Droplet evaporation is ubiquitous to multitude of applications such as microfluidics, surface patterning and ink-jet printing. In many of the process like food processing tiny concentrations of suspended particles may alter the behavior of an evaporating droplet remarkably, leading to partially viscous and partially elastic dynamical characteristics. This, in turn, may lead to some striking mechanical instabilities, such as buckling and rupture. In this thesis, we provide a comprehensive physical description of the vaporization, self-assembly, agglomeration and buckling kinetics of sessile nanofluid droplet pinned on a hydrophobic substrate in various configurations. We have deciphered five distinct regimes of droplet lifecycle. Regime I-III consists of evaporation induced preferential agglomeration that leads to the formation of unique dome shaped inhomogeneous shell with stratified varying density liquid core. Regime IV involves capillary pressure initiated shell buckling and stress induced shell rupture. Regime V marks rupture induced cavity inception and growth. We provide a regime map explaining the droplet morphology and buckling characteristics for droplets evaporating on various substrates. Specifically, we find that final droplet volume and radius of curvature at buckling onset are universal functions of particle concentration. Furthermore, flow characteristics inside the heated and unheated droplets are investigated and found to be driven by the buoyancy effects. Velocity magnitudes are observed to increase by an order at higher temperatures with self-similar flow profiles. With an increase in the surface temperature, droplets exhibit buckling from multiple sites over a larger sector in the top half of the droplet. In addition, irrespective of the initial nanoparticle concentration and substrate temperature, hydrophobicity and roughness, growth of daughter cavity (subsequent to buckling) inside the droplet is found to be controlled by the solvent evaporation rate from the droplet periphery. The results are of great significance to a plethora of applications like DNA deposition and nanofabrication. In the next part of the thesis, we deploy the droplet in a rectangular channel. The rich physics governing the universality in the underlying dynamics remains grossly elusive. Here, we bring out hitherto unexplored universal features of the evaporation dynamics of a sessile droplet entrapped in a 3D confined fluidic environment. Increment in channel length delays the completion of the evaporation process and leads to unique spatio-temporal evaporation flux and internal flow. We show, through extensive set of experiments and theoretical formulations, that the evaporationtimescale for such a droplet can be represented by a unique function of the initial conditions. Moreover, using same theoretical considerations, we are able to trace and universally merge the volume evolution history of the droplets along with evaporation lifetimes, irrespective of the extent of confinement. These results are explained in the light of increase in vapor concentration inside the channel due to greater accumulation of water vapor on account of increased channel length. We have formulated a theoretical framework which introduces two key parameters namely an enhanced concentration of the vapor field in the vicinity of the confined droplet and a corresponding accumulation lengthscale over which the accumulated vapor relaxes to the ambient concentration. Lastly, we report the effect of confinement on particle agglomeration and buckling dynamics. Compared to unconfined scenario, we report non-intuitive suppression of rupturing beyond a critical confinement. We attribute this to confinement-induced dramatic alteration in the evaporating flux, leading to distinctive spatio-temporal characteristics of the internal flow leading to preferential particle transport and subsequent morphological transitions. We present a regime map quantifying buckling & non-buckling pathways. These results may turn out to be of profound importance towards achieving desired morphological features of a colloidal droplet, by aptly tuning the confinement space, initial particle concentration, as well as the initial droplet volume. These findings may have implications in designing functionalized droplet evaporation devices for emerging engineering and biomedical applications.

Les courants turbulents de bord ouest sont l'un des phénomènes les plus dominants des océans, il en existe aux faibles latitudes aussi. Ils sont caractérisés par une dynamique très turbulente avec une forte production d'énergie cinétique, et une forte variabilité interne. Plusieurs régions existent où les courants de bord ouest se rétrofléchissent (décollage de la côte) pour former des structures cohérentes: des anticyclones, des bursts (arrachements) et des dipoles. Circulant le long de la côte, les courants de bord ouest interagissent très fortement avec le bord ouest et la bathymétrie et sont donc un problème de couche limite. Cependant aucune étude du point de vue de la théorie de couche limite n'a été jamais été faite. Cette thèse aborde le problème d'un point de vue de couche limite par l'utilisation d'un modèle idéalisé "shallow water" à très haute résolution (2.5km) afin d'isoler et de comprendre les processus. Les résultats sont ensuite appliqués à des sorties de modèle réaliste Drakkar (~10km) basé sur le code NEMO. Le courant de Somali est ensuite pris pour cette application. / Strong western boundary currents are one of dominant features of the world oceans, also at low latitudes. They exhibit a turbulent dynamics and their region is a source of strong kinetic energy production and internal variability of the worlds oceans. Several places exists where the western boundary currents retrofect (i.e separation from the coast) and generate coherent structures as anticyclonic eddies, bursts and dipoles. The dynamics of turbulent western boundary currents has so far not been extensively studied in the viewpoint of turbulent boundary-layer theory. The approach followed in this thesis is to use a fine resolution (2.5km) reduced-gravity shallow water model to understand the turbulent boundary-layer processes and then apply these findings to the Ocean General Circulation Model NEMO in the Drakkar configuration (~10km). The case of the Somali Currentis considered for this application.

