Dissertations / Theses on the topic 'Oceanic response to the monsoons'

The response to ship induced motions has been predicted for four towed underwater vehicles. The purpose of the study is to determine the suitability of these vehicles for their use as oceanic microstructure sensor carrying platforms. All have been used in the past for oceanic studies, and these four vehicles show the most promise for microstructure work. Transfer functions which describe the response of a towed vehicle have been determined, for longitudinal motions. Also, the motion spectra of the vehicles have been predicted for the longitudinal mode as a result of being towed by a typical research vessel in a sea state three.<br>M.S.

Thesis (PhD)--Stellenbosch University, 2013.<br>ENGLISH ABSTRACT: Invasive species are among the most important global conservation threats. Their management is one of the key conservation challenges that will have to be addressed in the next few decades. The study of real invasions and their management in natural ecosystems provides an opportunity to gain important information on theoretical and applied aspects of biological invasions. This project focuses on the broader ecological context of invasive ant management in an ecologically sensitive island habitat. The thesis has three main components: 1) assessing the role of the invasive ant Pheidole megacephala in the ecosystem and evaluating its threat to the system, 2) evaluating a low-impact management program for the ant, and 3) using a community-level approach to assess ecosystem response to ant removal. The ant occupied almost 30% of the island‘s total land area and reached extremely high densities in some areas. The ant was associated with exotic hemipteran scale insects through trophobiotic mutualisms that facilitated high ant and hemipteran abundances. The highly destructive scale insect Pulvinaria urbicola was among the hemipterans that benefited from ant attendance. High levels of hemipteran feeding resulted in dieback of functionally important and threatened native Pisonia trees, which represented a significant threat to the forest ecosystem. A management program was initiated in response to this threat, consisting of baiting with selective hydramethylnon-based bait delivered in bait stations, accompanied by detailed pre-and post-baiting monitoring. The method was highly effective at suppressing the ants, whilst preventing bait uptake by non-target organisms. It was also cost-effective and adaptable to ant density in the field, but was only effective over short distances. The method may be applicable to other sensitive environments with similar challenges. After ant control, the ant-scale mutualism was decoupled and the Pu. urbicola population collapsed. There were variable responses in different taxa to the removal of these highly abundant exotic species, the most important of which was the recovery in Pisonia trees. Shoot condition and foliage density improved and there was a decrease in sooty mold. Herbivory on Pisonia increased due to recovery of native canopy herbivores, but the overall impact was far less than that of the exotic hemipterans. Soil surface arthropods, a group that may have been vulnerable to the treatment method, were unaffected by baiting. Instead, they increased significantly after ant removal, confirming the ant‘s impact on other arthropods. Other ant diversity and non-ant arthropod abundance increased post-baiting, including the endemic ant Pheidole flavens farquharensis and some functionally important insects such as the Indian cockroach. Natural enemies that interacted predictably with the mutualists were influenced by management. Predators of hemipterans increased significantly after ant removal and were instrumental in the scale population collapse, whereas parasitoids of hemipterans that benefited from the mutualism declined. Additionally, groups that were unrelated to the mutualism were indirectly influenced by management. The natural enemy assemblage as a whole showed recovery to pre-invasion conditions. The study shows how widely interconnected and influential the ant was in the ecosystem. It highlights the threat of the species in natural systems as well as the complex responses following invasive ant removal. Yet, it also demonstrates the potential to safely and effectively manage the species, thereby raising the opportunity for ecosystem recovery.