Neste estudo investigou-se a variabilidade da Monção Sul-Americana (MSA) ao longo dos últimos 38ka, por meio de um registro em alta resolução de \'\'delta\' POT.18\'O baseado em três espeleotemas da caverna Shatuca, localizada no norte do Peru (~ 5ºS). O registro da caverna Shatuca é um dos primeiros registros paleoclimáticos da zona de altitude intermediária no flanco oriental dos Andes setentrionais (1960m). O registro isotópico da Shatuca compreende espeleotemas bem datados e de alta resolução que são usados para investigar a atividade da MSA no passado, em resposta tanto ao ciclo de precessão da insolação, como às mudanças na circulação oceânica, ocorridas durante o último período Glacial - Deglacial, as quais são definidas nos testemunhos marinhos e de gelo do Hemisfério Norte. Os registros de espeleotemas da caverna Shatuca, não mostram nenhum controle claro da insolação sobre a MSA nos Andes entre 38-11 ka AP, o que pode ser explicado por um controle predominante das condições de contorno glaciais sobre a MSA. Mudanças abruptas, entre períodos mais úmidos e mais secos da MSA, em escalas de tempo milenar, são observadas no registro de espeleotemas de Shatuca através de valores de \'\'delta\' POT.18\'O anormalmente baixos e altos, respectivamente. Estes eventos são interpretados como uma resposta aos eventos Heinrich (H) e Dansgaard-Oeschger (D-O) através de deslocamentos latitudinais da Zona de Convergência Intertropical (Intertropical Convergence Zone-ITCZ). No entanto, a intensidade da resposta a esses ciclos foi variável. Em particular, os episódios climáticos mais extremos foram aqueles relacionados aos eventos Heinrich 1 e 2. O período de ocorrência e a estrutura do evento Heinrich 1 (H1) são mais precisamente descritos nos espeleotemas da caverna Shatuca do que em registros anteriores dos Andes e da Bacia de Cariaco. O evento H1 é caracterizado por valores isotópicos baixos entre 18.0 e 14.7 ka AP, o que indica condições predominantemente úmidas; mas um pico, nunca antes registrado, de valores de \'\'delta\' POT.18\'O altos foi registrado em 16.2 ka AP. Este resultado é particularmente importante dado que a ITCZ poderia ter estado deslocada mais ao sul do que 5ºS. Além disso, a estrutura dos períodos do Bølling-Allerød (B/A) e Younger Dryas (YD) assemelha-se à dos testemunhos de gelo da Groenlândia. Durante o Holoceno, o clima da região da caverna Shatuca foi controlado pela insolação, consistente com outros registros de isótopos de diferentes altitudes nos Andes peruanos. O Holoceno Inferior é marcado pelo severo enfraquecimento da MSA na região da Shatuca, sendo seguido por uma tendência de aumento gradual das condições de umidade em direção ao Holoceno Superior, esta tendência climática, em longo prazo, ocorreu em união à tendência de aumento da insolação modulada pelo ciclo de precessão. Condições particularmente úmidas foram sentidas na região da caverna Shatuca após 5.0 ka AP. Várias mudanças abruptas ocorridas, em escalas de tempo centenárias e multidecenais, durante o Holoceno, são descritas pela primeira vez nos Andes. Durante o Holoceno Inferior, o caso mais extremo, é o registrado em 9.5 ka AP, mas outros eventos úmidos ocorreram também, tais como o registrado em 8.1 ka AP. Por outro lado, durante o Holoceno Médio, a comparação com outros registros andinos, na região afetada pela MSA, aponta para uma série de eventos abruptos que ocorreram entre 5.1 e 5.0 ka AP. Finalmente, um resultado importante do presente estudo é a semelhança observada, durante o Holoceno Superior, entre o registro da caverna Shatuca com o do lago Pallcacocha, situado no sul dos Andes equatorianos e amplamente utilizado como um proxy da frequência do fenômeno El Niño Oscilação Sul (El Niño Southern Oscillation -ENSO). O registro Shatuca não apresenta nenhuma evidência clara de ter sofrido algum controle climático influenciado por ENSO. Pelo contrário, propõe-se que ambos registros, o lago Pallcacocha e a caverna Shatuca, indicam um aumento