O objetivo da presente tese é o de documentar, em escala milenar, as variações paleoclimáticas e paleoceanográficas superficiais do Atlântico Sul Ocidental desde o Último Máximo Glacial. Com esta finalidade, foram obtidas estimativas de paleotemperatura, paleossalinidade e paleoprodutividade baseadas na análise de fauna de foraminíferos planctônicos, isótopos estáveis de oxigênio e carbono em Globigerinoides ruber e nove datações de radiocarbono em amostras de um testemunho coletado na Bacia de Santos. Os dados foram complementados com a reanálise de outro testemunho previamente coletado em outro setor da mesma Bacia. Os resultados demonstram que as mudanças paleoceanográficas registradas no período analisado podem ser parcialmente explicadas por variações na exportação de calor e sal para o hemisfério norte relacionadas à atividade da célula de transporte meridional e à configuração das correntes superficiais associadas à circulação atmosférica. Outra parte significativa da variabilidade parece estar relacionada à intensidade do Sistema de Monção da América do Sul resultante de variações na insolação de acordo com o ciclo de precessão. Os principais desvios da tendência de variação da composição isotópica da água do mar esperada em função das mudanças de insolação coincidem com os grandes pulsos de degelo ocorridos em torno de 19, 14 e 8,2 mil anos AP. / The goal of this thesis is to document the paleoclimatic and paleoceanographic changes occurred at the millennial scale since the Last Glacial Maximum at the surface Western South Atlantic. Paleotemperature, paleosalinity and paleoproductivity estimates were obtained for this purpose based on faunal changes of foraminifera assemblages, carbon and oxygen stable isotopes in Globigerinoides ruber and nine radiocarbon datings in a core retrieved at Santos Basin. These data were supplemented by the reanalysis of a second core previously analyzed from a different sector of the same basin. The results demonstrate that paleoceanographic changes recorded in the analyzed period may be partially explained by changes in the heat and salt export to the northern hemisphere related to the meridional overturning cell and the surface currents related to atmospheric circulation. Another significant portion of the observed variability seems to be related to changes in the strength of the South American Monsoon System resulting from insolation changes according to the precessional cycle. The main departures from the expected trend of variation in the isotopic composition of seawater related to insolation changes coincide with great meltwater pulses occurred around 19, 14, and 8.2 kyr BP.

This thesis examines the structure of the Tropical Easterly Jet (TEJ) in a General Circulation Model (GCM). The TEJ is observed only during the Indian summer monsoon period and is strongest during July and August. The jet structure simulated by an atmospheric GCM (CAM-3.1) in July has been compared with reanalysis data. The simulated TEJ was displaced westward by ~ 25◦ when compared to observations. The removal of orography had no impact on the jet structure. This demonstrated that the Tibetan Plateau did not play an important role in the location and structure of the jet. The changes in cumulus scheme in the GCM had a large influence on the location of the jet maxima. To examine the factors which control the location and structure of the jet, a series of experiments were conducted using an aqua-planet version of the model. The impact of different sea surface temperature (SST) profiles was studied. The rainfall in the GCM was primarily in the regions where the SST attained a maximum. By altering the location of SST maximum (and hence the rainfall maximum), the impact of location of rainfall maximum on the location and structure of the jet was studied. When the rainfall maximum was located close to the equator, it did not generate a strong jet but had an influence on the vertical structure of the jet. A large number of simulations were conducted with multiple rainfall maxima and the need for these was demonstrated since only then was the observed jet structure well simulated. Based on the simulations, it was concluded that the simulation of the TEJ by CAM-3.1 was unrealistic because of large unrealistic rainfall over Saudi Arabia in this GCM. Equatorial heating has been shown to be important to simulate proper jet structure. The zonal structure of the jet was also influenced by rainfall in the Pacific Ocean. Although the aqua-planet configuration of the CAM-3.1 GCM provided several useful insights, the simulation was not perfect on account of errors in the simulation of the temperature profile in the lower troposphere. An ideal-physics configuration of the GCM was used. This removed the cumulus physics and instead imposed the observed heating pro-files. Both upper tropospheric friction and radiative-convective atmospheric temperatures were required to simulate the TEJ. The problems with the simulation of structure in the jet exit region was corrected by using radiative-convective atmospheric temperatures that were qualitatively similar to those observed in northern hemisphere summer time. The ideal-physics configuration reconfirmed that the Saudi Arabian rainfall was responsible for the westward shift of the TEJ in the simulations. The ideal-physics simulations showed that the simple analytical model proposed by Gillin1980 was not suitable for the simulation of TEJ. The above the simulations indicate that a shift in the location of the jet is related to a shift in the rainfall pattern. Based on this insight one would expect that the jet location will be different in good and bad monsoon periods. This is indeed the case. In July 2002 the Indian monsoon failed after beginning well in June. In June the TEJ is consequently located west ward compared to July. The same situation prevails even in good and poor monsoon years. In a good monsoon year (July 1988) the jet maximum is located westward when compared to a bad monsoon year (July 2002). In this thesis we have clearly demonstrated the role of anomalous rainfall on the location of the TEJ. This thesis has shown that an accurate simulation of the TEJ depends upon the accurate simulation of various rainfall centers that act as multiple heat sources in the atmosphere. The rainfall in the equatorial region does not influence the strength of the TEJ but alters the vertical structure of the jet. The strength the jet is dependent on the intensity of rainfall and the latitudinal distance from the equator. The complex vertical structure of the jet is not simulated by simple analytical models of the jet.

