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1
Poblete, Arnobio Germán, et Juan Leonidas Minetti. « Propuesta de un índice de sequía para valuar la posible expansión de la celda de Hadley y su relación con la crisis hídrica de las regiones subtropicales del hemisferio sur ». Geográfica digital 12, no 24 (24 octobre 2017) : 1. http://dx.doi.org/10.30972/geo.12242158.
Texte intégralRésumé :
<p>La celda de Hadley (HC) es la más importante de la circulación general atmosférica por su rol fundamental en el sistema climático, y por ende su impacto sobre el clima global. Sus principales componentes son: La Convergencia Intertropical de la circulación en superficie -alisios- (ITCZ) con el</p><p>consiguiente ascenso, condensación y posible precipitación; y la circulación de altura formada por los Contralisios subsidentes (CAS) en las regiones subtropicales de ambos hemisferios, provocando calentamiento y secamiento del aire descendente, con el consiguiente aumento de masa.</p><p>Recientemente se ha puesto especial énfasis en los cambios latitudinales de la ITCZ y CAS posiblemente como resultado de un proceso de expansión de la HC producido por el calentamiento global (CG). Esta intensificación/debilitamiento de la HC a su vez está asociada/o con el crecimiento/disminución de las precipitaciones en la ITCZ y disminución/crecimiento de las precipitaciones en la banda subtropical, contribuyendo al incremento de las frecuencias de sequías en</p><p>latitudes medias de ambos hemisferios. En ese marco, se buscó una explicación sobre la intensificación y desplazamiento hacia altas latitudes de la posición del anticiclón subtropical del Pacífico Sur (LSAP).</p><p>Para comprobar el impacto descripto del CG sobre la celda de Hadley y de esta en la subsidencia sobre los anticiclones subtropicales del Hemisferio Sur (HS), se construye un índice de sequía hemisférico, con el objeto de relacionarlo con las principales variables asociadas a los procesos precipitantes en el mismo, y de esa manera contextualizar la crisis hídrica que están soportando dichas regiones.</p><p> </p>
2
Dorta Antequera, Pedro. « Las inversiones térmicas en Canarias ». Investigaciones Geográficas, no 15 (15 juin 1996) : 109. http://dx.doi.org/10.14198/ingeo1996.15.01.
Texte intégralRésumé :
La localización geográfica del archipiélago canario, en latitudes subtropicales y con un predominio casi absoluto de los vientos alisios, determina una estructura vertical de la baja troposfera con una presencia constante de inversiones térmicas, de subsidencia en la mayor parte de los casos, que explican la fuerte tendencia a la estabilidad de toda esta región. Realizar un primer análisis estadístico de estas inversiones así como estudiar su origen supone el tema de estudio de este trabajo.
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Antonio Arz, José, et Eustoquio Molina. « Late Campanian and Maastrichtian bitostratigraphy based on planctonic foraminifera from temperate to subtropical latitudes (Spain, France and Tunisia) ». Neues Jahrbuch für Geologie und Paläontologie - Abhandlungen 224, no 2 (24 mai 2002) : 161–95. http://dx.doi.org/10.1127/njgpa/224/2002/161.
Texte intégral
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LEBOEUF, B., J. A. DELGADILLO, E. MANFREDI, A. PIACERE, V. CLEMENT, P. MARTIN, M. T. PELLICER-RUBIO, P. BOUÉ et R. DE CREMOUX. « Place de la maîtrise de la reproduction dans les schémas de sélection en chèvres laitières ». INRAE Productions Animales 21, no 5 (27 novembre 2008) : 391–402. http://dx.doi.org/10.20870/productions-animales.2008.21.5.3414.
Texte intégralRésumé :
La saisonnalité de la reproduction chez les chèvres originaires des latitudes tempérées ou subtropicales peut maintenant être contrôlée par des changements artificiels de la photopériode. Les jours courts stimulent l’activité sexuelle tandis que les jours longs l’inhibent. Ces connaissances ont permis le développement de traitements photopériodiques pour le contrôle de l’activité sexuelle des chèvres et des boucs.
En France, l’Insémination Artificielle (IA) des chèvres joue un rôle central pour le contrôle des appariements et l’organisation du schéma de sélection. La plupart des chèvres sont inséminées en dehors de la saison sexuelle avec de la semence cryoconservée, après induction hormonale de l’ovulation seule ou en combinaison avec des traitements photopériodiques. Les taux de fertilité sont en moyenne de 65%. De nouvelles stratégies sont en cours d’expérimentation. Elles sont basées sur l’IA après un effet mâle pour réduire l’utilisation des hormones.
Le schéma de sélection s’est développé grâce aux progrès de l’IA. Ce schéma repose sur des plans d’accouplements entre reproducteurs d’élite, le testage sur descendance en fermes et la diffusion des semences de boucs améliorateurs. Après les caractères laitiers, les caractères fonctionnels sont désormais pris en compte. Actuellement, l’accent est mis sur la morphologie de la mamelle. La résistance à certaines maladies est à l’étude. Outre cette approche de génétique quantitative, de nouvelles perspectives basées sur une approche moléculaire permettront de détecter des gènes économiquement intéressants pour l’élevage caprin.
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PAIVA, M. P., et M. F. ANDRADE-TUBINO. « Distribuição e abundância de peixes bentônicos explotados pelos linheiros ao largo do sudeste do Brasil (1986-1995) ». Revista Brasileira de Biologia 58, no 4 (novembre 1998) : 619–32. http://dx.doi.org/10.1590/s0034-71081998000400009.
Texte intégralRésumé :
Este trabalho trata da distribuição e abundância das principais espécies de peixes bentônicos explotados pelos linheiros ao largo do sudeste do Brasil (latitudes 18º-26ºS), no período 1986-1995, listadas a seguir, com as respectivas participações no total das capturas: batata = Lopholatilus vilariii Ribeiro = 33,4%, namorado = Pseudopercis numida Ribeiro = 18,4%, cherne = Epinephelus niveatus (Valenciennes) = 12,1%, garoupa = Epinephelus marginatus (Lowe) = 4,1% e badejo = Mycteroperca bonaci (Poey) = 3,1%. As pescarias ocorreram em águas tropicais (latitudes 18º- 23ºS) e subtropicais (latitudes 23º-26ºS), tendo como divisor zoogeográfico a ressurgência de Cabo Frio. Os dados foram agrupados por faixas de latitude e de profundidade, segundo espécies e estações do ano. As pescarias estiveram grandemente concentradas nas águas subtropicais, com tendência(s) de produtividade decrescente, à medida que aumenta(m) a latitude e/ou a profundidade dos pesqueiros explotados. Nas águas tropicais, as espécies dominantes foram a garoupa e o badejo, de forma indiscutível entre as latitudes 18º-20ºS, com maiores capturas em profundidades de até 80 metros. Nas águas subtropicais, as espécies dominantes sempre foram o batata, o namorado e o cherne, com maiores capturas em profundidades superiores a 100 metros, incluindo a parte superior do talude continental. A produção das cinco espécies principais apresentou tendências de variação estacional, mas apenas o cherne evidenciou deslocamento sazonal ao longo da costa. Foram comprovadas a menor diversidade e a maior biomassa de peixes bentônicos nas águas subtropicais.
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Floyd, Matthew, Masaru Mizuyama, Masami Obuchi, Brigitte Sommer, Mark GR Miller, Iori Kawamura, Hiroki Kise, James D. Reimer et Maria Beger. « Functional diversity of reef molluscs along a tropical-to-temperate gradient ». Coral Reefs 39, no 5 (1 juillet 2020) : 1361–76. http://dx.doi.org/10.1007/s00338-020-01970-2.
Texte intégralRésumé :
Abstract Global warming is leading to range shifts of marine species, threatening the structure and functioning of ecological communities and human populations that rely on them. The largest changes are seen in biogeographic transition zones, such as subtropical reef communities, where species range shifts are already causing substantial community reorganisation. This causes functional changes in communities over subtropical latitudes, though a baseline functional understanding remains elusive for many taxa. One key marine taxon are molluscs, which provide many ecosystem services, are important prey for fishes and are also fisheries targets themselves, but remain largely unstudied. Here, we examine the trait composition, functional diversity, and functional redundancy of mollusc assemblages along the tropical-to-temperate transition in Japan (25° to 35° Northern latitude). Specifically, we use a trait database of 88 mollusc species from 31 subtropical reefs along the Pacific coast of Japan to show that trait composition of mollusc assemblages changes continuously along the latitudinal gradient. We discover that functional diversity of mollusc assemblages decreases with increasing latitude, a pattern associated with declines in functional dispersion. Moreover, we find a clear distinction between tropical and subtropical mollusc assemblages, with substrate-attached, suspension feeding bivalves more abundant in the tropics and free-living gastropod grazers more prevalent at higher latitudes. Our trait-based evidence in this study shows a contraction and almost complete shift in the functioning of marine mollusc assemblages at biogeographic transition zones and our trait database facilitates further study. Our findings provide evidence of the changing taxonomic and functional composition of extant mollusc communities with latitude, pointing to potential pertinent changes and tropicalisation of these communities with rapid ocean warming.
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Bosmans, J. H. C., F. J. Hilgen, E. Tuenter et L. J. Lourens. « Obliquity forcing of low-latitude climate ». Climate of the Past 11, no 10 (9 octobre 2015) : 1335–46. http://dx.doi.org/10.5194/cp-11-1335-2015.
Texte intégralRésumé :
Abstract. The influence of obliquity, the tilt of the Earth's rotational axis, on incoming solar radiation at low latitudes is small, yet many tropical and subtropical palaeoclimate records reveal a clear obliquity signal. Several mechanisms have been proposed to explain this signal, such as the remote influence of high-latitude glacials, the remote effect of insolation changes at mid- to high latitudes independent of glacial cyclicity, shifts in the latitudinal extent of the tropics, and changes in latitudinal insolation gradients. Using a sophisticated coupled ocean–atmosphere global climate model, EC-Earth, without dynamical ice sheets, we performed two idealized experiments of obliquity extremes. Our results show that obliquity-induced changes in tropical climate can occur without high-latitude ice sheet fluctuations. Furthermore, the tropical circulation changes are consistent with obliquity-induced changes in the cross-equatorial insolation gradient, suggesting that this gradient may be used to explain obliquity signals in low-latitude palaeoclimate records instead of the classical 65° N summer insolation curve.