da umidade entre 3.5 e 2.5 ka AP, resultado do controle da alta insolação de verão austral sobre a MSA, e de uma profunda reorganização do sistema climático ocorrido na borda oeste da MSA, entre terras altas e intermediárias dos Andes. / this study, we investigated the South American Summer Monsoon (SASM) variability through the last 38 ky with a high-resolution \'\'delta\' POT.18\'O record based on three speleothems from Shatuca cave, located in northern Peru (~ 5ºS). The Shatuca cave record is one of the first paleoclimate records from mid-altitude (1960m) sites in the northeastern Andean slopes. The Shatuca isotope record comprises well-dated and high-resolution speleothems that were used to investigate the past activity of SASM, in response to both insolation precession cycle and changes in oceanic circulation during the last Glacial-Deglacial period, defined in ice cores and marine core records from the northern Hemisphere. The speleothem records from Shatuca cave show no clear insolation control over the SASM between 38-11 ky BP, which could be explained by a prevailing control of the glacial boundary conditions over SASM. Abrupt millennial shifts between wetter and drier monsoon phases are observed in Shatuca speleothem record based on abnormally low and high values of \'\'delta\' POT.18\'O, respectively. These events are interpreted as a response to Heinrich (H) and Dansgaard-Oeschger (D-O) events through latitudinal displacements in the Intertropical Convergence Zone (ITCZ). However, the response intensity to these events was variable. In particular, the most extreme climate episodes were those related to the Heinrich events 1 and 2. The structure and timing of the Heinrich event 1 (H1) event are more precisely described in Shatuca speleothems than in previous records from Andes and Cariaco Basin. The H1 event is characterized by low ?18O values from 18.0 to 14.7 ky BP, indicative of predominantly wet conditions; but a peak, never reported before, of high \'\'delta\' POT.18\'O values is recorded at 16.2 ky BP. This result is of particular importance given that the ITCZ was probably displaced even more to the south than 5ºS. In addition, the structure of the Bølling-Allerød (B/A) and Younger Dryas (YD) periods resembles that of the Greenland ice cores. Insolation control on climate at Shatuca site is evident during the Holocene, which is consistent with other Andean isotope records from different altitudes in the Peruvian Andes. The early Holocene is marked by a extremely weak SASM activity over Shatuca area, that is followed by a gradual increasing trend toward wetter conditions at the late Holocene period, this long term climate trend occurred in union with increasing insolation trend modulated by the precession cycle. Particularly wet conditions were felt in Shatuca site after 5.0 ky BP. During the Holocene, several abrupt multidecadal to centennial events are for the first time described in Andes. During the early Holocene, the most extreme event is the one logged at 9.5 ky BP, however other wet events occurred, such as the one logged at 8.1 ky BP. On the other side, during the mid Holocene, the comparison with other Andean records affected by the SASM, points out to a striking series of events that occurred between 5.1 and 5.0 ky BP. Finally, one important result from the present study is the similarity observed during the late Holocene between Shatuca cave and the Pallcacocha lake record in southern Equadorian Andes, a record that has been widely used as a proxy for El Niño Southern Oscillation (ENSO) frequency during Holocene. Shatuca record presents no clear evidence for climate control by ENSO. On the contrary, it is proposed that the increase in moisture logged between 3.5 and 2.5 ky BP, in both Pallcacocha lake and Shatuca cave records, resulted from high austral insolation control over the SASM and a major reorganization of the climatic system in the western border of the SASM at mid- to high altitudes of the Andes.