O Sistema de Monção Sul-americana (SMSA) e a Zona de Convergência Intertropical (ZCIT) são dois dos mais importantes sistemas de circulação que afetam o clima da América do Sul (AS). Enquanto houve um grande progresso no número de registros paleoclimáticos relacionados com a precipitação do SMSA nos Andes tropicais e nas porções sudeste, centroleste e nordeste do Brasil, ainda pouco se sabe sobre mudanças de paleo-pluviosidade na região do Centro-Oeste do Brasil. Na presente tese, foram estudados novos registros de paleo-precipitação baseados em mais de 5000 análises isotópicas de oxigênio e carbono em estalagmites coletadas em cavernas da região Centro-Oeste que cobrem os últimos ~33 mil anos, cuja cronologia se baseia em aproximadamente 200 análises U/Th. Esse estudo mostrou que a região do Mato Grosso do Sul foi mais úmida durante o Último Máximo Glacial (LGM - do inglês Last Glacial Maximum) em relação ao Holoceno médio. A correspondência desse cenário paleoclimático com o que foi documentado em outras áreas da AS indica a presença de um corredor de umidade durante o período do LGM que se estende desde o oeste amazônico até o sudeste do Brasil. Os eventos milenares caracterizados por oscilações na temperatura das regiões das altas latitudes do globo claramente afetaram a precipitação na região. O registro isotópico mostra que a região esteve mais úmida durante os eventos frios do Hemisfério Norte (Younger Dryas e Heinrich events) e mais seca durante os eventos quentes (Bolling-Allerod e Dansgaard-Oeschger). Essa relação teve como provável causa o fortalecimento/enfraquecimento do SMSA em função do posicionamento da ZCIT em resposta ao gradiente térmico inter-hemisférico. Durante o evento Heinrich 1 a região teve a presença de uma proeminente fase seca entre duas úmidas, esse fato foi reportado em outras regiões do Brasil e não parece estar conectado com as condições de temperatura das altas latitudes. A partir dos dados de alta resolução temporal das estalagmites coletadas no estado do Mato Grosso, foi realizado o estudo da variabilidade climática associada com atividade do SMSA ao longo nos últimos milênios que inclui os períodos de anomalias climáticas da Pequena Idade do Gelo e da Anomalia Climática Medieval, caracterizados na região como úmido e seco, respectivamente. Com o uso de técnicas de análises espectrais foi identificada uma persistente periodicidade de aproximadamente 210 anos na variabilidade do SMSA, a qual foi associada a influência da variabilidade solar no clima Em complemento, foi realizado um monitoramento isotópico e ambiental durante 4 anos nos sistemas cársticos onde as estalagmites foram coletadas. A comparação entre o \'delta\'\'POT.18\'O da água da chuva com a quantidade de precipitação e temperatura do ar evidencia o efeito quantidade (amount effect) como o principal modulador da variação isotópica na atmosfera. A relação isotópica entre o carbonato e a água meteórica, associados as condições do microclima da caverna, indica que o sinal isotópico da água da chuva foi preservado de forma suavizada nos espeleotemas, o que suporta o uso dos isótopos de oxigênio como indicador paleoambiental nas estalagmites dessas cavernas. / The South American Monsoon System (SAMS) and the Inter-Tropical Convergence Zone (ITCZ) are the two most important circulation systems affecting climate over tropical South America (SA). While an increasing number of paleoclimatic archives related to SAMS precipitation have recently been published, most of these paleoclimate reconstructions are located along the eastern slope of the tropical Andes or in southeastern SA. In central SA, most proxy records consist of pollen-based vegetation reconstructions, but these show significant disagreements when compared to precipitation records further to the east and west. Here we present a new paleo-rainfall record based in more than 5.000 oxygen and carbon isotope samples from speleothems collected on two study sites from mid-west Brazil (states of Mato Grosso and Mato Grosso do Sul), chronologically constrained by almost 200 U/Th ages that cover the last ~33.000 years BP. This study shows wet conditions in the regions of Mato Grosso do Sul during the Last Glacial Maximum (LGM) in relationship to mid-Holocene. The comparison of our new record with others paleoclimate records from SA indicates a wet corridor during the LGM over west Amazon and southeast Brazil. The millennial events documented in the high latitudes of the northern hemisphere were evident in paleoprecipitation of Mato Grosso do Sul, the isotopic record shows wet conditions for the region during could period of the northern hemisphere (Yonger Dryas and Heinrich events) and dry conditions during warm periods (Bolling-Allerod and Dansgaard-Oeschger). This was due to strengthening/weakening of SAMS, which is ITCZ position dependent. During the event Heinrich 1, our record shown a dry phase between two wet intervals. This was also documented in