<br>AFRIKAANSE OPSOMMING: Indringerspesies is van die belangrikste globale bedreigings vir natuurbewaring. Hulle bestuur is van die grootste bewaringsuitdagings wat in die volgende paar dekades aangespreek moet word. Die studie van werklike invalle en hul bestuur in natuurlike ekosisteme bied 'n geleentheid om belangrike inligting te verkry oor teoretiese en toegepaste aspekte van biologiese indringing. Hierdie projek fokus op die breër ekologiese konteks van uitheemse mier bestuur in 'n ekologies sensitiewe eiland habitat. Die tesis het drie hoofkomponente: 1) die beoordeling van die rol van die indringer mier Pheidole megacephala in die ekosisteem en evaluering van sy bedreiging vir die sisteem, 2) die evaluering van 'n lae-impak bestuursprogram vir die mier, en 3) die gebruik van 'n gemeenskaps-vlak benadering om ekosisteem reaksie op mierverwydering te assesseer. Die mier het byna 30% van die totale landoppervlak van die eiland beslaan en het in party areas baie hoë digthede bereik. Die mier was geassosieer met uitheemse dopluis spesies in mutualismes wat hoë mier en dopluis getalle gefasiliteer het. Die hoogs beskadigende dopluis Pulvinaria urbicola was een van die spesies wat bevoordeel is deur die mutualisme. Hoë vlakke van dopluis voeding het die terugsterwe van funksioneel belangrike, bedreidge inheemse Pisonia bome veroorsaak, wat ʼn groot bedreiging vir die ekosisteem verteenwoordig het. ‗n Bestuursprogram is geïmplimenteer as gevolg van hierdie bedreiging, wat bestaan het uit selektiewe hidrametielnoon-gebaseerde lokaas wat in die veld geplaas is in lokaashouers, vergesel deur intensiewe monitering voor en na lokaasplasing. Die metode was hoogs effektief in die onderdrukking van die miere en het lokaasinname deur nie-teiken organismes verhoed. Dit was ook koste-effektief en aanpasbaar volgens mierdigtheid in die veld, maar was slegs effektief oor kort afstande. Die metode mag van toepassing wees in ander sensitiewe omgewings met soortgelyke uitdagings. Na mierbeheer is die mier-dopluis mutualisme ontkoppel en die Pu. urbicola bevolking het drasties verminder. Daar was verskillende reaksies in verskillende taxa tot die verwydering van die oorvloedryke eksotiese spesies, maar die belangrikste reaksie was die herstel van Pisonia bome. Spruittoestand en blaardigtheid het verbeter en daar was ʼn afname in roetskimmel. Herbivorie op Pisonia het toegeneem as gevolg van ʼn herstel in inheemse herbivore, maar die algehele impak was veel minder as dié van die eksotiese dopluis. Grondoppervlak gelidpotiges, 'n groep wat kwesbaar kon wees vir die behandelingsmetode, was onaangeraak deur die lokaas, maar het beduidend na mierverwydering vermeerder. Mierdiversiteit het vermeerder en die Seychelles endemiese mier Pheidole flavens farquharensis is hervestig. Ander gelidpotiges het ook vermeerder, insluitend funksioneel belangrike spesies soos die Indiese kakkerlak. Natuurlike vyande wat geassosieer was met die mutualiste is beïnvloed deur die mierbestuur. Predatore van dopluis het beduidend toegeneem na mierverwydering en was hoogs betrokke by die vermindering van dopluis, terwyl parasiete van dopluis, wat voordeel getrek het uit die mutualisme, gedaal het. Daarbenewens is groepe wat onverwant was aan die mutualisme indirek beïnvloed deur mierbestuur. Die algehele natuurlike vyand gemeenskap het herstel na pre-indringing toestand. Die studie toon hoe wydverbind en invloedryk die mier was in die ekosisteem. Dit beklemtoon die bedreiging van die spesies in natuurlike stelsels asook die komplekse reaksies wat uitheemse mierverwydering volg. Tog demonstreer dit die potensiaal om die spesies veilig en doeltreffend te bestuur, en sodoende die geleentheid vir ekosisteemherstel te skep.