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Bosmans, J. H. C., F. J. Hilgen, E. Tuenter et L. J. Lourens. « Obliquity forcing of low-latitude climate ». Climate of the Past Discussions 11, no 1 (11 février 2015) : 221–41. http://dx.doi.org/10.5194/cpd-11-221-2015.
Texte intégralRésumé :
Abstract. The influence of obliquity, the tilt of the Earth's rotational axis, on incoming solar radiation at low latitudes is small, yet many tropical and subtropical paleoclimate records reveal a clear obliquity signal. Several mechanisms have been proposed to explain this signal, such as the remote influence of high-latitude glacials, the remote effect of insolation changes at mid- to high latitudes independent of glacial cyclicity, shifts in the latitudinal extent of the tropics, and changes in latitudinal insolation gradients. Using a sophisticated coupled ocean–atmosphere global climate model, EC-Earth, without dynamical ice sheets, we performed two experiments of obliquity extremes. Our results show that obliquity-induced changes in tropical climate can occur without high-latitude ice sheet fluctuations. Furthermore, the tropical circulation changes are consistent with obliquity-induced changes in the cross-equatorial insolation gradient, implying that this gradient may be used to explain obliquity signals in low-latitude paleoclimate records instead of the classic 65° N summer insolation curve.
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Upchurch, Garland R. « Cretaceous vegetational change : a biomal perspective ». Paleontological Society Special Publications 6 (1992) : 295. http://dx.doi.org/10.1017/s2475262200008558.
Texte intégralRésumé :
The Cretaceous rise of flowering plants marked an important transition in the modernization of terrestrial ecosystems. Well documented is the diversification of angiosperm pollen during the mid-Cretaceous and the migration of angiosperms from low latitudes to middle and high latitudes during the Barremian to Cenomanian. Global compilations of “species” diversity indicate a rapid rise in angiosperm diversity during the Albian to Cenomanian. This rise parallels a decline in the species diversity of archaic pteridophytes and the gymnosperm orders Cycadales, Bennettitales, Ginkgoales, Czekanowskiales, and Caytoniales. Late Cretaceous floras show more gradual trends in species diversity than mid-Cretaceous floras.Megafloral reconstructions of vegetation and climate for North America and other continents indicate warm temperatures in coastal regions of middle to high latitudes. Cretaceous biomes, however, often cannot be compared closely with Recent biomes. During much of the Cretaceous, conifers and other gymnosperms shared dominance with angiosperms in tropical and subtropical vegetation, unlike the Recent. During the Late Cretaceous, tropical rainforest was areally restricted. The few known leaf megafloras from equatorial regions indicate subhumid, rather than rainforest, conditions. Desert and semi-desert were widespread at lower latitudes and are documented by the occurrence of evaporite minerals in China, Africa, Spain, Mexico, and South America. Mid-latitude vegetation consisted of open-canopy broadleaved and coniferous evergreen woodlands that existed under subhumid conditions and low seasonality. High-latitude vegetation of the Northern Hemisphere consisted of coniferous and broadleaved deciduous forest, rather than boreal forest and tundra. High-latitude vegetation from coastal regions of the Southern Hemisphere consisted of evergreen conifers and angiosperms. Rainforest conditions appear to have been largely restricted to polar latitudes.Data on relative abundance, though often incomplete, indicate that angiosperms became ecologically important in tropical to warm subtropical broadleaved evergreen forests and woodlands by the Cenomanian. However, their rise to dominance took longer in other biomes. Conifers formed an important component of many Late Cretaceous biomes, and the persistence of archaic gymnosperms was strongly influenced by climate. Deciduous Ginkgoales, Czekanowskiales, Bennettitales, and Caytoniales are rare to absent in Late Cretaceous megafloras from warm subtropical to tropical climates, but they persist in megafloras from cooler climates. Archaic conifers such as Frenelopsis occur in megafloras representing low-latitude desert and semi-desert, but they are generally absent in more humid assemblages. Within mid-latitude broadleaved and coniferous evergreen woodland from North America, conifers show evidence for co-dominance with angiosperms into the early Maastrichtian. However, this co-dominance appears to have ended by latest Maastrichtian, which implies that vegetational reorganization occurred during the last few million years of the Cretaceous in North America.
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Chen, Gang, et Pablo Zurita-Gotor. « The Tropospheric Jet Response to Prescribed Zonal Forcing in an Idealized Atmospheric Model ». Journal of the Atmospheric Sciences 65, no 7 (1 juillet 2008) : 2254–71. http://dx.doi.org/10.1175/2007jas2589.1.
Texte intégralRésumé :
Abstract This paper explores the tropospheric jet shift to a prescribed zonal torque in an idealized dry atmospheric model with high stratospheric resolution. The jet moves in opposite directions for torques on the jet’s equatorward and poleward flanks in the troposphere. This can be explained by considering how the critical latitudes for wave activity absorption change, where the eastward propagation speed of eddies equals the background zonal mean zonal wind. While the increased zonal winds in the subtropics allow the midlatitude eddies to propagate farther into the tropics and result in the equatorward shift in the critical latitudes, the increased winds in the midlatitudes accelerate the eastward eddy phase speeds and lead to the poleward shift in the critical latitudes. In contrast, the jet moves poleward when a westerly torque is placed in the extratropical stratosphere irrespective of the forcing latitude. The downward penetration of zonal winds to the troposphere displays a poleward slope for the subtropical torque, an equatorward slope for the high-latitude torque, and less tilting for the midlatitude torques. The stratospheric eddies play a key role in transferring zonal wind anomalies downward into the troposphere. It is argued that these stratospheric zonal wind anomalies can affect the tropospheric jet by altering the eastward propagation of tropospheric eddies. Additionally, the zonal wind response to a subtropical zonal torque in this idealized model is of value in understanding the tropospheric jet sensitivity to the orographic gravity wave drag parameterization in a realistic climate model.
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LOCATELLI, Y., et P. MERMILLOD. « Caractéristiques et maîtrise de la fonction de reproduction chez les cervidés ». INRAE Productions Animales 18, no 1 (14 mars 2005) : 3–25. http://dx.doi.org/10.20870/productions-animales.2005.18.1.3505.
Texte intégralRésumé :
Ces dernières années, l’élevage de cervidés s’est considérablement développé en Nouvelle-Zélande pour devenir une industrie profitable. Alors que quelques cervidés (daim d’Europe et cerf élaphe) font l’objet de domestication et d’élevage dans un but commercial, d’autres espèces et sous-espèces de cervidés sont menacées d’extinction dans le milieu naturel. Les travaux de recherches effectués afin de mieux caractériser et maîtriser la fonction de reproduction de ces ruminants sont présentés dans cette étude bibliographique.
Il apparaît qu’à l’instar de nos ruminants domestiques (ovins, caprins), les cervidés originaires des zones tempérées sont généralement caractérisés par un saisonnement marqué de leur fonction de reproduction. Ces variations saisonnières d’activité sexuelle sont dictées par les variations photopériodiques et permettent la naissance des jeunes en fin de printemps. Chez les cervidés, les différences entre les périodes d’activité et de repos sexuels semblent beaucoup plus marquées en comparaison de nos ruminants domestiques. La période d’activi-té sexuelle est variable d’une espèce à l’autre (été, automne ou début de l’hiver) mais très fixe pour une espèce donnée. La période de repos sexuel traduit des modifications importantes dans les sécrétions de gonadotrophines et se caractérise notamment par une aspermie complète chez le mâle. Chez la femelle, l’état d’anoestrus est profond et associé à une absence d’ovulation. La durée de gestation est également variable d’une espèce à l’autre mais est remarquablement fixe pour une espèce donnée.
Chez les cervidés originaires des zones subtropicales et selon les espèces, les variations d’activité sexuelle sont plus discrètes permettant une répartition des mises bas plus ou moins homogène au cours de l’année, y compris lorsque les animaux sont transportés sous des latitudes élevées.
Dans le cas des espèces de cervidés menacés d’extinction, l’utilisation des biotechnologies de la reproduction et des méthodes de procréation assistée pourrait, à terme, faciliter la réalisation des programmes conservatoires. Les techniques classiques de production in vivo d’embryons basées sur l’ovulation multiple, l’insémination artificielle et le transfert embryonnaire se sont avérées difficilement applicables aux cervidés. Aussi, les recherches portent actuellement sur le développement de techniques de production in vitro d’embryons.
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Simpson, Isla R., Michael Blackburn et Joanna D. Haigh. « A Mechanism for the Effect of Tropospheric Jet Structure on the Annular Mode–Like Response to Stratospheric Forcing ». Journal of the Atmospheric Sciences 69, no 7 (1 juillet 2012) : 2152–70. http://dx.doi.org/10.1175/jas-d-11-0188.1.
Texte intégralRésumé :
Abstract For many climate forcings the dominant response of the extratropical circulation is a latitudinal shift of the tropospheric midlatitude jets. The magnitude of this response appears to depend on climatological jet latitude in general circulation models (GCMs): lower-latitude jets exhibit a larger shift. The reason for this latitude dependence is investigated for a particular forcing, heating of the equatorial stratosphere, which shifts the jet poleward. Spinup ensembles with a simplified GCM are used to examine the evolution of the response for five different jet structures. These differ in the latitude of the eddy-driven jet but have similar subtropical zonal winds. It is found that lower-latitude jets exhibit a larger response due to stronger tropospheric eddy–mean flow feedbacks. A dominant feedback responsible for enhancing the poleward shift is an enhanced equatorward refraction of the eddies, resulting in an increased momentum flux, poleward of the low-latitude critical line. The sensitivity of feedback strength to jet structure is associated with differences in the coherence of this behavior across the spectrum of eddy phase speeds. In the configurations used, the higher-latitude jets have a wider range of critical latitude locations. This reduces the coherence of the momentum flux anomalies associated with different phase speeds, with low phase speeds opposing the effect of high phase speeds. This suggests that, for a given subtropical zonal wind strength, the latitude of the eddy-driven jet affects the feedback through its influence on the width of the region of westerly winds and the range of critical latitudes on the equatorward flank of the jet.
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Siu, Leong Wai, et Kenneth P. Bowman. « Forcing of the Upper-Tropospheric Monsoon Anticyclones ». Journal of the Atmospheric Sciences 76, no 7 (19 juin 2019) : 1937–54. http://dx.doi.org/10.1175/jas-d-18-0340.1.
