. Some supplementary results on the evolution of monsoonal rainfall extremes over the last sixty years are also presented.
Um Extremereignisse in der Dynamik des indischen Sommermonsuns (ISM) in der geologischen Vergangenheit zu identifizieren, schlage ich einen neuartigen Ansatz basierend auf der Quantifikation von Fluktuationen in einem nichtlinearen Ähnlichkeitsmaß vor. Dieser reagiert empfindlich auf Zeitabschnitte mit deutlichen Veränderungen in der dynamischen Komplexität kurzer Zeitreihen. Ein mathematischer Zusammenhang zwischen dem neuen Maß und dynamischen Invarianten des zugrundeliegenden Systems wie fraktalen Dimensionen und Lyapunovexponenten wird analytisch hergeleitet. Weiterhin entwickle ich einen statistischen Test zur Schätzung der Signifikanz der so identifizierten dynamischen Übergänge. Die Stärken der Methode werden durch die Aufdeckung von Bifurkationsstrukturen in paradigmatischen Modellsystemen nachgewiesen, wobei im Vergleich zu den traditionellen Lyapunovexponenten eine Identifikation komplexerer dynamischer Übergänge möglich ist. Wir wenden die neu entwickelte Methode zur Analyse realer Messdaten an, um ausgeprägte dynamische Veränderungen auf Zeitskalen von Jahrtausenden in Klimaproxydaten des südasiatischen Sommermonsunsystems während des Pleistozäns aufzuspüren. Dabei zeigt sich, dass viele dieser Übergänge durch den externen Einfluss der veränderlichen Sonneneinstrahlung, sowie durch dem Klimasystem interne Einflussfaktoren auf das Monsunsystem (Eiszeitzyklen der nördlichen Hemisphäre und Einsatz der tropischenWalkerzirkulation) induziert werden. Trotz seiner Anwendbarkeit auf allgemeine Zeitreihen ist der diskutierte Ansatz besonders zur Untersuchung von kurzen Paläoklimazeitreihen geeignet. Die während des ISM über dem indischen Subkontinent fallenden Niederschläge treten, bedingt durch die zugrundeliegende Dynamik der atmosphärischen Zirkulation und topographische Einflüsse, in äußerst komplexen, raumzeitlichen Mustern auf. Ich stelle eine detaillierte Analyse der Sommermonsunniederschläge über der indischen Halbinsel vor, die auf Ereignissynchronisation (ES) beruht, einem Maß für die nichtlineare Korrelation von Punktprozessen wie Niederschlagsereignissen. Mit hierarchischen Clusteringalgorithmen identifiziere ich zunächst Regionen mit besonders kohärenten oder homogenen Monsunniederschlägen. Dabei können auch die Zeitverzögerungsmuster von Regenereignissen rekonstruiert werden. Darüber hinaus führe ich weitere Analysen auf Basis der Theorie komplexer Netzwerke durch. Diese Studien ermöglichen wertvolle Einsichten in räumliche Organisation, Skalen und Strukturen von starken Niederschlagsereignissen oberhalb der 90% und 94% Perzentilen während des ISM (Juni bis September). Weiterhin untersuche ich den Einfluss von verschiedenen, kritischen synoptischen Systemen der Atmosphäre sowie der steilen Topographie des Himalayas auf diese Niederschlagsmuster. Die vorgestellte Methode ist nicht nur geeignet, die Struktur extremer Niederschlagsereignisse zu visualisieren, sondern kann darüber hinaus über der Region atmosphärische Transportwege von Wasserdampf und Feuchtigkeitssenken auf dekadischen Skalen identifizieren.Weiterhin wird ein einfaches, auf komplexen Netzwerken basierendes Verfahren zur Entschlüsselung der räumlichen Feinstruktur und Zeitentwicklung von Monsunniederschlagsextremen während der vergangenen 60 Jahre vorgestellt.