central-east of Brazil and seems decoupled of high latitude climate conditions. For stalagmites collected in Mato Grosso state, we focus on the climate variability over the last two millennia using high resolution sampling (approximately 1 years sample spacing). This new record shows abrupt fluctuations in rainfall tied to variations in the intensity of SAMS, including the periods corresponding to the Little Ice Age (LIA) and the Medieval Climate Anomaly (MCA). Using spectral analyses, we show that changes in SASM activity is linked to solar variability with a distinct periodicity of 210 years. We also show our isotope and environmental monitoring over 4 years at the cave site, in order to support our paleoclimatic interpretation of the isotopic profiles obtained from the stalagmites. The systematic measurement of the oxygen isotopic composition of rainfall, drip water and associated fresh calcite at the caves indicates that the stalagmites from this region can be used as a proxy of the SAMS activity. The effect of seasonal temperature variability in the atmosphere is suppressed by the \"amount effect\" on the \'delta\'\'POT.18\'O values of rainfall. Inside the cave, the microclimate variability has a small effect on calcite \'delta\'\'POT.18\'O, not being significant over long time scales.

Utilizando registros isotópicos (\'delta\' \'POT.18\'O e \'delta\'\'POT.13\'C) de alta resolução, razões de elementos-traço em relação ao cálcio e taxas de crescimento de espeleotemas, precisamente datados pelo método U-Th, foi feita uma reconstrução da intensidade do Sistema de Monções Sul-Americano (SMSA) nos últimos 1500 anos na região do município de Bonito, Mato Grosso do Sul. O estudo do sinal climático, interpretado por meio de variações geoquímicas dos espeleotemas, foi realizado com base em estudos e comparações com dados instr umentais de uma estação meteorológica de Bonito, com dados de monitoramentos do IAEA - GNIP de Campo Grande e Cuiabá e com informações de modelos computacionais que simulam a variação de \'delta\'\'POT.18\'O em relação à intensidade das monções na América do Sul, o qual e stá relacionado com variação regional de pluviosidade. Apesar destes dados mostrarem que o amount effect é um dos fatores determinantes na variação das razões isotópicas dos espeleotemas, outros fatores como o degree of rainout upstream e a fonte de umidade (Amazônia x oceano Atlântico sul) também devem ser considerados importantes. Assim, a interpretação das razões de \'delta\'\'POT.18\'O é atribuída à intensidade do sistema de monções e, desta forma, à atuação da Zona de Convergência do Atlântico Sul (ZACS), principalmente associada à propagação de chuvas da região Amazônica para o centro-oeste e sudeste brasileiro. O registro paleoclimático de Bonito mostrou excelente coerência com variados tipos de registros de outros locais da América do Sul, e também com dados do Hemisfério Norte, evidenciando a influência de eventos de escala secular como MCA e LIA na intensidade das monções na América do Sul. A boa correlação entre algumas razões de elementos-traço com a precipitação mostra que o \"amount effect\" não controla totalmente os registros isotópicos. Análises de estatísticas de séries temporais aplicadas aos registros isotópicos e aos registros de elementos-traço permitiram a observação de vários eventos de freqüêcia decadal a multidecadal influenciando as monções da América do Sul. Os principais destes eventos são a NAO (North Atlantic Oscillation) e a AMO (Atlantic Multidecadal Oscillation), os quais, pelas flutuações de TSM no Atlântico Norte, desencadeiam teleconexões que têm resultados no posicionamento da Zona de Convergência Intertropical (Intertropical Convergence Zone - ITCZ) e da ZACS. / High-resolution isotope records (\'delta\'\'POT.18\'O e \'delta\'\'POT.13\'C), growth rates, elemental ratios in precisely dated speleothems by U/Th method are used to reconstruct the intensity of the South American Monsoon System (SAMS) during the 1500 years in Bonito Region, Mato Grosso do Sul. The interpretations of geochemical variations in speleothems are based on comparative studies with instrumental climatic data and at isotope measurements in rain water at IAEA - GNIP stations in Campo Grande and Cuiabá, as well as from climate model studies using oxygen isotope ratios as tracers of monsoon rainfall intensity. Despite of these data suggest that the \"amount effect\" is a dominant factor controlling the isotope variations in preci pitation, the \"degree of rainout upstream\" is considered also important because the distal influence of SAMS and South Atlantic Convergence Zone (SACZ) along the moisture transport tra jectory from Amazon to SE Brazilian Coast. The other significant factor to isotope composition to precipitation, therefore to speleothems, is the difference in moisture source area from Amazon Region (summer monsoonal precipitation) and Atlantic Ocean (extratropical regime). Bonito\'s paleoclimate Record