Texte intégralRésumé :
Abstract During the boreal warm season (May–September), the circulation in the upper troposphere and lower stratosphere is dominated by two large anticyclones: the Asian monsoon anticyclone (AMA) and North American monsoon anticyclone (NAMA). The existence of the AMA has long been linked to Asian monsoon precipitation using the Matsuno–Gill framework, but the origin of the NAMA has not been clearly understood. Here the forcing mechanisms of the NAMA are investigated using a simplified dry general circulation model. The simulated anticyclones are in good agreement with observations when the model is forced by a zonally symmetric meridional temperature gradient plus a realistic geographical distribution of heating based on observed tropical and subtropical precipitation in the Northern Hemisphere. Model experiments show that the AMA and NAMA are largely independent of one another, and the NAMA is not a downstream response to the Asian monsoon. The primary forcing of the NAMA is precipitation in the longitude sector between 60° and 120°W, with the largest contribution coming from the subtropical latitudes within that sector. Experiments with idealized regional heating distributions reveal that the extratropical response to tropical and subtropical precipitation depends approximately linearly on the magnitude of the forcing but nonlinearly on its latitude. The AMA is stronger than the NAMA, primarily because precipitation in the subtropics over Asia is much heavier than at similar latitudes in the Western Hemisphere.
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Davis, Christopher A. « Simulations of Subtropical Cyclones in a Baroclinic Channel Model ». Journal of the Atmospheric Sciences 67, no 9 (1 septembre 2010) : 2871–92. http://dx.doi.org/10.1175/2010jas3411.1.
Texte intégralRésumé :
Abstract The present study considers a variety of cyclone developments that occur in an idealized, baroclinic channel model featuring full condensation heating effects over an ocean with prescribed sea surface temperature variation. The geostrophic basic-state jet is specified by the tropopause shape, and horizontal shear is included by specifying the meridional variation of zonal wind on the lower boundary. The horizontal shear induces anticyclonic wave breaking of baroclinic waves. Normal mode perturbations are computed using a “fake-dry” version of the model but integrated forward using full physics. Low-latitude moist convection is particularly strong in simulations with strong surface easterlies that destabilize the troposphere through water vapor fluxes from the ocean surface. Deep convection produces a locally elevated dynamic tropopause and an associated anticyclone. This modified zonal flow supports moist baroclinic instability. The resulting cyclones, identified as subtropical cyclones, occur in deep westerly vertical wind shear but are nearly devoid of lower-tropospheric baroclinicity initially. These systems are distinguished from baroclinically dominated secondary cyclones that also form at relatively low latitudes in the simulations. For weak jets and strong subtropical surface easterlies, subtropical cyclone development dominates formation on the midlatitude jet. For strong westerly jets or weak horizontal shear, the situation is reversed and the midlatitude baroclinic wave can help or hinder the ultimate intensification of the subtropical cyclone. The similarity of this cross-latitude influence to the extratropical transition of tropical cyclones is noted.
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Delibes, M., S. C. Zapata, M. C. Blázquez et R. Rodríguez-Estrella. « Seasonal food habits of bobcats (Lynx rufus) in subtropical Baja California Sur, Mexico ». Canadian Journal of Zoology 75, no 3 (1 mars 1997) : 478–83. http://dx.doi.org/10.1139/z97-058.
Texte intégralRésumé :
The annual and seasonal diet of the bobcat (Lynx rufus) was determined from analysis of 188 feces in the Cape region of Baja California, Mexico, an arid zone with numerous subtropical elements in its flora and fauna. Bobcats fed mainly on lagomorphs, which reached 74% of occurrence, followed by rodents (40%), reptiles (15%), and birds (12%). No seasonal variations were observed. The results were consistent with those of studies elsewhere, indicating that bobcats still rely upon lagomorphs for much of their food in southern latitudes. This supports the hypothesis that lynx have evolved to prey on hares and rabbits. The prevalence of reptiles as prey of bobcats in our study area was the highest ever reported. They were reported as bobcat prey in only 1 of 20 studies from north of latitude 40°, but in 14 of the 18 studies carried out south of this latitude. With regard to feeding on reptiles, the habits of bobcats in Baja California Sur resemble those of other similar-sized felids in tropical areas, such as ocelots (Felis pardalis) and servals (Leptailurus serval).
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Wang, Qi, et Rui Xin Huang. « Decadal Variability of Pycnocline Flows from the Subtropical to the Equatorial Pacific* ». Journal of Physical Oceanography 35, no 10 (1 octobre 2005) : 1861–75. http://dx.doi.org/10.1175/jpo2791.1.
Texte intégralRésumé :
Abstract A method based on isopycnal trajectory analysis is proposed to quantify the pathways from the subtropics to the Tropics. For a continuous stratified ocean a virtual streamfunction is defined, which can be used to characterize these pathways. This method is applied to the climatological dataset produced from a data-assimilated model. Analysis indicates that in each layer contours of the virtual streamfunction are a good approximation for streamlines, even if there is a cross-isopycnal mass flux. The zonal-integrated meridional transport per unit layer thickness through each pathway varies in proportion to 1/sinθ, where θ is latitude. The vertical-integrated total transport through pathways behaves similarly. Transport through pathways has a prominent decadal variability. Results suggest that in decadal time scales the interior pathway transport (IPT) anomaly may be mainly caused by the wind stress anomaly at low latitude. The western boundary pathway transport (WBPT) anomaly often has a sign opposite to the IPT anomaly, reflecting compensation between the IPT and the WBPT. However, more often than not the wind stress anomaly within tropical latitudes can also be used to explain the WBPT anomaly.
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Tiedje, Bente, Armin Köhl et Johanna Baehr. « Potential Predictability of the North Atlantic Heat Transport Based on an Oceanic State Estimate ». Journal of Climate 25, no 24 (15 décembre 2012) : 8475–86. http://dx.doi.org/10.1175/jcli-d-11-00606.1.
Texte intégralRésumé :
Abstract This paper investigates the potential predictability of the meridional heat transport (MHT) in the North Atlantic on interannual time scales using hindcast ensembles based on an oceanic data assimilation product. The work analyzes the prognostic potential predictability (PPP), using the ocean synthesis of the German partner of the consortium for Estimating the Circulation and Climate of the Ocean (GECCO) as initial conditions and as boundary conditions. The PPP of the MHT varies with latitude: local maxima are apparent within the subpolar and the subtropical gyres, and a minimum is apparent at the boundary between the gyres. This PPP minimum can also be seen in the PPP structure of the Atlantic meridional overturning circulation (AMOC), although it is considerably less pronounced. The decomposition of the MHT shows that within the subpolar gyre, the gyre component of the MHT influences the PPP structure of the MHT. Within the subtropical gyre, the overturning component of the MHT characterizes the PPP structure of the MHT. At the boundary between the subpolar and the subtropical gyres, the dynamics of the Ekman heat transport limit the predictable lead times of the MHT. At most latitudes, variations in the velocity field control the PPP structure of the MHT. The PPP structure of the AMOC can also be classified into gyre and gyre-boundary regimes, but the predictable lead times within the gyres are only similar to those of the overturning component of the MHT. Overall, the analysis provides a reference point for the latitude dependence of the MHT’s PPP structure and relates it to the latitude dependence of the AMOC’s PPP structure.
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Wilson, Aaron B., David H. Bromwich, Keith M. Hines et Sheng-hung Wang. « El Niño Flavors and Their Simulated Impacts on Atmospheric Circulation in the High Southern Latitudes* ». Journal of Climate 27, no 23 (1 décembre 2014) : 8934–55. http://dx.doi.org/10.1175/jcli-d-14-00296.1.
Texte intégralRésumé :
Abstract Two El Niño flavors have been defined based on whether warm sea surface temperature (SST) anomalies are located in the central or eastern tropical Pacific (CP or EP). This study further characterizes the impacts on atmospheric circulation in the high latitudes of the Southern Hemisphere associated with these types of El Niño events though a series of numerical simulations using the National Center for Atmospheric Research Community Atmosphere Model (CAM). Comparing results with the Interim ECMWF Re-Analysis (ERA-Interim), CAM simulates well the known changes to blocking over Australia and a southward shift in the subtropical jet stream across the eastern Pacific basin during CP events. More importantly for the high southern latitudes, CAM simulates a westward shift in upper-level divergence in the tropical Pacific, which causes the Pacific–South American stationary wave pattern to shift toward the west across the entire South Pacific. These changes to the Rossby wave source region impact the South Pacific convergence zone and jet streams and weaken the high-latitude blocking that is typically present in the Amundsen-Bellingshausen Seas during EP events. Anticyclonic flow becomes established farther west in the south central Pacific, modifying high-latitude heat and momentum fluxes across the South Pacific and South Atlantic associated with the ENSO–Antarctic dipole.
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Hudson, R. D. « Measurements of the movement of the jet streams at mid-latitudes, in the Northern and Southern Hemispheres, 1979 to 2010 ». Atmospheric Chemistry and Physics Discussions 11, no 11 (22 novembre 2011) : 31067–90. http://dx.doi.org/10.5194/acpd-11-31067-2011.
Texte intégralRésumé :
Abstract. Previous studies have shown that the mean latitude of the subtropical jet streams in both hemispheres have shifted toward the poles over the last few decades. This paper presents a study of the movement of both the subtropical and Polar fronts, the location of the respective jet streams, between 1979 and 2010 at mid-latitudes, using total ozone measurements to identify the sharp horizontal boundary that occurs at the position of the fronts. Previous studies have shown that the two fronts are the boundaries of three distinct regimes in the stratosphere, corresponding to the Hadley. Ferrel, and Polar meridionally overturning circulation cells in the troposphere, each of which has a distinct temperature profile. Over the period of study the horizontal area of the Hadley cell has increased at latitudes between 20 and 60 degrees while the area of the Polar cell has decreased. A linear regression analysis was performed to identify the major factors associated with the movement of the subtropical jet streams. These were: (1) changes in the Tropical land/ocean temperature, (2) direct radiative forcing from greenhouse gases in the troposphere, (3) changes in the temperature of the lower Tropical stratosphere, (4) the Quasi-Biennial Oscillation, and (5) volcanic eruptions. The dominant mechanism was the direct radiative forcing from greenhouse gases. Over the period of study the poleward movement of the subtropical jet streams was 3.7±0.3 degrees in the Northern Hemisphere and 6.5±0.2 degrees in the Southern Hemisphere, with a net expansion of the Tropical belt of 10.2 degrees. Previous studies have shown that weather systems tend to follow the jet streams. The observed poleward movement in both hemispheres over the past thirty years represents a significant change in the position of the subtropical jet streams, which should lead to significant latitudinal shifts in the global weather patterns, temperatures, precipitation and the hydrologic cycle.