The Kukkal basin, Tamil Nadu, India, receives most of its rain from the southwest monsoon (SWM). A sediment core from Kukkal Lake preserves a continuous sediment record from the early-Holocene to present (9000 yr BP to present). The present lake is situated at an elevation of similar to 1887m a.s.l., in a small basin that appears to have alternated between a and wetland depositional environment. Climate proxies, including sediment texture, total organic carbon (TOC), total nitrogen (TN), C/N, pollen and geochemical composition indicate a steady progression to wetter conditions, with two stepwise changes at about 8000, and between 3200 and 1800 yr BP. The change at 8000 yr BP appears to correspond to a brief (100-150years) dry spell recorded elsewhere in India. The change at 3200-1800 yr BP consisted in a rapid intensification of the SWM, and may correlate with the initiation of the Roman Warm Period'. There is no clear evidence of changes at the times of the Medieval Warm Period' (MWP') and the Little Ice Age' (LIA'). The C/N ratio of the sediments ranges from 14.02 to 8.31, indicating that the organic matter originated from a mixture of lacustrine algae, vascular and terrestrial plants. Chemical weathering indices (Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), and Plagioclase Index of Alteration (PIA)) are consistent with extreme silicate weathering. Pollen data show a development from savanna vegetation prior to about 8000 yr BP, followed by grassland with palms, the appearance of ferns just prior to 3200 yr BP and the establishment of the tropical humid forest between 3200 and about 1800 yr BP.

Le Golfe du Bengale (GdB), dans l'océan indien Nord, est sous l'influence d'intenses vents de mousson, qui se renversent saisonnièrement. Les fortes pluies et les apports fluviaux associés à la mousson de Sud-Ouest font du GdB l'une des régions les moins salées des océans tropicaux. La forte stratification haline proche de la surface qui en découle contribue à limiter le mélange vertical, ce qui maintient des températures de surface élevées et favorise la convection atmosphérique et les pluies. Cette stratification en sel a ainsi des implications profondes sur les échanges air-mer et sur le climat des pays riverains. L'objectif de ma thèse est d'améliorer la description de la variabilité de la salinité de surface (SSS) du GdB, et de comprendre ses mécanismes aux échelles de temps saisonnières à interannuelles. Les climatologies existantes ont permis de mettre en évidence un cycle saisonnier marqué de la SSS, avec un dessalement intense de la partie Nord du bassin pendant l'automne, suivi par une expansion de ces eaux dessalées le long du bord Ouest du bassin. Cette langue dessalée s'érode finalement pendant l'hiver, pour revenir à son extension minimale au printemps. Cependant, la rareté des observations in-situ de SSS ne permet d'observer les fluctuations interannuelles autour de ce cycle saisonnier que de manière parcellaire dans le GdB. Le développement récent de la télédétection spatiale de la SSS (missions SMOS et AQUARIUS) a ouvert de nouvelles opportunités à cet égard. Cette technologie reste toutefois délicate dans le cas d'un bassin de petite taille tel que le GdB, du fait des contaminations éventuelles du signal de SSS par les interférences radio et par les sources d'origine continentale. Une validation systématique des produits satellites par comparaison à un jeu de données in-situ exhaustif montre qu'Aquarius capture de façon réaliste les évolutions saisonnières et interannuelles de la SSS partout dans le GdB. A l'inverse, SMOS ne parvient pas à restituer une salinité meilleure que les climatologies existantes
Located in the Northern Indian Ocean, the Bay of Bengal (BoB) is forced by intense seasonally reversing monsoon winds. Heavy rainfall and strong river runoffs associated with the southwest monsoon makes the bay one of the freshest regions in the tropical ocean. This surface fresh water flux induces strong near surface salinity stratification, which reduces vertical mixing and maintains high sea surface temperatures and deep atmospheric convection and rainfall. This intense near surface haline stratification has therefore profound implications on the air-sea exchanges, and on the climate of the neighboring countries. The goal of my thesis is to improve the description of the Sea surface salinity (SSS) variability in the BoB and to understand the oceanic and atmospheric processes driving this variability at seasonal and interannual timescales. Existing climatologies reveal a marked seasonal cycle of SSS with an intense freshening of the northern part of the basin during fall that subsequently spreads along the western boundary. This fresh pool finally erodes during winter, to reach its minimal extent in spring. The paucity of in-situ SSS observations however prevented to monitor the interannual fluctuations around this seasonal picture with a good spatial coverage. The recent development of SSS remote-sensing capabilities (with SMOS and AQUARIUS satellites) may help with that regard. However this is particularly challenging for a small semi-enclosed basin such as the Bay of Bengal, because of the potential contamination of the SSS signal by radio frequency interferences and land effects in the near coastal environment. A thorough validation of these satellite products to an exhaustive gridded in-situ dataset shows that Aquarius reasonably captures the large-scale observed seasonal and interannual SSS evolution everywhere in the BoB while SMOS does not perform better than existing climatologies, advocating for improvements of its SSS retrieval algorithm there