indicate very good coherence with other records in South America and Northern Hemisphere, particularly in the response to the global scale climate events such as Medieval Climate Anomaly (MCA) e Little Ice Age (LIA) on summer precipitation. In these events were evidenced dryer and wetter conditions, respectively, not just from the stable isotope ratios but also from Sr/Ca and Ba/Ca ratios. Time series statistical analysis in the isotope and trace -element records showed that the SAMS is largely influenced by decadal to multidecadal events. In this regard, the most influent climate modes are the NAO (North Atlantic Oscillation) and the AMO (Atlantic multi Decadal Oscillation), with significant influence on the precipitation variability. In both cases these changes in precipitation are resulted from teleconection with Atlantic Ocean, which impacts the location of ITCZ and consequently the SACZ activity in Central-West Brazil.

Reconstruction studies of seasonal rainfall utilizing stable isotope based proxy approach suffer from the limitations of time resolutions. Conventional methods and archives limit the achievable resolution to annual scales. However, high resolution reconstruction (seasonal to sub-weekly scale) can be achieved in proxy records where growth rates are high enough to leave spatial signatures in an organically or inorganically deposited layer such as growth bands. In this study, aragonitic skeleton of the gastropod Lissachatina fulica (Bowdich, Giant African Land Snails) is investigated with an aim to achieve sub-weekly scale reconstruction of the Indian monsoon rainfall. These terrestrial gastropods are native of Africa and highly invasive. Their evolution in the geological time period dates back to the Pliocene and is presently distributed across the tropical belt. They exhibit a high growth rate in the presence of water and high relative humidity in the environment. As a result, they are ideally suited for the task of palaeo seasonality reconstruction. The isotopic patterns recorded in their growth bands reveal composition of environmental water at seasonal time scales. In vitro studies were carried out on L. fulica to estimate their growth rates and growth responses to changes in the physical conditions within the culture chamber. The Indian monsoon rainfall exhibits characteristic dry spells that are generally sandwiched between periods of active phases of high rainfall during the South West monsoon season. These dry spells are typically characterized by rainfall with low intensity. Isotope fingerprinting of the rain water at daily time resolution, covering the years of 2007-10 exhibited distinct isotopic ratios for the dry and wet spells. Dry spells were clearly demarcated in the record with isotopically enriched signature. In addition, the study indentified the role of three distinct moisture sources on δ18O of rain water at Bangalore, India. The variability in the oxygen isotopic composition of the Indian monsoon rainfall is predominantly controlled by this source moisture variability at inter annual time scales, while temperature and amount of rainfall tend to dominate the variability in the precipitation isotopes at seasonal and weekly scales. Simultaneous isotopic analyses of both rainwater and shell carbonates growth bands were undertaken to understand their relationship to aid in high resolution reconstruction. Carbonate found in the growth bands of the gastropods, which is precipitated under equilibrium condition from rainwater, preserves the signature of rainfall. This provides an opportunity to reconstruct rainfall parameters (i.e. amount and moisture sources) knowing the variability in shell carbonates. Stable isotopic ratios measured across the growth bands of live shell specimens collected from the southern and eastern Indian regions (Bangalore and Kolkata, respectively) were compared with the rainfall isotope ratios at these two locations; signature of dry spells were clearly identified from the study of isotopic composition in the growth bands of the gastropod specimens. The approach was also extended to older samples from historical archives from eastern Indian region (Kolkata, East India). Individual specimens belonging to the same species of gastropod, which were collected during the monsoon season of the year 1918 were used for reconstructing the seasonal pattern in monsoon rainfall over the region. The record of variation in the isotopic composition seen in the shell was compared with the rainfall data from Indian Metrological Division observatory at Kolkata station. The year 1918 was characterized as a major drought year and the signature of dry period was seen preserved in the specimen. The work under taken in this thesis will widen the scope of seasonality reconstruction using terrestrial shell fossils from palaeo records, which have been rarely investigated in paleoclimate studies from the perspective of understanding the seasonal precipitation variability.