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Yang, Hu, Gerrit Lohmann, Xiaoxu Shi et Chao Li. « Enhanced Mid-Latitude Meridional Heat Imbalance Induced by the Ocean ». Atmosphere 10, no 12 (27 novembre 2019) : 746. http://dx.doi.org/10.3390/atmos10120746.
Texte intégralRésumé :
The heat imbalance is the fundamental driver for the atmospheric circulation. Therefore, it is crucially important to understand how it responds to global warming. In this study, the role of the ocean in reshaping the atmospheric meridional heat imbalance is explored based on observations and climate simulations. We found that ocean tends to strengthen the meridional heat imbalance over the mid-latitudes. This is primarily because of the uneven ocean heat uptake between the subtropical and subpolar oceans. Under global warming, the subtropical ocean absorbs relatively less heat as the water there is well stratified. In contrast, the subpolar ocean is the primary region where the ocean heat uptake takes place, because the subpolar ocean is dominated by upwelling, strong mixing, and overturning circulation. We propose that the enhanced meridional heat imbalance may potentially contribute to strengthening the water cycle, westerlies, jet stream, and mid-latitude storms.
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Ishizu, Miho, Yasumasa Miyazawa, Tomohiko Tsunoda et Xinyu Guo. « Seasonal variability in the inorganic ocean carbon cycle in the Northwest Pacific evaluated using a biogeochemical and carbon model coupled with an operational ocean model ». Climatic Change 162, no 2 (9 juillet 2020) : 877–902. http://dx.doi.org/10.1007/s10584-020-02779-2.
Texte intégralRésumé :
Abstract Here, we investigate the seasonal variability in the dissolved inorganic carbon (DIC) cycle in the Northwest Pacific using a high-resolution biogeochemical and carbon model coupled with an operational ocean model. Results show that the contribution to DIC from air–sea CO2 exchange is generally offset by vertical mixing at the surface at all latitudes, with some seasonal variation. Biological processes in subarctic regions are evident at the surface, whereas in the subtropical region these processes take place within the euphotic layer and then DIC consumption deepens southward with latitude. Such latitudinal differences in biological processes lead to marked horizontal and vertical contrasts in the distribution of DIC, with modulation by horizontal and vertical advection–diffusion processes.
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Lyu, Kewei, Xuebin Zhang, John A. Church et Quran Wu. « Processes Responsible for the Southern Hemisphere Ocean Heat Uptake and Redistribution under Anthropogenic Warming ». Journal of Climate 33, no 9 (1 mai 2020) : 3787–807. http://dx.doi.org/10.1175/jcli-d-19-0478.1.
Texte intégralRésumé :
AbstractThe Southern Hemisphere oceans absorb most of the excess heat stored in the climate system due to anthropogenic warming. By analyzing future climate projections from a large ensemble of the CMIP5 models under a high emission scenario (RCP8.5), we investigate how the atmospheric forcing and ocean circulation determine heat uptake and redistribution in the Southern Hemisphere oceans. About two-thirds of the net surface heat gain in the high-latitude Southern Ocean is redistributed northward, leading to enhanced and deep-reaching warming at middle latitudes near the boundary between the subtropical gyres and the Antarctic Circumpolar Current. The projected magnitudes of the ocean warming are closely related to the magnitudes of the wind and gyre boundary poleward shifts across the models. For those models with the simulated gyre boundary biased equatorward, the latitude where the projected ocean warming peaks is also located farther equatorward and a larger poleward shift of the gyre boundary is projected. In a theoretical framework, the subsurface ocean changes are explored using three distinctive processes on the temperature–salinity diagram: pure heave, pure warming, and pure freshening. The enhanced middle-latitude warming and the deepening of isopycnals are attributed to the pure heave and pure warming processes, likely related to the wind-driven heat convergence and the accumulation of extra surface heat uptake by the background ocean circulation, respectively. The equatorward and downward subductions of the surface heat and freshwater input at high latitudes (i.e., pure warming and pure freshening processes) result in cooling and freshening spiciness changes on density surfaces within the Subantarctic Mode Water and Antarctic Intermediate Water.
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Zhuravleva, Anastasia, et Henning A. Bauch. « Last interglacial ocean changes in the Bahamas : climate teleconnections between low and high latitudes ». Climate of the Past 14, no 10 (1 octobre 2018) : 1361–75. http://dx.doi.org/10.5194/cp-14-1361-2018.
Texte intégralRésumé :
Abstract. Paleorecords and modeling studies suggest that instabilities in the Atlantic Meridional Overturning Circulation (AMOC) strongly affect the low-latitude climate, namely via feedbacks on the Atlantic Intertropical Convergence Zone (ITCZ). Despite the pronounced millennial-scale overturning and climatic variability documented in the subpolar North Atlantic during the last interglacial period (MIS 5e), studies on cross-latitudinal teleconnections remain very limited. This precludes a full understanding of the mechanisms controlling subtropical climate evolution across the last warm cycle. Here, we present new planktic foraminiferal assemblage data combined with δ18O values in surface and thermocline-dwelling foraminifera from the Bahamas, a region ideally suited to studying past changes in the subtropical ocean and atmosphere. Our data reveal that the peak sea surface warmth during early MIS 5e was intersected by an abrupt millennial-scale cooling/salinification event, which was possibly associated with a sudden southward displacement of the mean annual ITCZ position. This atmospheric shift is, in turn, ascribed to the transitional climatic regime of early MIS 5e, which was characterized by persistent ocean freshening in the high latitudes and an unstable AMOC mode.
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Hudson, R. D. « Measurements of the movement of the jet streams at mid-latitudes, in the Northern and Southern Hemispheres, 1979 to 2010 ». Atmospheric Chemistry and Physics 12, no 16 (30 août 2012) : 7797–808. http://dx.doi.org/10.5194/acp-12-7797-2012.
Texte intégralRésumé :
Abstract. Previous studies have shown that the mean latitude of the sub-tropical jet streams in both hemispheres have shifted toward the poles over the last few decades. This paper presents a study of the movement of both the subtropical and Polar fronts, the location of the respective jet streams, between 1979 and 2010 at mid-latitudes, using total ozone measurements to identify the sharp horizontal boundary that occurs at the position of the fronts. Previous studies have shown that the two fronts are the boundaries of three distinct regimes in the stratosphere, corresponding to the Hadley, Ferrel, and polar meridionally overturning circulation cells in the troposphere. Over the period of study the horizontal area of the Hadley cell has increased at latitudes between 20 and 60 degrees while the area of the Polar cell has decreased. A linear regression analysis was performed to identify the major factors associated with the movement of the subtropical jet streams. These were: (1) changes in the Tropical land plus ocean temperature, (2) direct radiative forcing from greenhouse gases in the troposphere, (3) changes in the temperature of the lower tropical stratosphere, (4) the Quasi-Biennial Oscillation, and (5) volcanic eruptions. The dominant mechanism was the direct radiative forcing from greenhouse gases. Between 1979 and 2010 the poleward movement of the subtropical jet streams was 3.7 ± 0.3 degrees in the Northern Hemisphere and 6.5 ± 0.2 degrees in the Southern Hemisphere. Previous studies have shown that weather systems tend to follow the jet streams. The observed poleward movement in both hemispheres over the past thirty years represents a significant change in the position of the sub-tropical jet streams, which should lead to significant latitudinal shifts in the global weather patterns and the hydrologic cycle.
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Pena, L. D., G. Francés, P. Diz, M. A. Nombela et I. Alejo. « Climate fluctuations during the Holocene in NW Iberia : high and low latitude linkages ». Climate of the Past Discussions 3, no 6 (3 décembre 2007) : 1283–309. http://dx.doi.org/10.5194/cpd-3-1283-2007.
Texte intégralRésumé :
Abstract. High resolution benthic foraminiferal oxygen and carbon stable isotopes (δ18O, δ13C) from core EUGC-3B are used here to infer rapid climatic changes for the last 8500 yr in the Ría de Muros (NW Iberian Margin). Benthic foraminiferal δ18O and δ13C potentially register migrations in the position of the hydrographic front formed between two different intermediate water masses: Eastern North Atlantic Central Water of subpolar origin (ENACWsp), and subtropical origin (ENACWsp). The isotopic records have been compared with two well established North Atlantic marine Holocene paleoceanographic records from low (Sea Surface Temperatures anomalies off Cape Blanc, NW Africa) and high latitudes (Hematite Stained Grains percentage, subpolar North Atlantic). This comparison clearly demonstrates that there is a strong link between high- and low-latitude climatic perturbations at centennial-millennial time scales during the Holocene. Spectral analyses also points at a pole-to-equator propagation of the so-called 1500 yr cycles. Our results demonstrate that during the Holocene, the NW Iberian Margin has undergone a series of cold episodes which are likely triggered at high latitudes in the North Atlantic and are rapidly propagated towards lower latitudes. Conceivably, the propagation of these rapid climatic changes involves a shift of atmospheric and oceanic circulatory systems and so a migration of the hydrographical fronts and water masses all along the North Atlantic area.
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Lambert, Dominique, et Jean-Pierre Cammas. « Rossby wave interactions with Mediterranean and subtropical latitudes ». Meteorology and Atmospheric Physics 108, no 3-4 (7 septembre 2010) : 83–94. http://dx.doi.org/10.1007/s00703-010-0081-0.
Texte intégral
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Korty, Robert L., Kerry A. Emanuel, Matthew Huber et Ryan A. Zamora. « Tropical Cyclones Downscaled from Simulations with Very High Carbon Dioxide Levels ». Journal of Climate 30, no 2 (janvier 2017) : 649–67. http://dx.doi.org/10.1175/jcli-d-16-0256.1.