The Miocene of the Lidam area, Eastern Province, Saudi Arabia, was studied to examine the interaction of glacio-esustasy during moderate Antarctic glaciation, within a small back bulge basin on the slowly subsiding distal Arabian foreland, distal from the active Zagros fold-thrust belt. Low subsidence rates of 1 to 4 cm/k.y generated the long-term accommodation, which were considerable slower than those in the proximal foredeep in Iran. Deposition of the siliciclastics was driven by lowered sea levels, and moderately humid to arid climate. Rising sea levels pushed the siliciclastics updip allowing mixed siliciclastics and carbonates to form downdip, under semi-arid climate and locally hypersaline conditions. Maximum transgression slightly predated the Middle Miocene climatic optimum when prograding siliciclastics migrated across the platform. Falling triggered siliciclastic deposition under semi-arid climate. Sequences appear to relate to long-term obliquity (~1.2 m.y. cycles) and long-term eccentricity (400 k.y.) cycles. The succession contains numerous missing beats reflecting the updip position of the study area, and sea level changes of tens of meters that frequently exposed the platform. Siliciclastic units commonly are incised into muddy sediments beneath sequence boundaries. Multiple exposure surfaces occur within Hadrukh brecciated palustrine carbonates. Within Dam carbonates, parasequence boundaries commonly are capped by tidal flat laminites (some of which are incipiently brecciated). High frequency negative excursions of ∂¹³C within the succession appear to relate to near-surface diagenesis by soil gas depleted in ∂¹³C beneath sequence boundaries. Positive C isotope excursions in the Lidam Miocene section can be tied to similar excursions in Qatar and UAE, where Sr isotope dates constrain the ages of the units. The overall C isotope profile at Lidam shows depleted values early in the Miocene to heavy values in the Middle Miocene, becoming lighter again in the late Miocene. The profile appears to follow the long-term global ∂¹³C curve. Incursion of meteoric groundwaters into the study area was driven by the long-term global sea level changes. Oxygen isotopes are surprisingly light, extending down to -12.5 ‰VPDB. The very light δ¹⁸O values of the meteoric waters may be explained by rainfall associated with enhanced Miocene Indian monsoons, and with far travelled air mass trajectories migrating across north Africa and from the polar region.
Ph. D.

The ENSO-monsoon system is known to have a biennial component. Here we show using high resolution satellite data, mainly daily rainfall and sea surface temperature (SST) from the Tropical Rainfall Measuring Mission (TRMM), and daily scatterometer surface winds from QuickSCAT, that there is a clear biennial oscillation (TBO) in summer monsoon rainfall over Central India – Bay of Bengal (Cl-BoB) and the far west Pacific in the period 1999-2005. Summer (JJAS) mean rainfall oscillates between high and low values in alternate years; the rainfall is high in the odd years 1999, 2001, 2003, and 2005, and low in even years 2000, 2002 and 2004. The amplitude of the oscillation is significant, as measured against the long term standard deviation of seasonal rain based on 1979-2005 Global Precipitation Climatology Project (GPCP) data. We find that the TBO in rainfall is associated with TBO of SST over the tropical Indian, west Pacific and Atlantic Oceans in different seasons. There is no TBO in east Pacific SST, and no strong El Nino in this period. The TBO of SST is related to change in evaporation due to TBO of surface wind speed. A TBO of the surface branch of the Walker circulation in the eastern Indian and western Pacific basins is clearest in the autumn season during 1999-2005. There is a clear relation between a large-amplitude TBO of winter surface air temperature over north Asia associated with TBO of the Arctic oscillation (AO), and the TBO of summer monsoon rainfall. High rainfall over CI-BoB lin summer is followed by a relatively high value of the AO Index, and warm air termperature over north Asia in the succeeding winter. The Inter Tropical Convergence Zone(ITCZ) over the central Pacific and Atlantic Oceans shift north by about two degrees when the northern hemisphere is warm, reminiscent of the behaviour of the climate system of ENSO, decadal and palaeoclimate time scales. In this thesis we document the biennial oscillation of monsoon rain and its spatial structure in the recent period, and its relation with biennial oscillation of surface climate over the global tropics and extratropical regions. The existence of TBO in the tropical Atlantic, and its relation with the monsoon, is a new finding. We demonstrate that the interannual variability of the summer monsoon during 1999-2005, including the drought of 2002, is part of a pervasive TBO of global surface climate.