Reconstruction studies of seasonal rainfall utilizing stable isotope based proxy approach suffer from the limitations of time resolutions. Conventional methods and archives limit the achievable resolution to annual scales. However, high resolution reconstruction (seasonal to sub-weekly scale) can be achieved in proxy records where growth rates are high enough to leave spatial signatures in an organically or inorganically deposited layer such as growth bands. In this study, aragonitic skeleton of the gastropod Lissachatina fulica (Bowdich, Giant African Land Snails) is investigated with an aim to achieve sub-weekly scale reconstruction of the Indian monsoon rainfall. These terrestrial gastropods are native of Africa and highly invasive. Their evolution in the geological time period dates back to the Pliocene and is presently distributed across the tropical belt. They exhibit a high growth rate in the presence of water and high relative humidity in the environment. As a result, they are ideally suited for the task of palaeo seasonality reconstruction. The isotopic patterns recorded in their growth bands reveal composition of environmental water at seasonal time scales. In vitro studies were carried out on L. fulica to estimate their growth rates and growth responses to changes in the physical conditions within the culture chamber. The Indian monsoon rainfall exhibits characteristic dry spells that are generally sandwiched between periods of active phases of high rainfall during the South West monsoon season. These dry spells are typically characterized by rainfall with low intensity. Isotope fingerprinting of the rain water at daily time resolution, covering the years of 2007-10 exhibited distinct isotopic ratios for the dry and wet spells. Dry spells were clearly demarcated in the record with isotopically enriched signature. In addition, the study indentified the role of three distinct moisture sources on δ18O of rain water at Bangalore, India. The variability in the oxygen isotopic composition of the Indian monsoon rainfall is predominantly controlled by this source moisture variability at inter annual time scales, while temperature and amount of rainfall tend to dominate the variability in the precipitation isotopes at seasonal and weekly scales. Simultaneous isotopic analyses of both rainwater and shell carbonates growth bands were undertaken to understand their relationship to aid in high resolution reconstruction. Carbonate found in the growth bands of the gastropods, which is precipitated under equilibrium condition from rainwater, preserves the signature of rainfall. This provides an opportunity to reconstruct rainfall parameters (i.e. amount and moisture sources) knowing the variability in shell carbonates. Stable isotopic ratios measured across the growth bands of live shell specimens collected from the southern and eastern Indian regions (Bangalore and Kolkata, respectively) were compared with the rainfall isotope ratios at these two locations; signature of dry spells were clearly identified from the study of isotopic composition in the growth bands of the gastropod specimens. The approach was also extended to older samples from historical archives from eastern Indian region (Kolkata, East India). Individual specimens belonging to the same species of gastropod, which were collected during the monsoon season of the year 1918 were used for reconstructing the seasonal pattern in monsoon rainfall over the region. The record of variation in the isotopic composition seen in the shell was compared with the rainfall data from Indian Metrological Division observatory at Kolkata station. The year 1918 was characterized as a major drought year and the signature of dry period was seen preserved in the specimen. The work under taken in this thesis will widen the scope of seasonality reconstruction using terrestrial shell fossils from palaeo records, which have been rarely investigated in paleoclimate studies from the perspective of understanding the seasonal precipitation variability.

In this thesis, we have presented a systematic analysis of the change of cloud properties due to variation in aerosol concentration over Indian region using satellite observations, and Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) simulations. The Tropical Rainfall Measurement Mission (TRMM) based Microwave Imager (TMI) estimates (2A12) have been used to compare and contrast the characteristics of cloud liquid water and ice over the Indian land region and the surrounding oceans, during the pre-monsoon (May) and monsoon (June–September) seasons. Based on the spatial homogeneity of rainfall, we have selected five regions for our study (three over ocean, two over land). In general, we find that the mean cloud liquid water and cloud ice content of land and oceanic regions are different, with the ocean regions showing higher amount of CLW. A comparison across the ocean regions suggests that the cloud liquid water over the or graphically influenced Arabian Sea (close to the Indian west coast) behaves differently from the cloud liquid water over a trapped ocean (Bay of Bengal) or an open ocean (Equatorial Indian Ocean). Specifically, the Arabian Sea region shows higher liquid water for a lower range of rainfall, whereas the Bay of Bengal and the Equatorial Indian Ocean show higher liquid water for a higher range of rainfall. Apart from geographic differences, we also documented seasonal differences by comparing cloud liquid water profiles between monsoon and pre-monsoon periods, as well as between early and peak phases of the monsoon. We find that the cloud liquid water during the lean periods of rainfall (May or June) is higher than during the peak and late monsoon season (July-September) for raining clouds over central India. However, this is not true over the ocean. As active and break phases are important signatures of the monsoon progression, we also analyzed the differences