Texte intégralRésumé :
A method to simulate thousands of tropical cyclones using output from a global climate model is applied to simulations that span very high surface temperatures forced with high levels of carbon dioxide (CO2). The climatology of the storms downscaled from a simulation with modern-day conditions is compared to that of events downscaled from two other simulations featuring 8 and 32 times preindustrial-era levels of CO2. Storms shift poleward with warming: genesis locations and track densities increase in subtropical and higher latitudes, and power dissipation increases poleward of 20°S and 30°N. The average latitude at which storms reach their maximum intensity shifts poleward by more than 1.5° latitude in the 8 × CO2 experiment and by more than 7° latitude in the 32 × CO2 case. Storms live longer and are more numerous in both of the warmer climates. These increases come largely from an expansion of the area featuring favorable conditions into subtropics and midlatitudes, with some regions of the Arctic having the thermodynamic conditions necessary to sustain systems in the hottest case. Storms of category 5 intensity are 52% more frequent in the 8 × CO2 experiment but 40% less so in the 32 × CO2 case, largely owing to a substantial decline in low-latitude activity associated with increases in a normalized measure of wind shear called the ventilation index. Changes in genesis and track densities align well with differences in the ventilation index, and environmental conditions become substantially more favorable poleward of about 20° latitude in the warmer climates.
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Ramos, A. M., R. Nieto, R. Tomé, L. Gimeno, R. M. Trigo, M. L. R. Liberato et D. A. Lavers. « Atmospheric rivers moisture transport from a Lagrangian perspective ». Earth System Dynamics Discussions 6, no 2 (17 décembre 2015) : 2617–43. http://dx.doi.org/10.5194/esdd-6-2617-2015.
Texte intégralRésumé :
Abstract. An automated atmospheric rivers (ARs) detection algorithm is used for the North Atlantic Ocean Basin allowing the identification of the major ARs that affected western European coasts between 1979 and 2014 over the winter half-year (October to March). The entire west coast of Europe was divided into five domains, namely, the Iberian Peninsula (9.75° W; 36–43.75° N), France (4.5° W; 43.75–50° N), UK (4.5° W; 50–59° N), southern Scandinavia and the Netherlands (5.25° E; 50–59° N), and northern Scandinavia (5.25° E; 59–70° N). Following the identification of the main ARs that made landfall in western Europe, a Lagrangian analysis was then applied in order to identify the main sources of moisture that reach each domain. The Lagrangian dataset used was obtained from the FLEXPART model global simulation from 1979 to 2012, where the atmosphere was divided into approximately 2.0 million parcels, and it was forced by ERA-Interim reanalysis on a 1° latitude–longitude grid. Results show that, in general, for all regions considered, the major climatological source of moisture extends along the subtropical North Atlantic, from the Florida Peninsula (northward of 20° N), to each sink region, with the nearest coast to each sink region always appearing as a local maximum of evaporation. In addition, during the AR events, the Atlantic subtropical source is reinforced and displaced, with a slight northward movement of the moisture sources is found when the sink region is positioned at higher latitudes. In conclusion, the results confirm the advection of moisture linked to ARs from subtropical ocean areas, but also the existence of a tropical one, and the mid-latitude sources further the analysed longitude along the North Atlantic is located eastward.
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Kaser, Georg. « Glacier-climate interaction at low latitudes ». Journal of Glaciology 47, no 157 (2001) : 195–204. http://dx.doi.org/10.3189/172756501781832296.
Texte intégralRésumé :
AbstractIn the low latitudes there is an absence of major thermal seasonality, yet there are three different climate regimes related to global circulation patterns and their seasonal oscillation: the humid inner tropics, the dry subtropics and, intermediate between these two, the outer tropics. For the respective glacier regimes the vertical profiles of specific mass balance (VBPs) are modeled considering vertical gradients of accumulation, air temperature and albedo, the duration of the ablation period and a factor for the ratio between melting and sublimation. The model is first calibrated with data from Hintereisferner, Austrian Alps, and is then applied to tropical conditions. The simulated VBP matches well the measured profiles from Irian jaya and Mount Kenya. Due to lack of field evidence, the subtropical VBP cannot be verified directly. However, application of the respective model versions separately to the humid and dry seasons of the outer-tropical Glaciar Uruashraju, Cordillera Blanca, Peru, provides reasonable results. Glaciers in the humid inner tropics are considered to be most sensitive to variations in air temperature, while dry subtropical glaciers are most sensitive to changes in air humidity. The two seasons of the outer tropics have to be viewed from these different perspectives.
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Eyring, V., M. Dameris, V. Grewe, I. Langbein et W. Kouker. « Climatologies of subtropical mixing derived from 3D models ». Atmospheric Chemistry and Physics 3, no 4 (14 juillet 2003) : 1007–21. http://dx.doi.org/10.5194/acp-3-1007-2003.
Texte intégralRésumé :
Abstract. Fingerlike structures reaching from lower into extra-tropical latitudes significantly contribute to the tropical-extratropical exchange of air masses. This is also an exchange of upper tropospheric and stratospheric air. Those so called streamers can, on a horizontal plane, be detected in N2O or O3 since they are characterised by high N2O or low O3 values compared to undisturbed mid-latitude values. A climatology of streamer events has been established, employing the chemical-transport model KASIMA, which is driven by ECMWF re-analyses (ERA) and operational analyses. For the first time, the seasonal and geographical distribution of streamer frequencies has been determined on the basis of 9 years of meteorological analyses. For the current investigation, a meridional gradient criterion has been newly formulated and applied to the N2O distributions calculated with KASIMA. A climatology has been derived by counting all streamer events between 21 and 25 km for the years 1990 to 1998. The results have been compared with a streamer climatology which has been established in the same way employing data of a multi-year simulation with the coupled chemistry-climate model ECHAM4.L39(DLR)/CHEM (E39/C). Both climatologies are qualitatively in agreement, in particular in the northern hemisphere, where much higher streamer frequencies are found in winter than in summer. In the southern hemisphere, the KASIMA analyses indicate strongest streamer activity in September. E39/C streamer frequencies clearly displays an offset from June to October, pointing to model deficiencies with respect to tropospheric dynamics. KASIMA and E39/C results agree well from November to May. Some of the findings give strong indications that the streamer events found in the altitude region between 21 and 25 km are mainly forced from the troposphere and are not directly related to the dynamics of the stratosphere, in particular not to the dynamics of the polar vortex. Sensitivity simulations with E39/C, which represent recent and possible future atmospheric conditions, have been employed to answer the question how climate change would alter streamer frequencies. This shows that the seasonal cycle does not change but that significant changes occur in months of minimum and maximum streamer frequencies. This could have an impact on the mid-latitude distribution of chemical tracers and compounds.
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Fukumori, Ichiro, Tong Lee, Benny Cheng et Dimitris Menemenlis. « The Origin, Pathway, and Destination of Niño-3 Water Estimated by a Simulated Passive Tracer and Its Adjoint ». Journal of Physical Oceanography 34, no 3 (1 mars 2004) : 582–604. http://dx.doi.org/10.1175/2515.1.
Texte intégralRésumé :
Abstract The nature of subtropical–tropical water mass exchange in the Pacific Ocean is investigated, focusing on the origin, pathway, and destination of water occupying the surface layer of the eastern equatorial Pacific Ocean (Niño-3 region; 5°S–5°N, 150°–90°W). Simulated passive tracers and their adjoint are employed to explicitly follow the circulation of specific water masses accounting for advective and diffusive effects and their time variabilities. The evolution of the forward passive tracer and adjoint passive tracer can be identified as describing where the tracer-tagged water mass goes and from where it comes, respectively. Over 10 years on average, water mass of the Niño-3 region can be traced back to eastern subtropical thermocline waters of the Northern (27%) and Southern Hemispheres (39%). The Niño-3 water subsequently returns to these subtropical latitudes in the upper ocean. In contrast to the hypothesized “subtropical cell,” however, this circulation is an open circuit with water returning to the western regions of the two hemispheres (subtropical gyres) and to the Indian Ocean, instead of returning to its origins. The representative transit time scale from the subtropics to the Tropics is 10–15 yr. Temporal variability causes the tropical circulation inferred from a time-mean state to differ significantly from the average circulation. In particular, stirring due to nonseasonal, intra-annual variability significantly enhances the transport magnitude of the so-called interior pathways relative to that of the circuitous low-latitude western boundary pathways. Such short-circuit in the subtropical–tropical exchange may help better to explain tracer distributions, such as the observed midbasin tritium maximum in the equatorial Pacific Ocean. Significant differences in circulation pathways are also identified that are associated with El Niño and La Niña events. The strength of the subtropical–tropical water mass exchange is estimated to have weakened during the 1990s.
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Garny, H., G. E. Bodeker et M. Dameris. « Trends and variability in stratospheric mixing : 1979–2005 ». Atmospheric Chemistry and Physics 7, no 21 (14 novembre 2007) : 5611–24. http://dx.doi.org/10.5194/acp-7-5611-2007.
Texte intégralRésumé :
Abstract. Changes in climate are likely to drive changes in stratospheric mixing with associated implications for changes in transport of ozone from tropical source regions to higher latitudes, transport of water vapour and source gas degradation products from the tropical tropopause layer into the mid-latitude lower stratosphere, and changes in the meridional distribution of long-lived trace gases. To diagnose long-term changes in stratospheric mixing, global monthly fields of Lyapunov exponents were calculated on the 450 K, 550 K, and 650 K isentropic surfaces by applying a trajectory model to wind fields from NCEP/NCAR reanalyses over the period 1979 to 2005. Potential underlying geophysical drivers of trends and variability in these mixing fields were investigated by applying a least squares regression model, which included basis functions for a mean annual cycle, seasonally dependent linear trends, the quasi-biennial oscillation (QBO), the solar cycle, and the El Niño Southern Oscillation (ENSO), to zonal mean time series of the Lyapunov exponents. Long-term positive trends in mixing are apparent over southern middle to high latitudes at 450 K through most of the year, while negative trends over southern high latitudes are apparent at 650 K from May to August. Wintertime negative trends in mixing over northern mid-latitudes are apparent at 550 K and 650 K. Over low latitudes, within 40° of the equator, the QBO exerts a strong influence on mixing at all three analysis levels. This QBO influence is strongly modulated by the annual cycle and shows a phase shift across the subtropical mixing barrier. Solar cycle and ENSO influences on mixing are generally not significant. The diagnosed long-term changes in mixing should aid the interpretation of trends in stratospheric trace gases.
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Garny, H., G. E. Bodeker et M. Dameris. « Trends and variability in stratospheric mixing : 1979–2005 ». Atmospheric Chemistry and Physics Discussions 7, no 3 (10 mai 2007) : 6189–228. http://dx.doi.org/10.5194/acpd-7-6189-2007.