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

The thesis is concerned with development of efficient regional frequency analysis (RFA) approaches to estimate quantiles of hydrometeorological events. The estimates are necessary for various applications in water resources engineering. The classical approach to estimate quantiles involves fitting frequency distribution to at-site data. However, this approach cannot be used when data at target site are inadequate or unavailable to compute parameters of the frequency distribution. This impediment can be overcome through RFA, in which sites having similar attributes are identified to form a region, and information is pooled from all the sites in the region to estimate the quantiles at target site. The thesis proposes new approaches to RFA of precipitation, meteorological droughts and floods, and demonstrates their effectiveness. The approach proposed for RFA of precipitation overcomes shortcomings of conventional approaches with regard to delineation and validation of homogeneous precipitation regions, and estimation of precipitation quantiles in ungauged and data sparse areas. For the first time in literature, distinction is made between attributes/variables useful to form homogeneous rainfall regions and to validate the regions. Another important issue is that some of the attributes considered for regionalization vary dynamically with time. In conventional approaches, there is no provision to consider dynamic aspects of time varying attributes. This may lead to delineation of ineffective regions. To address this issue, a dynamic fuzzy clustering model (DFCM) is developed. The results obtained from application to Indian summer monsoon and annual rainfall indicated that RFA based on DFCM is more effective than that based on hard and fuzzy clustering models in arriving at rainfall quantile estimates. Errors in quantile estimates for the hard, fuzzy and dynamic fuzzy models based on the proposed approach are shown to be significantly less than those computed for Indian summer monsoon rainfall regions delineated in three previous studies. Overall, RFA based on DFCM and large scale atmospheric variables appeared promising. The performance of DFCM is followed by that of fuzzy and hard clustering models. Next, a new approach is proposed for RFA of meteorological droughts. It is suggested that homogeneous precipitation regions have to be delineated before proceeding to develop drought severity - areal extent - frequency (SAF) curves. Drought SAF curves are constructed at annual and summer monsoon time scales for each of the homogeneous rainfall regions that are newly delineated in India based on the proposed approach. They find use in assessing spatial characteristics and frequency of meteorological droughts. It overcomes shortcomings associated with classical approaches that construct SAF curves for political (e.g., state, country) and physiographic regions (e.g., river basin), based on spatial patterns of at-site values of drought indices in the study area, without testing homogeneity in rainfall. Advantage of the new approach can be noted especially in areas that have significant variations in temporal and spatial distribution of precipitation (possibly due to variations in topography, landscape and climate). The DFCM is extended to RFA of floods, and its effectiveness in prediction of flood quantiles is demonstrated by application to Godavari basin in India, considering precipitation as time varying attribute. Six new homogeneous regions are formed in Godavari basin and errors in quantile estimates based on those regions are shown to be significantly less than those computed based on sub-zones delineated in Godavari basin by Central Water Commission in a previous study.

Indian summer monsoon (ISM) is an important component of the tropical climate system, known for its regular seasonality and abundance of rainfall over the country. The droughts and floods associated with the year-to-year variation of the average seasonal rainfall have devastating effect on people, agriculture and economy of this region. The demand for prediction of seasonal monsoon rainfall, therefore, is overwhelming. A number of attempts to predict the seasonal mean monsoon have been made over a century, but neither dynamical nor empirical models provide skillful forecasts of the extremes of the monsoon such as the unprecedented drought of 2002. This study investigates the problems and prospects of extended range monsoon prediction. An evaluation of the potential predictability of the ISM with the aid of an ensemble of Atmospheric General Circulation Model (AGCM) simulations indicates that the interannual variability (IAV) of ISM is contributed equally by the slow boundary forcing (‘externally’ forced variability) and the inherent climate noise (‘internal’ variability) in the atmosphere. Success in predicting the ISM would depend on our ability to extract the predictable signal from a background of noise of comparable amplitude. This would be possible only if the ‘external’ variability is separable from the ‘internal’ variability. A serious effort has been made to understand and isolate the sea surface temperature (SST) forced component of ISM variability that is not strongly influenced by the ‘internal’ variability. In addition, we have investigated to unravel the mechanism of generation of ‘internal’ IAV so that the method of isolating it from forced variability may be found. Since the primary forcing mechanism of the monsoon is the large-scale