The transcriptional response of freshwater Synechococcus sp. PCC 7942 and oceanic Synechococcus sp. WH 7803 to ammonium, nitrate or iron was studied in single or multiple factor limited cultures. Both strains showed maximum production of NiR mRNA when grown in nitrate-containing media. When grown in ammonium-containing medium, they did not show any signal for NiR mRNA synthesis. The influence on the transcription of NiR mRNA by iron as sole limiting nutrient was also evaluated. Iron increased the NiR mRNA whether or not the positive effect of nitrate was already present. The hybridization signal of mRNA for the large subunit of ribulose bisphosphate carboxylase/oxygenase enzyme (rbcl), was increased in both strains when iron was added. The relative response of NiA was larger than that of rbcL to iron addition. In the final set of experiments, combinations of iron, ammonium, or nitrate nutrient limitation were studied to understand their interactive effect on NiR and rbcL mRNA production. The ammonium-grown cells were allowed to starve for iron and then transferred to +iron+nitrate, +iron-nitrate, or -iron+nitrate cultures. rbcL mANA was increased gradually up to 168 hr. NiR mRNA increased initially but, for unknown reasons, decreased after 24 hr to a minimum that was similar for +iron+nitrate, +iron-nitrate, and -iron+nitrate cultures. A comparison of the rbcL or NiR mRNA from multiple nutrient limitation with that of single nutrient limitation showed different time dependent patterns of synthesis. The response to iron starvation could be different when cells experience a co-limitation with nitrate. This response is also influenced by species specific differences by evolutionary adaptation to different environments. This difference is confirmed in the NiR and rbcL mRNA response in two strains of marine and freshwater Synechococcus for single (iron) or multiple (iron+nitrate) limitation. Physiological studies commonly assume a single factor will constrain cell growth at any moment and when conditions change, the limiting factor will switch. The differential response in the synthesis of NiA and rbcl mANA observed shows that cells exhibit unique responses to combined limitations.

Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1988.<br>Includes bibliographical references (leaves 163-172).<br>A freshwater plume often forms when a river or an estuary discharges water onto the continental shelf. Freshwater plumes are ubiquitous features of the coastal ocean and usually leave a striking signature in the coastal hydrography. The present study combines both hydrographic data and idealized numerical simulations to examine how ambient currents and winds influence the transport and mixing of plume waters. The first portion of the thesis considers the alongshore transport of freshwater using idealized numerical simulations. In the absence of any ambient current, the downstream coastal current only carries a fraction of the discharged fresh water; the remaining fraction recirculates in a continually growing "bulge" of fresh water in the vicinity of the river mouth. The fraction of fresh water transported in the coastal current is dependent on the source conditions at the river mouth. The presence of an ambient current augments the transport in the plume so that its freshwater transport matches the freshwater source. For any ambient current in the same direction as the geostrophic coastal current, the plume will evolve to a steady-state width. A key result is that an external forcing agent is required in order for the entire freshwater volume discharged by a river to be transported as a coastal current. The next section of the thesis addresses the wind-induced advection of a river plume, using hydrographic data collected in the western Gulf of Maine. The observations suggest that the plume's cross-shore structure varies markedly as a function of fluctuations in alongshore wind forcing. Consistent with Ekman dynamics, upwelling favorable winds spread the plume offshore, at times widening it to over 50 km in offshore extent, while downwelling favorable winds narrow the plume width to a few Rossby radii. Near-surface current meters show significant correlations between cross-shore currents and alongshore wind stress, consistent with Ekman theory. Estimates of the terms in the alongshore momentum equation calculated from moored current meter arrays also indicate an approximate Ekman balance within the plume. A significant correlation between alongshore currents and alongshore wind stress suggests that interfacial drag may be important. The final section of the thesis is an investigation of the advection and mixing of a surface-trapped river plume in the presence of an upwelling favorable wind stress, using a three-dimensional model in a simple, rectangular domain. Model simulations demonstrate that the plume thins and is advected offshore by the cross shore Ekman transport. The thinned plume is susceptible to significant mixing due to the vertically sheared horizontal currents. The first order plume response is explained by Ekman dynamics and a Richardson number mixing criterion.<br>by Derek Allen Fong.<br>Ph.D.