Texte intégralRésumé :
Abstract. Changes in climate are likely to drive changes in stratospheric mixing with associated implications for changes in transport of ozone from tropical source regions to higher latitudes, transport of water vapour and source gas degradation products from the tropical tropopause layer into the mid-latitude lower stratosphere, and changes in the meridional distribution of long-lived trace gases. To diagnose long-term changes in stratospheric mixing, global monthly fields of Lyapunov exponents were calculated on the 450 K, 550 K, and 650 K isentropic surfaces by applying a trajectory model to wind fields from NCEP/NCAR reanalyses over the period 1979 to 2005. Potential underlying geophysical drivers of trends and variability in these mixing fields were investigated by applying a least squares regression model, which included basis functions for a mean annual cycle, seasonally dependent linear trends, the quasi-biennial oscillation (QBO), the solar cycle, and the El Niño Southern Oscillation (ENSO), to zonal mean time series of the Lyapunov exponents. Long-term positive trends in mixing are apparent over southern middle to high latitudes at 450 K through most of the year, while negative trends over southern high latitudes are apparent at 650 K from May to August. Wintertime negative trends in mixing over northern mid-latitudes are apparent at 550 K and 650 K. Over low latitudes, within 40° of the equator, the QBO exerts a strong influence on mixing at all three analysis levels. This QBO influence is strongly modulated by the annual cycle and shows a phase shift across the subtropical mixing barrier. Solar cycle and ENSO influences on mixing are generally not significant. The diagnosed long-term changes in mixing should aid the interpretation of trends in stratospheric trace gases.
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Poulard, Jean-Charles, et Fabian Blanchard. « The impact of climate change on the fish community structure of the eastern continental shelf of the Bay of Biscay ». ICES Journal of Marine Science 62, no 7 (1 janvier 2005) : 1436–43. http://dx.doi.org/10.1016/j.icesjms.2005.04.017.
Texte intégralRésumé :
Abstract Many fish species are at the southern or northern limit of their distribution range in the Bay of Biscay, where large-scale hydroclimatic changes have occurred in recent decades. We attempt here to identify the impact of these changes on the fish community of the eastern continental shelf of the Bay of Biscay. Data collected during 14 autumn groundfish surveys in 1973 and from 1987 to 2002 are used. The study area is between latitudes 48°30′N and 43°30′N while the depth ranges from 15 to 200 m. Annual abundance indices (number of individuals per km2) of 56 fish taxa present on average in at least 5% of the tows are computed. Multivariate analysis is used to detect temporal trends in these species' abundance indices. Assuming that increased water temperature may favour subtropical species and hinder temperate ones, knowledge about the latitudinal distribution range is used to interpret time trends. Results show an increasing abundance trend with time for fish species having a wide distribution range in latitude (mainly subtropical ones), whereas the abundance of temperate and the least widely distributed species decreased steadily.
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Davidson, Noel E., Kevin J. Tory, Michael J. Reeder et Wasyl L. Drosdowsky. « Extratropical–Tropical Interaction during Onset of the Australian Monsoon : Reanalysis Diagnostics and Idealized Dry Simulations ». Journal of the Atmospheric Sciences 64, no 10 (1 octobre 2007) : 3475–98. http://dx.doi.org/10.1175/jas4034.1.
Texte intégralRésumé :
Abstract The onset of the Australian monsoon is examined using (i) reanalysis data for seasons when enhanced observational networks were available and (ii) a 15-yr onset composite. Similar to previous findings, onset is characterized by a sudden strengthening and deepening in tropical westerly winds, which are overlain with upper-tropospheric easterlies. All onsets are preceded by up to a 7-day preconditioning period of enhanced vertical motion and moistening. During the transition season, the 6 weeks prior to onset, a number of moist westerly events occur. Generally they are only sustained for short periods and overlain by upper-level westerly winds, suggesting an association with midlatitude troughs, which protrude into the deep Tropics. For individual years and for a 15-yr composite, monsoon onset is associated with major cyclogenesis events over the southwest Indian Ocean in the presence of a subtropical jet over the eastern Indian Ocean. The proposed mechanism for extratropical–tropical interaction is northeastward Rossby wave propagation from the cyclogenesis region toward the Tropics at upper levels. At these levels, westerly winds extend to nearly 10°S and provide a favorable background flow for such propagation. The process eventually results in the amplification of an equatorward-extending midlatitude upper trough and tropical ridge, which appears to trigger the development of the underlying monsoon trough. To test the hypothesis, the influence of high-latitude cyclogenesis on the tropical circulation is investigated with the aid of an idealized, dry, three-dimensional, baroclinic wave channel model. The initial state consists of (i) a zonally constant baroclinic region centered on 40°S, from which the high-latitude cyclogenesis develops, (ii) a weak monsoon trough at 15°S, and (iii) a subtropical jet at 25°S. The major findings from the simulations are as follows: 1) There is evidence of northeastward Rossby wave propagation from the cyclogenesis region toward low latitudes. 2) Consistent with theoretical studies, the subtropical jet plays a key role by providing a favorable westerly background flow for group propagation into the Tropics. 3) High-latitude cyclogenesis in the presence of a subtropical jet can influence the meridional location, zonal structure, vorticity, and divergence in the monsoon trough. 4) Vorticity and divergence changes are consistent with enhancement of the monsoon trough (increases in low-level cyclonic vorticity) and the potential for triggering a large-scale convective outbreak (changes in upper-level divergence).
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Delgadillo, José Alberto, Gonzalo Fitz-Rodríguez, Gerardo Duarte, Francisco Gerardo Véliz, Evaristo Carrillo, José Alfredo Flores, Jesús Vielma, Horacio Hernandez et Benoît Malpaux. « Management of photoperiod to control caprine reproduction in the subtropics ». Reproduction, Fertility and Development 16, no 4 (2004) : 471. http://dx.doi.org/10.1071/rd04030.
Texte intégralRésumé :
Reproductive seasonality is observed in some breeds originating from or adapted to subtropical latitudes. In ‘photoperiodic flexible breeds’, such as Australian cashmere goats, the annual breeding season can be manipulated through nutrition, whereas in ‘photoperiodic rigid breeds’, such as Creole goats from subtropical Mexico, sexual activity can be controlled by altering the photoperiod. In males from the latter breed, artificial long days, whether or not accompanied by the administration of melatonin, stimulate sexual activity during the non-breeding season. These treated males are able to induce the sexual activity of anoestrous females through the male effect under intensive or extensive conditions. Photoperiodic treatments and the male effect can be easily integrated into different breeding management systems in subtropical latitudes.
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Maher, Penelope, Michael E. Kelleher, Philip G. Sansom et John Methven. « Is the subtropical jet shifting poleward ? » Climate Dynamics 54, no 3-4 (31 décembre 2019) : 1741–59. http://dx.doi.org/10.1007/s00382-019-05084-6.
Texte intégralRésumé :
AbstractThe tropics are expanding poleward at about $$0.5{^\circ }$$0.5∘ per decade in observations. This poleward expansion of the circulation is consistently reported using Hadley cell edge metrics and lower-atmospheric tropical edge metrics. However, some upper-atmospheric tropical metrics report smaller trends that are often not significant. One such upper-atmospheric metric is the subtropical jet latitude, which has smaller trends compared to the Hadley cell edge. In this study we investigate the robustness of the weak trends in the subtropical jet position by introducing a new method for locating the subtropical jet, and examining the trends and variability of the subtropical jet latitude. We introduce the tropopause gradient method based on the peak gradient in potential temperature along the dynamic tropopause. Using this method we find the trends in the subtropical jet latitude are indeed much smaller than $$0.5{^\circ }$$0.5∘ per decade, consistent with previous studies. We also find that natural variability within the subtropical jet latitude would not prevent trends from being detected if they were similar to the Hadley cell edge, as trends greater than 0.24$${^\circ }$$∘ per decade could reliably be detected using monthly data or 0.09$${^\circ }$$∘ per decade using daily data. Despite the poleward expansion of the tropics, there is no robust evidence to suggest the subtropical jet is shifting poleward in either hemisphere. Neither the current diagnostic methods nor natural variability can account for the small subtropical jet trends. The most likely explanation, which requires further investigation, is that the subtropical jet position is not tied dynamically to the Hadley cell edge.
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Li, Muyuan, Yao Yao, Dehai Luo et Linhao Zhong. « The Linkage of the Large-Scale Circulation Pattern to a Long-Lived Heatwave over Mideastern China in 2018 ». Atmosphere 10, no 2 (20 février 2019) : 89. http://dx.doi.org/10.3390/atmos10020089.
Texte intégralRésumé :
In this study, the large-scale circulation patterns (a blocking high, wave trains and the western Pacific subtropical high (WPSH)) associated with a wide ranging and highly intense long-lived heatwave in China during the summer of 2018 are examined using both observational data and reanalysis data. Four hot periods are extracted from the heatwave and these are related to anticyclones (hereafter referred to as heatwave anticyclone) over the hot region. Further analysis shows a relationship between the heatwave anticyclone and a synthesis of low, mid- and high latitude circulation systems. In the mid-high latitudes, a midlatitude wave train and a high latitude wave train are associated with a relay process which maintains the heatwave anticyclone. The midlatitude wave train acts during 16–21 July, whereas the high latitude wave train takes affect during 22–28 July. The transition between the two wave trains leads to the northward movement of the hot region. With the help of a wave flux analysis, it was found that both wave trains originate from the positive North Atlantic Oscillation (NAO+) which acts as an Atlantic wave source. Serving as a circulation background, the blocking situated over the Scandinavia-Ural sector is maintained for 18 days from 14 to 15 August, which is accompanied by the persistent wave trains and the heatwave anticyclone. Additionally, the abnormal northward movement of the WPSH and its combination with the high latitude wave train lead to the occurrence of extreme hot weather in north-eastern China occurring during the summer of 2018.
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Nullet, Dennis, et James O. Juvik. « GENERALISED MOUNTAIN EVAPORATION PROFILES FOR TROPICAL AND SUBTROPICAL LATITUDES ». Singapore Journal of Tropical Geography 15, no 1 (juin 1994) : 17–24. http://dx.doi.org/10.1111/j.1467-9493.1994.tb00242.x.
Texte intégral
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Krüger, K., U. Langematz, J. L. Grenfell et K. Labitzke. « Climatological features of stratospheric streamers in the FUB-CMAM with increased horizontal resolution ». Atmospheric Chemistry and Physics 5, no 2 (21 janvier 2005) : 547–62. http://dx.doi.org/10.5194/acp-5-547-2005.