The ENSO-monsoon system is known to have a biennial component. Here we show using high resolution satellite data, mainly daily rainfall and sea surface temperature (SST) from the Tropical Rainfall Measuring Mission (TRMM), and daily scatterometer surface winds from QuickSCAT, that there is a clear biennial oscillation (TBO) in summer monsoon rainfall over Central India – Bay of Bengal (Cl-BoB) and the far west Pacific in the period 1999-2005. Summer (JJAS) mean rainfall oscillates between high and low values in alternate years; the rainfall is high in the odd years 1999, 2001, 2003, and 2005, and low in even years 2000, 2002 and 2004. The amplitude of the oscillation is significant, as measured against the long term standard deviation of seasonal rain based on 1979-2005 Global Precipitation Climatology Project (GPCP) data. We find that the TBO in rainfall is associated with TBO of SST over the tropical Indian, west Pacific and Atlantic Oceans in different seasons. There is no TBO in east Pacific SST, and no strong El Nino in this period. The TBO of SST is related to change in evaporation due to TBO of surface wind speed. A TBO of the surface branch of the Walker circulation in the eastern Indian and western Pacific basins is clearest in the autumn season during 1999-2005. There is a clear relation between a large-amplitude TBO of winter surface air temperature over north Asia associated with TBO of the Arctic oscillation (AO), and the TBO of summer monsoon rainfall. High rainfall over CI-BoB lin summer is followed by a relatively high value of the AO Index, and warm air termperature over north Asia in the succeeding winter. The Inter Tropical Convergence Zone(ITCZ) over the central Pacific and Atlantic Oceans shift north by about two degrees when the northern hemisphere is warm, reminiscent of the behaviour of the climate system of ENSO, decadal and palaeoclimate time scales. In this thesis we document the biennial oscillation of monsoon rain and its spatial structure in the recent period, and its relation with biennial oscillation of surface climate over the global tropics and extratropical regions. The existence of TBO in the tropical Atlantic, and its relation with the monsoon, is a new finding. We demonstrate that the interannual variability of the summer monsoon during 1999-2005, including the drought of 2002, is part of a pervasive TBO of global surface climate.