Marine communities are affected by oceanographic processes, which influence ecological interactions, such as recruitment rates, that are essential regulators of community dynamics. These relationships are not constant; they change in space and time or among taxa. We defend the thesis that oceanographic processes of climatic origin influencing larval abundance at the study region, regulate and establish the trends in settlement and recruitment of invertebrates (cirripeds and bivalves) at rocky shore intertidal. We first investigated the recruitment at different temporal scales and its relationships with physical forcings, chlorophyll-a concentration and sea surface temperature. Second, we focused on the spatial synchrony and contrasts of recruitment, and interspecific trends. Third, we described and evaluated the co-variation between cold fronts and the larval abundance and settlement. We concluded that there is a high degree of correlation between recruitment/settlement and the variation of the wind field, which set temporal trends. Cold fronts are important regulators of settlement, but higher recruitment was associated to NE-E winds. Barnacle recruitment is more susceptible to the environmental variations compared to bivalves. Regional recruitment is not spatially synchronic with differences in the scale of 100 km. This study highlights the importance of oceanic-climatic phenomena as predictors of spatio-temporal trends of recruitment showing that climatic fluctuations might have contrasting effects on rocky shore communities.<br>As comunidades marinhas são afetadas por processos oceanográficos que influenciam as interações ecológicas, como as taxas de recrutamento, reguladores essenciais da dinâmica dessas comunidades. Essas relações não são constantes, elas mudam no espaço e no tempo, ou entre taxa. Aqui nós defendemos a tese que processos oceanográficos de origem climática, por influenciarem a abundância larval região de estudo, regulam e estabelecem tendências do assentamento e recrutamento de invertebrados (cirripedes e bivalves) do entremarés de costas rochosas. Primeiramente, nós investigamos o recrutamento em diferentes escalas de tempo e sua relação com forçantes físicas, concentração de clorofila-a e temperatura da superfície do mar. Em um segundo momento, nós focamos na sincronia e nos contrastes espaciais do recrutamento, e as tendências inter-específicas. Por fim, descrevemos e avaliamos a co-variância entre frentes frias, abundância larval e assentamento. Concluímos que há um alto grau de correlação entre recrutamento/ assentamento e a variação do campo de ventos, o qual estabelece as tendências temporais. As frentes frias são reguladores importantes do assentamento, mas o recrutamento mais alto está associado a ventos de NE-E. O recrutamento de cirripedes é mais susceptível às variações ambientais se comparado aos bivalves. O recrutamento regional não é sincrônico no espaço, com diferenças na escala de 100km. Este estudo destaca a importância dos fenômenos oceano-climáticos na previsão de tendências espaço-temporais do recrutamento, mostrando que flutuações climáticas podem ter efeitos contrastantes nas comunidades de costas rochosas.

The West African Monsoon (WAM) is a significant component of the global monsoon system and plays a key role in the annual cycle of precipitation in the Sahel region of Africa (10°N to 20°N) during the summer months (July to September). Rainfall in the Sahel varies on timescales ranging from seasons to millennia as a result of changes in the WAM. In the last century, the Sahel experienced a relatively wet period (prior to the 1960s) followed by a period of severe drought (1970s-1980s) with higher-frequency variability superimposed on this low-frequency background signal. Understanding precipitation variability like that which occurred over the 20th Century and its impact on Sahel precipitation is critically important for skillful hydroclimate predictions and disaster preparedness in the region. Previous work has shown that the WAM responds to both internal atmospheric variability and external oceanic forcing. A large fraction of 20th Century Sahel rainfall variability has been linked to nearby and remote oceanic forcing from the Atlantic, Pacific, and Indian Oceans, suggesting that the ocean is the primary driver of variability. However, the mechanisms underlying the influence of sea surface temperature (SST) forcing to land based precipitation and the relative importance of the roles of different basins are not as well understood. To this end, the work completed in this thesis examines the physical mechanisms linking oceanic forcing to recent precipitation variability in the Sahel and identifies them alongside large-scale environmental conditions. A series of moisture budget decomposition studies are performed for the Sahel in order to understand the processes that govern regional hydroclimate variability on decadal and interannual time