Texte intégralRésumé :
Abstract. The purpose of this study is to investigate horizontal transport processes in the winter stratosphere using data with a resolution relevant for chemistry and climate modeling. For this reason the Freie Universität Berlin Climate Middle Atmosphere Model (FUB-CMAM) with its model top at 83 km altitude, increased horizontal resolution T42 and the semi-Lagrangian transport scheme for advecting passive tracers is used. A new approach of this paper is the classification of specific transport phenomena within the stratosphere into tropical-subtropical streamers (e.g. Offermann et al., 1999) and polar vortex extrusions hereafter called polar vortex streamers. To investigate the role played by these large-scale structures on the inter-annual and seasonal variability of transport processes in northern mid-latitudes, the global occurrence of such streamers was calculated based on a 10-year model climatology, concentrating on the existence of the Arctic polar vortex. For the identification and counting of streamers, the new method of zonal anomaly was chosen. The analysis of the months October-May yielded a maximum occurrence of tropical-subtropical streamers during Arctic winter and spring in the middle and upper stratosphere. Synoptic maps revealed highest intensities in the subtropics over East Asia with a secondary maximum over the Atlantic in the northern hemisphere. Furthermore, tropical-subtropical streamers exhibited a higher occurrence than polar vortex streamers, indicating that the subtropical barrier is more permeable than the polar vortex barrier (edge) in the model, which is in good correspondence with observations (e.g. Plumb, 2002; Neu et al., 2003). Interesting for the total ozone decrease in mid-latitudes is the consideration of the lower stratosphere for tropical-subtropical streamers and the stratosphere above ~20 km altitude for polar vortex streamers, where strongest ozone depletion is observed at polar latitudes (WMO, 2003). In the lower stratosphere the FUB-CMAM simulated a climatological maximum of 10% occurrence of tropical-subtropical streamers over East-Asia/West Pacific and the Atlantic during early- and mid-winter. The results of this paper demonstrate that stratospheric streamers e.g. large-scale, tongue-like structures transporting tropical-subtropical and polar vortex air masses into mid-latitudes occur frequently during Arctic winter. They can therefore play a significant role on the strength and variability of the observed total ozone decrease at mid-latitudes and should not be neglected in future climate change studies.
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Piva, Everson D., Manoel A. Gan et V. Brahmananda Rao. « An Objective Study of 500-hPa Moving Troughs in the Southern Hemisphere ». Monthly Weather Review 136, no 6 (1 juin 2008) : 2186–200. http://dx.doi.org/10.1175/2007mwr2135.1.
Texte intégralRésumé :
Abstract The location and movement of 500-hPa troughs using an automated method are studied with data from a 24-yr period with the objective of determining the trough formation and dissipation regions in the Southern Hemisphere. To identify the 500-hPa mobile troughs, an objective method that uses the Eulerian centripetal acceleration (ECA) is developed. On average, 868 troughs per year were identified by the method, with an increase in trend during the period studied. The troughs have an average lifetime of 4.3 days, being longer (shorter) in subtropical (high) latitudes. The average calculated phase velocity was 13.6 m s−1, being higher (lower) in middle (high) latitudes. The troughs are normally found in the 60°–40°S latitudinal band, with maximum occurrence at 50°S. The longitudinal distribution of trough genesis has three maximum regions: over the Drake Strait and the South Atlantic Ocean, over the Indian Ocean around 50°S, and over the southwestern Pacific Ocean between 150°E and 150°W. The trough dissipation regions are less concentrated than the genesis regions and also show three maxima: over the west of the Andes, south of the African continent, and south of Australia. The seasonal variation in the trough dissipation shows that the 30°–40°S band is more active during winter than in summer. The difference between the genesis and dissipation location is that formation occurs more in middle and high latitudes, while dissipation is more common in the 40°–30°S latitude belt.
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Zhang, Xuebin, Bruce Cornuelle et Dean Roemmich. « Sensitivity of Western Boundary Transport at the Mean North Equatorial Current Bifurcation Latitude to Wind Forcing ». Journal of Physical Oceanography 42, no 11 (1 novembre 2012) : 2056–72. http://dx.doi.org/10.1175/jpo-d-11-0229.1.
Texte intégralRésumé :
Abstract The bifurcation of the North Equatorial Current (NEC) plays an important role in the heat and water mass exchanges between the tropical and subtropical gyres in the Pacific Ocean. The variability of western boundary transport (WBT) east of the Philippine coast at the mean NEC bifurcation latitude (12°N) is examined here. A tropical Pacific regional model is set up based on the Massachusetts Institute of Technology general circulation model and its adjoint, which calculates the sensitivities of a defined meridional transport to atmospheric forcing fields and ocean state going backward in time. The adjoint-derived sensitivity of the WBT at the mean NEC bifurcation latitude to surface wind stress is dominated by curl-like patterns that are located farther eastward and southward with increasing time lag. The temporal evolution of the adjoint sensitivity of the WBT to wind stress resembles wind-forced Rossby wave dynamics but propagating with speeds determined by the background stratification and current, suggesting that wind-forced Rossby waves are the underlying mechanism. Interannual-to-decadal variations of the WBT can be hindcast well by multiplying the adjoint sensitivity and the time-lagged wind stress over the whole model domain and summing over time lags. The analysis agrees with previous findings that surface wind stress (especially zonal wind stress in the western subtropical Pacific) largely determines the WBT east of the Philippines, and with a time lag based on Rossby wave propagation. This adjoint sensitivity study quantifies the contribution of wind stress at all latitudes and longitudes and provides a novel perspective to understand the relationship between the WBT and wind forcing over the Pacific Ocean.
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Chen, Shun-Ying, Ching-Te Chien, Siti N. Hidayati et Jeffrey L. Walck. « Taiwanese montaneSambucus chinensisseeds require warm stratification, contrasting with other congeneric temperate members ». Seed Science Research 24, no 3 (22 mai 2014) : 217–28. http://dx.doi.org/10.1017/s0960258514000130.
Texte intégralRésumé :
AbstractMany temperate plant genera, likeSambucus, have species with range disjunctions among North America, Europe and/or Asia. Cold stratification (sometimes in combination with warm stratification) is the primary mechanism to break seed dormancy in these species. For some of these genera showing Northern Hemispheric disjunctions, members also occur in subtropical or tropical regions, mostly confined to higher elevations where climate and vegetation differ from those in northern latitudes. We made two hypotheses concerning germination for the subtropical TaiwaneseSambucus chinensis: (1) seeds from populations exposed to warm temperatures would require warm stratification, and (2) seeds from populations exposed to cold temperatures need cold stratification. We investigated the germination (including embryo growth) of non-stratified seeds over a range of temperatures and tested the effects of cold stratification and of gibberellins GA3and GA4on germination. The amount and timing of germination among populations varied substantially in response to temperature treatments. Seeds from all populations of this species required warm temperatures for dormancy break and germination, regardless of environmental conditions. As such, the majority of seeds had non-deep simple morphophysiological dormancy, which, until now, has not been reported in any members ofSambucus. The seed characteristics of the subtropicalS. chinensisare different from those of temperate members of the genus in which cold stratification is the predominate treatment to overcome dormancy.
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Pinto-Coelho, Ricardo, Bernadette Pinel-Alloul, Ginette Méthot et Karl E. Havens. « Crustacean zooplankton in lakes and reservoirs of temperate and tropical regions : variation with trophic status ». Canadian Journal of Fisheries and Aquatic Sciences 62, no 2 (1 février 2005) : 348–61. http://dx.doi.org/10.1139/f04-178.
Texte intégralRésumé :
The influence of trophic status on the crustacean zooplankton community was investigated in lakes and reservoirs in temperate and subtropical-tropical regions. We tested if there is a consistent relationship between crustacean species richness, assemblages, and abundance and trophic indices such as total phosphorus and chlorophyll a. We also examined if these patterns differ between regions. Cumulative species richness and assemblages varied among regions. The greatest number of crustacean species was found in the temperate oligotrophic region with the largest number of lakes sampled. However, cumulative species richness was similar in temperate and subtropicaltropical regions when comparing subsets with a similar number of lakes and reservoirs. The relationships between species richness and latitude or trophic status were difficult to assess owing to imbalance among regions in number of lakes and reservoirs sampled and to biogeography and fish predation potential influences. Trophic status was associated with changes in abundance of all major crustacean zooplankton groups. Eutrophic ecosystems supported greater crustacean abundances at all latitudes. However, cladocerans and cyclopoids were more abundant in eutrophic lakes and reservoirs, whereas calanoids were more abundant in temperate oligotrophic lakes. Total phosphorus was found to be a better predictor of the biomass of major crustacean groups than chlorophyll a in all regions.
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Wheeler, Elisabeth A., et Pieter Baas. « Wood evolution : Baileyan trends and Functional traits in the fossil record ». IAWA Journal 40, no 3 (8 janvier 2019) : 488–529. http://dx.doi.org/10.1163/22941932-40190230.
Texte intégralRésumé :
ABSTRACTWe revisited questions about changes in the incidences of functional wood anatomical traits through geologic time and compared the incidences of these traits in the fossil record with modern wood anatomical diversity patterns in order to test classical (“Baileyan”) and more recent ecophyletic hypotheses of xylem evolution. We contrast patterns through time for tropical and higher (paleo)latitudes. Data are from the InsideWood database. There are striking differences between woods from high and mid latitudes versus tropical (paleo)-latitudes. At temperate and subtropical latitudes (Laurasia and high latitude Gondwana), the epoch by epoch time series supports the Baileyan transformation series of vessel-bearing woody angiosperms (basal woody angiosperms and eudicots): “primitive” features such as scalariform perforations, exclusively solitary vessels, apotracheal diffuse parenchyma and heterocellular rays abound in the Cretaceous and become much less frequent in the Cenozoic, especially post-Eocene. In contrast, in the paleotropics hardly any changes occur in the incidences – each epoch has an equally “modern” spectrum of wood anatomical attributes. Although climatic gradients from the poles to the equator were less steep in the Cretaceous than in the late Cenozoic, the wood anatomical differences between mid-high latitude woods and tropical woods were much more pronounced in the Cretaceous than in later epochs. This seeming paradox is discussed but we cannot resolve it.We suggest that tropical conditions have accelerated xylem evolution towards greater hydraulic efficiency (simple perforations), biological defense and hydraulic repair (elaborate paratracheal parenchyma patterns) as evidenced by late Cretaceous tropical latitude woods having near modern incidences of almost all traits. At higher paleolatitudes of both the Northern and Southern Hemisphere “ancestral” features such as scalariform perforations were retained in cooler and frost-prone regions where they were not selected against in mesic habitats because of lower demands on conductive efficiency, and could even be advantageous in trapping freeze-thaw embolisms. The fossil wood record remains too incomplete for testing hypotheses on the wood anatomy of the earliest angiosperms. The low incidence of so-called “xerophobic” woods sensu Feild with scalariform perforations with numerous (over 40) closely spaced bars in the Cretaceous tropical fossil record is puzzling. At higher paleolatitudes such woods are common in the Cretaceous.Ring porosity, an indicator of seasonal climates and deciduousness, occurs at low levels in the Cretaceous and Paleogene at higher paleolatitudes only, and reaches modern levels in the Miocene. In Neogene and Recent temperate Northern Hemisphere, wide vessels are virtually restricted to ring-porous woods. In the tropics, there is a low incidence of ring porosity throughout all epochs.The fossil record indicates that ecophysiological adaptation to tropical or temperate conditions was already evident in the Cretaceous with considerable latitudinal differences.