in cloud liquid water during various phases of the monsoon, namely, active, break, active-to-break (a2b) and break-to-active (b2a) transition phases. We find that the cloud liquid water content during the b2a transition phase is significantly higher than that during the a2b transition phase over central India. We speculate that this could be attributed to higher amount of aerosol loading over this region during the break phase. We lend credence to this aerosol-liquid water/rain association by comparing the central Indian cloud liquid water with Southeast Asia (where the aerosol loading is significantly smaller) and find that in the latter region, there are no significant differences in cloud liquid water during the different phases of their monsoon. The second part of our study involves evaluating the ability of the Weather Research and Forecasting Model coupled with Chemistry (WRF-Chem) to simulate the observed variation of cloud liquid water and rain efficiency. We have used no chemistry option, and the model was run with constant aerosol concentration. The model simulations (at 4.5 km resolution) are done for the month of June–July 2004 since this period was particularly favorable for the study of an active–break cycle of the monsoon. We first evaluate the sensitivity of the model to different parameterizations (microphysical, boundary layer, land surface) on the simulation of rain over central India and Bay of Bengal. This is done to identify an “optimal” combination of parameterizations which reproduces the best correlation with observed rain over these regions. In this default configuration (control run), where the aerosol concentration is kept constant throughout the simulation period, the model is not able to reproduce the observed variations of cloud liquid water during the different phases of an active-break cycle. To this end, we proceeded to modify the model by developing an aerosol-rain relation, using Aerosol Robotic Network (AERONET) and TRMM 3B42 data that realistically captures the variation of aerosol with rain. It is worth highlighting here that our goal was to primarily isolate the indirect effect of aerosols in determining the observed changes in cloud liquid water (CLW) during the active-break phases of the Indian monsoon, without getting into the complexity of a full chemistry model such as that incorporated in WRF-Chem. Moreover, the proposed modification (modified run) is necessitated by the lack of realistic emission estimates over the Indian region as well as the presence of inherent biases in monsoon simulation in WRF. The main differences we find between the modified and control simulations is in the mean as well as spatial variability of CLW. We find that the proposed modification (i.e., rate of change of aerosol concentration as a function of rain rate) leads to a realistic variation in the CLW during the active-break cycle of Indian monsoon. Specifically, the peak value of CLW in the b2a (a2b) phase is larger (smaller) in the modified as compared to the control run. These results indicate a stronger change in CLW amount in the upper levels between the two transition phases in the modified scheme as compared to the control simulation. More significantly, we also observe a change in sign at the lower levels of the atmosphere, i.e., from a strong positive difference in the control run to a negative difference in the modified simulation, similar to that observed. Additionally, we investigated the impact of the proposed modification, via CLW changes, on cloud coverage, size of clouds and their spatial variability. We find that the transformation of optically thin clouds to thick clouds during the break phase was associated with larger cloud size in modified compared to the control simulation. Moreover, the higher rate of decay of the spatial variability of CLW with grid resolution, using the modified scheme, suggests that clusters of larger clouds are more in the modified compared to control simulation. Taken together, the interactive aerosol loading proposed in this thesis yields model simulations that better mimic the observed CLW variability between the transition phases.

Le but de cette thèse est de reconstruire l’évolution environnementale de la région nord de la mer de Chine du Sud au cours du Quaternaire récent. L'objectif est d'identifier des traceurs des interactions continent-océan, de la mousson est-asiatique et de la circulation océanique. Pour cela, des analyses des minéralogies argileuse et magnétique, de la composition en éléments majeurs, et la taille des grains ont été effectuées sur les sédiments de carotte MD12-3432 couvrant les derniers 400 ka avec un taux de sédimentation variant entre 4.5 et 24 cm/ka. Nous avons tout d'abord amélioré les calibrations de la composition en éléments majeurs obtenue par XRF core-scanning. En effet, celles-ci ne corrigeaient pas des variations de teneur en eau interstitielle observées la séquence sédimentaire étudiée. Nous avons donc proposé une correction polynomiale quadratique qui convertit maintenant correctement les données XRF core-scanning en concentrations précises d’éléments majeurs. La composition en élément majeurs, les minéralogies argileuse et magnétique, et la granulométrie indiquent que les changements climatiques à basse latitude influencent les diverses fractions terrigènes de différentes manières. Considérant d’abord les éléments majeurs et les argiles, nous observons que les rapports smectite/(illite+chlorite) et K₂O/Al₂O₃ présentent tous deux des cyclicités orbitales. Le rapport K₂O/Al₂O₃ qui reflète la contribution relative des apports de Taiwan par dénudation, augmente pendant les interglaciaires quand la mousson d’été asiatique est forte. Le rapport smectite/(illite+chlorite) présente des cyclicités également liées à l’excentricité et à la précession en phase avec les changements d’insolation de l’hémisphère nord en été. Connaissant les régions sources de