scales. The results show that the oceanic forcing of atmospheric mass convergence and divergence explains the moisture balance patterns in the region to first order on the timescales considered. On decadal timescales, forcing by the Indian and Atlantic Oceans correlate strongly with precipitation variability. The combination of a warm Indian Ocean and negative gradient across the Atlantic forces anomalous circulation patterns that result in net moisture divergence by the mean and transient flow. Together with negative moisture advection, these processes result in a strong drying of the Sahel during the later part of the 20th Century. Diagnosis of moisture budget and circulation components within the main rainbelt and along the monsoon margins show that changes to the mass convergence are related to the magnitude of precipitation that falls in the region, while the advection of dry air is associated with the maximum latitudinal extent of precipitation. On interannual timescales, results show that warm conditions in the Eastern Tropical Pacific remotely force anomalously dry conditions primarily through affecting the low-troposphere mass divergence field. This behavior is related to increased subsidence over the tropical Atlantic and into the Sahel and an anomalous westward flow of moisture from the continent, both resulting in a coherent drying pattern. The interannual signal is then further explored, particularly in light of the expected link between the El Niño Southern Oscillation and dry conditions in the Sahel, notably unseen during the historic El Niño event of 2015. Motivated by this, recent El Niño years and their precipitation signature in the Sahel along with the associated large-scale environmental conditions are examined. Two different outcomes for Sahel summer season are defined; an anomalously wet or an anomalously dry season coincident with El Niño conditions. The different precipitation patterns are distinguished by increased moisture supply for the wet years, which can be driven by both regional oceanic conditions that favor increased moisture convergence over the continent as well as weaker El Niño forcing. Finally, a series of new idealized SST-forced experiments that explore the causal link between oceanic forcing and the response of convection in the region on daily time resolution are discussed and preliminary results shown. These experiments aim to understand how convection in the Sahel responds to SST forcing using transient model simulations that track the evolving response of the WAM through time, day-by-day, under different oceanic conditions. Preliminary results show the stark differences in seasonal precipitation that occur when anomalies of opposite sign are applied in parts of the Atlantic and Pacific basin. There is also a suggestion of a difference in the timing of the rainy season when the model is run with different SST configurations.

碩士<br>國立臺灣大學<br>大氣科學研究所<br>98<br>The oceanic response to the idealized typhoon is studied using a 4th-order -accurate basin-scale ocean model. We examined the ocean response systematically under several wind strengths by adding idealized typhoons based on Rankine vortex. We set up the idealized typhoons with three different initial conditions: translation speed of typhoons, intensity of typhoons, and the thickness of initial ocean mixed layer. Nu-merical experiments showed that slower moving and stronger intensity typhoons lead to large Ekman pumping, which causes vigorous sea surface cooling. The faster moving typhoons lead to stronger mixing process and inertial oscillation, which enhances longer temperature drop and oscillation. Our results also showed that thicker initial mixed layer (larger momentum) has weaker ocean response to temperature drop. We also find that different turbulence parameterizations (Pacanowski and Philan-der, 1982) and Price et al., 1986) also lead to different sea temperature cooling in the numerical model. Our results show stronger and more intensive surface mixing process in PWP occurs at high wind speed, which is closer to the reality. Finally, three non-dimensional parameters are used to quantify the ocean response: Storm speed (S), Burger number (B), and Mach number (C). The Storm speed, S, which is the ratio of the local inertial period to the hurricane residence time, is expected to be large when the response of upper-ocean currents will include strong inertial motions. The Burger number, B, which is the pressure coupling between the mixed-layer current and the thermocline current, is expected to be large when the pressure coupling and the relaxation stage dynamics would be most pronounced. The Mach number, C, which is the ratio of sea surface current speed to typhoon moving speed, is expected to be large when the upwelling is strong. Our results showed that both Storm speed and Mach number show strong impacts on the ocean response to typhoons while the Burger number play only a minor role.