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Fu, Qiang, et Pu Lin. « Poleward Shift of Subtropical Jets Inferred from Satellite-Observed Lower-Stratospheric Temperatures ». Journal of Climate 24, no 21 (1 novembre 2011) : 5597–603. http://dx.doi.org/10.1175/jcli-d-11-00027.1.
Texte intégralRésumé :
Abstract One pronounced feature in observed latitudinal dependence of lower-stratospheric temperature trends is the enhanced cooling near 30° latitude in both hemispheres. The observed phenomenon has not, to date, been explained in the literature. This study shows that the enhanced cooling is a direct response of the lower-stratospheric temperature to the poleward shift of subtropical jets. Furthermore, this enhanced lower-stratospheric cooling can be used to quantify the poleward shift of subtropical jets. Using the lower-stratospheric temperatures observed by satellite-borne microwave sounding units, it is shown that the subtropical jets have shifted poleward by 0.6° ± 0.1° and 1.0° ± 0.3° latitude in the Southern and Northern Hemispheres, respectively, in last 30 years since 1979, indicating a widening of tropical belt by 1.6° ± 0.4° latitude.
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Pereira, Heloisa Ramos, Michelle Simões Reboita et Tércio Ambrizzi. « Características da Atmosfera na Primavera Austral Durante o El Niño de 2015/2016 ». Revista Brasileira de Meteorologia 32, no 2 (juin 2017) : 293–310. http://dx.doi.org/10.1590/0102-77863220011.
Texte intégralRésumé :
Resumo O El Niño (EN) é caracterizado pelo aquecimento anômalo das águas superficiais do oceano Pacífico Tropical central e leste que afeta o clima global e regional através das anomalias da circulação da atmosfera. O EN 2015/2016 foi classificado como um evento muito forte. Portanto, esse estudo tem como objetivo mostrar as características da atmosfera durante a ocorrência desse evento nos meses de setembro, outubro e novembro de 2015. Entre os resultados, têm-se o deslocamento para leste da célula de circulação zonal de Walker, intensificação dos jatos subtropical e polar, presença de anomalias positivas de altura geopotencial no oceano Pacífico próximas ao sul da América do Sul e anomalias negativas no sudeste desse continente e no oceano Atlântico. Além disso, há maior frequência de ciclones nas latitudes subtropicais do oceano Atlântico e convergência do fluxo de umidade integrado na vertical sobre o sudeste da América do Sul. Essas características contribuem para anomalias de precipitação e temperatura do ar na América do Sul.
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Krüger, K., U. Langematz, J. L. Grenfell et K. Labitzke. « Climatological features of stratospheric streamers in the FUB-CMAM with increased horizontal resolution ». Atmospheric Chemistry and Physics Discussions 4, no 5 (22 octobre 2004) : 6789–822. http://dx.doi.org/10.5194/acpd-4-6789-2004.
Texte intégralRésumé :
Abstract. The purpose of this study is to investigate horizontal transport processes in the winter stratosphere using data with a high spatial and temporal resolution. For this reason the Freie Universität Berlin Climate Middle Atmosphere Model (FUB-CMAM) with its model top at 83 km altitude, increased horizontal resolution T42 and the semi-Lagrangian transport scheme for advecting passive tracers is used. A new result of this paper is the classification of specific transport phenomena within the stratosphere into tropical-subtropical streamer (e.g. Offermann et al., 1999) and polar vortex extrusions hereafter called polar vortex streamers. To investigate the role played by these large-scale structures on the inter-annual and seasonal variability of the observed negative ozone trend in northern mid-latitudes, the global occurrence of such streamers were calculated based on a 10-year model climatology, concentrating on the existence of the Arctic polar vortex. For the identification and counting of streamers, the new method of zonal anomaly was chosen, which in comparison to other methods produced the best result in this study. The analysis of the months October–May yielded a maximum occurrence of tropical-subtropical streamers during Arctic winter and spring in the middle and upper stratosphere. Synoptic maps revealed highest intensities in the subtropics over East Asia with a secondary maximum over the Atlantic in the northern hemisphere. Furthermore, tropical-subtropical streamers exhibited about a four times higher occurrence than polar vortex streamers, indicating that the subtropical barrier is more permeable than the polar vortex barrier (edge) in the model, which is in good correspondence with observations (e.g. Plumb, 2002; Neu et al., 2003). Interesting for the total ozone loss in mid-latitudes is the consideration of the lower stratosphere, where strongest ozone depletion is observed at polar latitudes (WMO, 2003). In this particular region the FUB-CMAM simulated a climatological maximum of 10% occurrence of tropical-subtropical streamers over East-Asia/West Pacific and the Atlantic during early- and mid-winter. The results of this paper demonstrate that the regular occurrence of stratospheric streamers e.g., large-scale mixing processes of tropical-subtropical and polar vortex air masses into mid-latitudes, could play a significant role on the strength and variability of the observed total ozone decrease at mid-latitudes and should not be neglected in future climate change studies.
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Winters, Andrew C., Daniel Keyser et Lance F. Bosart. « Composite Vertical-Motion Patterns near North American Polar–Subtropical Jet Superposition Events ». Monthly Weather Review 148, no 11 (novembre 2020) : 4565–85. http://dx.doi.org/10.1175/mwr-d-20-0140.1.
Texte intégralRésumé :
AbstractA polar–subtropical jet superposition is preceded by the development of a polar cyclonic potential vorticity (PV) anomaly at high latitudes and a tropical anticyclonic PV anomaly at subtropical latitudes. A confluent large-scale flow pattern can lead to the juxtaposition of these respective PV anomalies at middle latitudes, resulting in the addition of the nondivergent circulations induced by each PV anomaly and an increase in upper-tropospheric wind speeds at the location of jet superposition. Once these PV anomalies become juxtaposed, vertical motion within the near-jet environment facilitates the advection and diabatic redistribution of tropopause-level PV, and the subsequent formation of the steep, single-step tropopause structure that characterizes a jet superposition. Given the importance of vertical motion during the formation of jet superpositions, this study adopts a quasigeostrophic (QG) diagnostic approach to quantify the production of vertical motion during three types of jet superposition events: polar dominant, eastern subtropical dominant, and western subtropical dominant. The diagnosis reveals that the geostrophic wind induced by polar cyclonic QGPV anomalies is predominantly responsible for QG vertical motion in the vicinity of jet superpositions. The QG vertical motion diagnosed from the along-isotherm component of the Q vector, which represents the vertical motion associated with synoptic-scale waves, is dominant within the near-jet environment. The QG vertical motion diagnosed from the across-isotherm component of the Q vector, which represents the vertical motion associated with frontal circulations in the vicinity of the jet, is subordinate within the near-jet environment, but is relatively more important during eastern subtropical dominant events compared to polar dominant and western subtropical dominant events.
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Hoskins, B. J., et K. I. Hodges. « A New Perspective on Southern Hemisphere Storm Tracks ». Journal of Climate 18, no 20 (15 octobre 2005) : 4108–29. http://dx.doi.org/10.1175/jcli3570.1.
Texte intégralRésumé :
Abstract A detailed view of Southern Hemisphere storm tracks is obtained based on the application of filtered variance and modern feature-tracking techniques to a wide range of 45-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) data. It has been checked that the conclusions drawn in this study are valid even if data from only the satellite era are used. The emphasis of the paper is on the winter season, but results for the four seasons are also discussed. Both upper- and lower-tropospheric fields are used. The tracking analysis focuses on systems that last longer than 2 days and are mobile (move more than 1000 km). Many of the results support previous ideas about the storm tracks, but some new insights are also obtained. In the summer there is a rather circular, strong, deep high-latitude storm track. In winter the high-latitude storm track is more asymmetric with a spiral from the Atlantic and Indian Oceans in toward Antarctica and a subtropical jet–related lower-latitude storm track over the Pacific, again tending to spiral poleward. At all times of the year, maximum storm activity in the higher-latitude storm track is in the Atlantic and Indian Ocean regions. In the winter upper troposphere, the relative importance of, and interplay between, the subtropical and subpolar storm tracks is discussed. The genesis, lysis, and growth rate of lower-tropospheric winter cyclones together lead to a vivid picture of their behavior that is summarized as a set of overlapping plates, each composed of cyclone life cycles. Systems in each plate appear to feed the genesis in the next plate through downstream development in the upper-troposphere spiral storm track. In the lee of the Andes in South America, there is cyclogenesis associated with the subtropical jet and also, poleward of this, cyclogenesis largely associated with system decay on the upslope and regeneration on the downslope. The genesis and lysis of cyclones and anticyclones have a definite spatial relationship with each other and with the Andes. At 500 hPa, their relative longitudinal positions are consistent with vortex-stretching ideas for simple flow over a large-scale mountain. Cyclonic systems near Antarctica have generally spiraled in from lower latitudes. However, cyclogenesis associated with mobile cyclones occurs around the Antarctic coast with an interesting genesis maximum over the sea ice near 150°E. The South Pacific storm track emerges clearly from the tracking as a coherent deep feature spiraling from Australia to southern South America. A feature of the summer season is the genesis of eastward-moving cyclonic systems near the tropic of Capricorn off Brazil, in the central Pacific and, to a lesser extent, off Madagascar, followed by movement along the southwest flanks of the subtropical anticyclones and contribution to the “convergence zone” cloud bands seen in these regions.