In the current study, we have presented a systematic analysis of the diurnal cycle of rainfall over the Indian region using satellite observations, and evaluated the ability of the Weather Research and Forecasting Model (WRF) to simulate some of the salient features of the observed diurnal characteristics of rainfall. Using high resolution simulations, we also investigate the underlying mechanisms of some of the observed diurnal signatures of rainfall. Using the Tropical Rain-fall Measuring Mission (TRMM) 3-hourly, 0.25 ×0.25 degree 3B42 rainfall product for nine years (1999-2007), we extract the finer spatial structure of the diurnal scale signature of Indian summer monsoon rainfall. Using harmonic analysis, we construct a signal corresponding to diurnal and sub-diurnal variability. Subsequently, the 3-hourly time-period or the octet of rain-fall peak for this filtered signal, referred to as the “peak octet,” is estimated with care taken to eliminate spurious peaks arising out of Gibbs oscillations. Our analysis suggests that over the Bay of Bengal, there are three distinct modes of the peak octet of diurnal rainfall corresponding to 1130, 1430 and 1730 IST, from north central to south Bay. This finding could be seen to be consistent with southward propagation of the diurnal rainfall pattern reported by earlier studies. Over the Arabian sea, there is a spatially coherent pattern in the mode of the peak octet (1430 IST), in a region where it rains for more than 30% of the time. In the equatorial Indian Ocean, while most of the western part shows a late night/early morning peak, the eastern part does not show a spatially coherent pattern in the mode of the peak octet, owing to the occurrence of a dual maxima (early morning and early/late afternoon). The Himalayan foothills were found to have a mode of peak octet corresponding to 0230 IST, whereas over the Burmese mountains and the Western Ghats (west coast of India) the rainfall peaks during late afternoon/early evening (1430-1730 IST). This implies that the phase of the diurnal cycle over inland orography (e.g., Himalayas) is significantly different from coastal orography (e.g., Western Ghats). We also find that over the Gangetic plains, the peak octet is around 1430 IST, a few hours earlier compared to the typical early evening maxima over land. The second part of our study involves evaluating the ability of the Weather Research and Fore-casting Model (WRF) to simulate the observed diurnal rainfall characteristics. It also includes conducting high resolution simulations to explore the underlying physical mechanisms of the observed diurnal signatures of rainfall. The model (at 54km resolution) is integrated for the month of July 2006 since this period was particularly favourable for the study of diurnal cycle. We first evaluate the sensitivity of the model to the prescribed sea surface temperature (SST) by using two different SST datasets, namely Final Analyses (FNL) and Real-time Global (RTG). The overall performance of RTG SST was found to be better than FNL, and hence it was used for further model simulations. Next, we investigated the impact of different parameterisations (convective, microphysical, boundary layer, radiation and land surface) on the simulation of diurnal cycle of rainfall. Following this sensitivity study, we identified the suite of physical parameterisations in the model that “best” reproduces the observed diurnal characteristics of Indian monsoon rainfall. The “best” model configuration was used to conduct two nested simulations with one-way, three-level nesting (54-18-6km) over central India and Bay of Bengal. While the 54km and 18km simulations were conducted for July 2006, the 6km simulation was carried out for the period 18-24 July 2006. This period was chosen for our study since it is composed of an active period (19-21 July 2006), followed by a break period (22-24 July 2006). At 6km grid-spacing the model is able to realistically simulate the active and break phases in rainfall. During the chosen active phase, we find that the observed rainfall over central India tends to reach a maximum in the late night/early morning hours. This is in contrast to the observed climatological diurnal maxima of late evening hours. Interestingly, the 6km simulation for the active phase is able to reproduce this late night/early morning maxima. Upon further analysis, we find that this is because of the strong moisture convergence at the mid-troposphere during 2030-2330 IST, leading to the rainfall peak seen during 2330-0230 IST. Based on our analysis, we conclude that during both active and break phases of summer monsoon, mid-level moisture convergence seems to be one of the primary factors governing the phase of the diurnal cycle of rainfall. Over the Bay of Bengal, the 6km model simulation is in very good agreement with observations, particularly during the active phase. The southward propagation observed during 19-20 July 2006, which was not captured by the coarse resolution simulation (54km), is exceedingly well captured by the 6km simulation. The positive anomalies in specific humidity attain a maxima during 2030-0230 IST in the north and during 0830-1430 IST in the south. This confirms the role of moisture convergence in the southward propagation of rainfall. Equally importantly we find that while low level moisture convergence is dominant in the north Bay, it is the mid-level moisture convergence that is predominant in the south Bay.