Global climate is controlled by two factors, the amount of heat energy received from the sun (solar insolation) and the way that heat is distributed Earth's surface. Solar insolation varies on timescales of 10s to 100s of thousands of years due to changes in the path of Earth's orbit about the sun (Milankovitch cycles). Earth's internal boundary conditions, such as paleogeography, the presence/absence of polar icecaps, atmospheric/oceanic chemistry and sea level, provide distribution and feedback mechanisms for the incoming heat. Variations in these internal boundary conditions may happen abruptly or, as in the case of plate tectonics, take millions of years. We use geochemical and sedimentological techniques to investigate the response of ocean chemistry, regional aridity and atmospheric and oceanic circulation patterns to climate change during both greenhouse and icehouse climates. To explore the connection between orbitally-forced changes in solar insolation, continental aridity and wind, we generated a high-resolution dust record for ~58 Myr old deep-sea sediments from Shatsky Rise. Our data provide the first evidence of a correlation between dust flux to the deep sea and orbital cycles during the Early Paleogene, indicating dust supply (regional aridity) responded to orbital forcing during the last major interval of greenhouse climate. The change in dust flux was comparable to that during icehouse climates implying subtle variations in solar insolation have a similar impact on climate during intervals of over-all warmth as they do during glacial-interglacial states. The Carboniferous Period (359-299 Ma) marks a critical time in Earth's history when a series of tectonic and biological events caused a shift in the mean climate state from a global "greenhouse" to an "icehouse". Geochemical records extracted from sedimentary rocks deposited in shallow epicontinental seaways are increasingly being used to infer relationships between tectonism, carbon cycling and climate and therefore are assumed to reflect global ocean processes. We analyzed radiogenic isotopes in biogenic apatite along a North American transect to constrain the degree of geochemical coupling between the epicontinental seas and the open ocean. Our results argue strongly for decoupling of North American seaways from the open ocean by latest Mississippian time.

The effects of a non-zero background state on atmospheric dynamics is explored through simple models and observations. Firstly, we examine the effects of moisture gradients on the stability and propagation of Rossby waves in a mid-latitude -plane. We begin by a consistent derivation of the forced quasi-geostrophic equations on a -plane to understand the constraints placed by geostrophy on the time scale of condensation. We see that the presence of meridional gradients of moisture results in a slowdown of the waves. On the introduction of zonal gradients of moisture, the waves become unstable, and for certain parameters which are representative of the real atmosphere, they propagate eastward and mature on an intra-seasonal timescale. The mechanism of the in hence of moisture on waves is understood by thinking of condensation as providing an \equivalent" potential vorticity (PV) gradient which opposes the dynamical PV gradient. Secondly, we look at the effects of a mean background ow on the Matsuno-Gill response in the spherical shallow water system. The mean ow is prescribed to resemble the climatological upper tropospheric zonal wind structure in the atmosphere. As the strength of the ow increases, the equatorially trapped Matsuno-Gill response rst transforms into a poleward propagating Rossby wavetrain. As the strength of the mean ow reaches values similar to that observed in the atmosphere, the stationary wave response becomes a zonally oriented quadrupole structure. This structure bears a striking resemblance to the observed upper level structure of the Madden-Julian oscillation (MJO). The time evolution of this quadrupole structure is quick enough to be relevant on MJO timescales, and the structure is quite robust across a range of values for the drag coefficient. Finally, we look at the role played by low frequency variability in the Pacific in the recent expansion of the Hadley cell. We find that the dominant effect of the low frequency variability is a stationary dispersive Rossby wavetrain extending from the tropical Paci. We further find that most of the observed expansion of the Hadley cell can be accounted for by this low frequency variability. We nd that large scale changes such as the changes in the equator-pole temperature gradient or midlatitude static stability need not be invoked to understand the observed expansion.