meridional gradient of deep tropospheric heat sources, large-scale changes in tropospheric temperature (TT) due to the boundary forcing can induce interannual variations of the timing and duration of the monsoon season. The concept of interannually varying monsoon season is introduced here, with the onset and withdrawal of monsoon definitions based on the reversal of meridional gradient of TT between north and south. This large scale definition of the monsoon season is representative of the planetary scale influence of the El Ni˜no Southern Oscillation (ENSO) on monsoon through the modification of TT and the cross equatorial pressure gradient over the ISM region. A sig- nificant relationship between ENSO and monsoon, that has remained steady over the decades, is discovered by which an El Ni˜no (La Ni˜na) delays (advances) the onset, advances (delays) the withdrawal and suppresses (enhances) the strength of the monsoon. The integral effect of the meridional gradient of TT from the onset to withdrawal proves to be a useful index of seasonal monsoon which isolates the boundary forced signal from the influence of internal variations that has remained steady even in the recent decades. However, consistent with the estimates of potential predictability, the boundary forced variability isolated with the above definitions explains only about 50% of the total interannual variability of ISM. Detailed diagnostics of the onset and withdrawal processes are performed to understand how the ENSO forcing modifies the onset and withdrawal, and thus the seasonal mean monsoon. It is found that during an El Ni˜no, the onset is delayed due to the enhanced adiabatic subsidence that inhibits vertical mixing of sensible heating from the warm landmass during pre-monsoon months, and the withdrawal is advanced due to the horizontal advective cooling. This link between ENSO and monsoon is realized through the advective processes associated with the stationary waves in the upper troposphere set up by the tropical ENSO heating. The remaining 50% of the monsoon IAV is governed by internal processes. To unravel the mechanism of the generation of internal IAV, we perform another set of AGCM simulations, forced with climatological monthly mean SSTs, to extract the pure internal IAV. We find that the spatial structure of the intraseasonal oscillations (ISOs) in these simulations has significant projection on the spatial structure of the seasonal mean and a common spatial mode governs both intraseasonal and interannual variability. Statistical average of ISO anomalies over the season (seasonal ISO bias) strengthens or weakens the seasonal mean. It is shown that interannual anomalies of seasonal mean are closely related to the seasonal mean of intraseasonal anomalies and explain about 50% of the IAV of the seasonal mean. The seasonal mean ISO bias arises partly due to the broadband nature of the ISO spectrum, allowing the intraseasonal time series to be aperiodic over the season and partly due to a non-linear process where the amplitude of ISO activity is proportional to the seasonal bias of ISO anomalies. The later relationship is a manifestation of the binomial character of the rainfall time series. The remaining part of IAV may arise due to the complex land-surface processes, scale interactions, etc. We also find that the ISOs over the ISM region are not significantly modulated by the Pacific and Indian Ocean SST variations. Thus, even with a perfect prediction of SST, only about 50% of the observed IAV of ISM could be predicted with the best model in forced mode. Even so, prediction of all India rainfall (AIR) representing the average conditions of the whole country and the season may not always serve the purposes of monsoon forecasting. One reason is the large inhomogeneities in the rainfall distribution during a normal seasonal monsoon. Agriculture and hydrological sector could benefit more if provided with regional scale forecasts of active/break spells 2-3 weeks ahead. Therefore, we advocate an alternative strategy to the seasonal prediction. Here, we present a method to estimate the potential predictability of active and break conditions from daily rainfall and circulation from observations for the recent 24 years. We discover that transitions from break to active conditions are much more chaotic than those from active to break, a fundamental property of the monsoon ISOs. The potential predictability limit of monsoon breaks (∼20 days) is significantly higher than that of the active conditions (∼10 days). An empirical real- time forecasting strategy to predict the sub-seasonal variations of monsoon up to 4 pentads (20 days) in advance is developed. The method is physically based, with the consideration that the large-scale spatial patterns and slow evolution of monsoon intraseasonal variations possess some similarity in their evolutions from one event to the other. This analog method is applied on NOAA outgoing longwave radiation (OLR) pentad mean data which is available on a near real time basis. The elimination of high frequency variability and the use of spatial and temporal analogs produces high and useful skill of predictions over the central and northern Indian region for a lead-time of 4-5 pentads. An important feature of this method is that, unlike other empirical methods to forecast monsoon ISOs, this uses minimal time filtering to avoid any possible end-point effects, and hence it has immense potential for real-time applications.

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.