ces argiles, nous suggérons que l’apport en smectite est étroitement lié à l'intensité de l’altération chimique et à la dénudation fluvial induite par les fortes précipitations de mousson, tandis que l’apport d’illite/chlorite depuis Taiwan répond principalement à la dénudation. Le rapport smectite/(illite+chlorite) reflète ainsi principalement l'intensité de l’altération chimique contemporaine (rapide) et donc l’intensité de la mousson d'été asiatique. Les résultats obtenus confirment que la mousson d'été asiatique est renforcée pendant les périodes interglaciaires et quand l'insolation d'été boréal est forte. Les propriétés magnétiques sédimentaires offrent des informations complémentaires sur les changements environnementaux passés dans cette région. La fraction magnétique de la carotte MD12-3432 est composée de magnétites, sulfures de fer et hématite. Des augmentations de teneur en hématite et des diminutions de la granulométrie sédimentaire sont observées aux minima de précession. Ces évènements qui ont lieu pendant les périodes arides pourraient illustrer des apports éoliens depuis le nord de la Chine, liés à des changements d'intensité et/ou de route des vents, probablement aussi liés à l’intensification de la mousson d'hiver. Outre les changements climatiques de basse latitude, le climat global et l'activité tectonique influencent aussi les apports terrigènes dans cette région. Les augmentations à long terme de la teneur en pyrrhotite par rapport à la magnétite et à l’hématite, de la teneur de illite/chlorite indiquent une contribution croissante de sédiment fine d'origine Taiwanaise au cours des derniers 400 ka. Ceci est très probablement lié à l’intensification de l'orogenèse taïwanais. A l'échelle glaciaire-interglaciaire, les variations synchrones des compositions argileuse et magnétique, de la taille des grains sédimentaires et magnétiques, et des taux de sédimentation sont attribuées aux changements de niveau marin. En effet, les bas niveaux marins pendant les périodes glaciaires exposent l’immense plateau continental et permettent à la Rivière des Perles de livrer au site du sédiment terrigène en plus grande quantité et plus grossier. / The aim of this study is to reconstruct late Quaternary environmental changes in the northern South China Sea by applying multi-disciplinary proxies of land-sea interaction, East Asian monsoon, and oceanic circulation. Investigations of clay and magnetic mineralogy, major element composition, and grain size were performed on marine sediment Core MD12-3432 retrieved from the continental slope of the northern South China Sea. The core covers the last 400 ka with a sedimentation rate varying between 4.5 and 24 cm/ka. We examined the accuracy of existing calibration methods on major element composition obtained by XRF core-scanning, because downcore variations in interstitial water content should strongly affect scanned element contents. We proposed a quadratic polynomial correction to account for this effect and implemented it in the calibration methods. Data from Core MD12-3432 show that the improved calibration process now correctly converts XRF core-scanning data into major element concentrations. Our results on high-resolution major element composition, clay and magnetic mineralogy, and grain size data indicate that low-latitude climate changes influence various terrigenous fractions in different ways. In bulk sediment, K₂O/Al₂O₃ ratio mainly reflects the relative contribution of detrital supply from Taiwan, and its variation exhibits eccentricity cycles. The ratio increases during interglacials, showing that strong precipitation and denudation are induced by enhanced East Asian summer monsoon. In clay fractions, smectite/(illite+chlorite) ratio presents both eccentricity and precession periodicities, in phase with the northern hemisphere summer insolation changes and therefore with the East Asian summer monsoon evolution. Based on the knowledge of sediment provenances, these results suggest that high smectite/(illite+chlorite) ratios illustrate contemporaneous chemical weathering intensity in Luzon. It supports the understanding that East Asian summer monsoon is enhanced during interglacial periods and when the boreal summer insolation is strong. Therefore, these two ratios are appropriate sedimentary tracers for East Asian summer monsoon evolution in the South China Sea. The magnetic fraction yields complementary information about environmental changes in the South China Sea. The magnetic mineral assemblage of Core MD12-3432 is mainly composed of magnetite, sulphide, and hematite, and the relative contributions of all these magnetic mineral contents change with time. Variations on precessional band related to the low latitude East Asian monsoon are observed in magnetic properties and grain size values. High magnetic inputs with high hematite proportion, which is part of fine-grain sediment, are observed during the precession minima. These events occurring during arid periods may illustrate enhanced eolian inputs caused by changes in intensity and/or winds pathway of winds, probably related to enhanced winter monsoon. Besides the low-latitude climate changes, global climate and tectonic activity also influence the terrigenous composition at the studied site at different timescales. A long-term increase in pyrrhotite content with respect to magnetite and hematite and in illite/chlorite contents indicates an increasing contribution of fine grained sediments from Taiwan. This is most likely related to the intensification of Taiwanese orogeny over the last 400 ka. On glacial-interglacial scale, coeval increases observed in sedimentation rate, magnetite/pyrrhotite content, kaolinite content, and grain size during glacial periods are attributed to sea-level changes. Low sea-level during glacial periods exposes the vast shelf and allows the Pearl River to deliver more and coarser terrigenous sediments to our site.