In the current study, we have presented a systematic analysis of the diurnal cycle of rainfall over the Indian region using satellite observations, and evaluated the ability of the Weather Research and Forecasting Model (WRF) to simulate some of the salient features of the observed diurnal characteristics of rainfall. Using high resolution simulations, we also investigate the underlying mechanisms of some of the observed diurnal signatures of rainfall. Using the Tropical Rain-fall Measuring Mission (TRMM) 3-hourly, 0.25 ×0.25 degree 3B42 rainfall product for nine years (1999-2007), we extract the finer spatial structure of the diurnal scale signature of Indian summer monsoon rainfall. Using harmonic analysis, we construct a signal corresponding to diurnal and sub-diurnal variability. Subsequently, the 3-hourly time-period or the octet of rain-fall peak for this filtered signal, referred to as the “peak octet,” is estimated with care taken to eliminate spurious peaks arising out of Gibbs oscillations. Our analysis suggests that over the Bay of Bengal, there are three distinct modes of the peak octet of diurnal rainfall corresponding to 1130, 1430 and 1730 IST, from north central to south Bay. This finding could be seen to be consistent with southward propagation of the diurnal rainfall pattern reported by earlier studies. Over the Arabian sea, there is a spatially coherent pattern in the mode of the peak octet (1430 IST), in a region where it rains for more than 30% of the time. In the equatorial Indian Ocean, while most of the western part shows a late night/early morning peak, the eastern part does not show a spatially coherent pattern in the mode of the peak octet, owing to the occurrence of a dual maxima (early morning and early/late afternoon). The Himalayan foothills were found to have a mode of peak octet corresponding to 0230 IST, whereas over the Burmese mountains and the Western Ghats (west coast of India) the rainfall peaks during late afternoon/early evening (1430-1730 IST). This implies that the phase of the diurnal cycle over inland orography (e.g., Himalayas) is significantly different from coastal orography (e.g., Western Ghats). We also find that over the Gangetic plains, the peak octet is around 1430 IST, a few hours earlier compared to the typical early evening maxima over land. The second part of our study involves evaluating the ability of the Weather Research and Fore-casting Model (WRF) to simulate the observed diurnal rainfall characteristics. It also includes conducting high resolution simulations to explore the underlying physical mechanisms of the observed diurnal signatures of rainfall. The model (at 54km resolution) is integrated for the month of July 2006 since this period was particularly favourable for the study of diurnal cycle. We first evaluate the sensitivity of the model to the prescribed sea surface temperature (SST) by using two different SST datasets, namely Final Analyses (FNL) and Real-time Global (RTG). The overall performance of RTG SST was found to be better than FNL, and hence it was used for further model simulations. Next, we investigated the impact of different parameterisations (convective, microphysical, boundary layer, radiation and land surface) on the simulation of diurnal cycle of rainfall. Following this sensitivity study, we identified the suite of physical parameterisations in the model that “best” reproduces the observed diurnal characteristics of Indian monsoon rainfall. The “best” model configuration was used to conduct two nested simulations with one-way, three-level nesting (54-18-6km) over central India and Bay of Bengal. While the 54km and 18km simulations were conducted for July 2006, the 6km simulation was carried out for the period 18-24 July 2006. This period was chosen for our study since it is composed of an active period (19-21 July 2006), followed by a break period (22-24 July 2006). At 6km grid-spacing the model is able to realistically simulate the active and break phases in rainfall. During the chosen active phase, we find that the observed rainfall over central India tends to reach a maximum in the late night/early morning hours. This is in contrast to the observed climatological diurnal maxima of late evening hours. Interestingly, the 6km simulation for the active phase is able to reproduce this late night/early morning maxima. Upon further analysis, we find that this is because of the strong moisture convergence at the mid-troposphere during 2030-2330 IST, leading to the rainfall peak seen during 2330-0230 IST. Based on our analysis, we conclude that during both active and break phases of summer monsoon, mid-level moisture convergence seems to be one of the primary factors governing the phase of the diurnal cycle of rainfall. Over the Bay of Bengal, the 6km model simulation is in very good agreement with observations, particularly during the active phase. The southward propagation observed during 19-20 July 2006, which was not captured by the coarse resolution simulation (54km), is exceedingly well captured by the 6km simulation. The positive anomalies in specific humidity attain a maxima during 2030-0230 IST in the north and during 0830-1430 IST in the south. This confirms the role of moisture convergence in the southward propagation of rainfall. Equally importantly we find that while low level moisture convergence is dominant in the north Bay, it is the mid-level moisture convergence that is predominant in the south Bay.