In this thesis, we investigated modes of intraseasonal variability (ISV) observed in the Indian monsoon rainfall and how these modes modulate rainfall over India. We identified a decreasing trend in the intensity of low-frequency intraseasonal mode with increasing strength in synoptic variability over India. We also made an attempt to understand the reason for these observed trends using numerical simulations. In the first part of the thesis, satellite rainfall estimates are used to understand the spatiotem-poral structures of convection in the intraseasonal timescale and their intensity during boreal sum-mer over south Asia. Two dominant modes of variability with periodicities of 10–20-days (high-frequency) and 20–60-days (low-frequency) are found, with the latter strongly modulated by sea surface temperature. The 20–60-day mode shows northward propagation from the equatorial In-dian Ocean linked with eastward propagating modes of convective systems over the tropics. The 10–20-day mode shows a complex space-time structure with a northwestward propagating anoma-lous pattern emanating from the Indonesian coast. This pattern is found to be interacting with a structure emerging from higher latitudes propagating southeastwards. This could be related to ver-tical shear of zonal wind over northern India. The two modes exhibit variability in their intensity on the interannual time scale and contribute a significant amount to the daily rainfall variability in a season. The intensities of the 20–60-day and 10–20-day modes show significantly strong inverse and direct relationship, respectively, with the all-India June–September rainfall. This study also establishes that the probability of occurrence of substantial rainfall over central India increases significantly if the two intraseasonal modes simultaneously exhibit positive anomalies over the region. There also exists a phase-locking between the two modes. In the second part of the thesis, we investigated the changing nature of these intraseasonal modes over Indian region, and their association with extreme rainfall events using ground based observed rainfall. We found that the relative strength of the northward propagating 20–60-day mode has a significant decreasing trend during the past six decades, possibly attributed to the weakening of large-scale circulation in the region during monsoon. This reduction is compensated by a gain in synoptic-scale (3–9 days) variability. The decrease in the low-frequency ISV is associated with a significant decreasing trend in the percentage of extreme events during the active phase of the monsoon. However, this decrease is balanced by a significant increasing trend in the percentage of extreme events in break phase. We also find a significant rise in occurrence of extremes during early- and late-monsoon months, mainly over the eastern coastal regions of India. We do not observe any significant trend in the high-frequency ISV. In the last part of the thesis, we used numerical simulations to understand the observed changes in the ISV features. Using the atmospheric component of a global climate model (GCM), we have performed two experiments: control experiment (CE) and heating experiment (HE). The CE is the default simulation for 10 years. In HE, we prescribed heating in the atmosphere in such a way that it mimics the conditions for extreme rainfall events as observed over central India during June– September. Heating is prescribed primarily during the break phase of the 20–60-day mode. This basically increases the number of extremes, majority of which are in break phase. The design of the experiment reflects the observed current scenario of increased extreme events during breaks. We found that the increased extreme events in the HE decreased the intensity of the 20–60-day mode over the Indian region. This reduction is associated with a reduction of rainfall in active phase and increase in the length of break phase. A reduction in the seasonal mean over India is also observed. The reduction of active phase rainfall is linked with an increased stability of the atmosphere over central India. Lastly, we propose a possible mechanism for the reduction of rainfall in active phase. We found that there is a significant reduction in the strength of the vertical easterly shear over the northern Indian region during break–active transition phase. This basically weakens the conditions for the growth of Rossby wave instability, thereby elongating break phase and reducing the rainfall intensity in the following active phase. This study highlights the redistribution of rainfall intensity among periodic (low-frequency) and non-periodic (extreme) modes in a changing climate scenario, which is further tested in a modeling study. The results presented in this thesis will provide a pathway to understand, using observations and numerical model simulations, the ISV and its relative contribution to the Indian summer monsoon. It can also be used for model evaluation.

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.

Smallholder farmers often have great entrepreneurial qualities that build on generations of experience. However, many farm management practices are poorly adapted to current climate change conditions. In order for farmers to understand the risks they are undertaking by following certain farming practices and to adapt accordingly, a decision support tool is being developed by researchers at TU Delft. The tool runs a socio-hydrological model, created in Python, in the back-end and provides farmer specific investment and profit data for different crops in the front-end. The aim of this study is to develop a risk assessment process that integrates hydro-climatic variability in the decision support tool, and to identify ways of communicating risk to smallholder farmers in Maharashtra, India. Two sources of variability were characterised based on a literature review of Indian farmers’ own risk perceptions; the untimely onset of the Indian Summer Monsoon and the frequency of dry spells. A sensitivity analysis was then carried out to investigate their respective effects on the farmers’ crop yields. The method proposed to evaluate these risks used a single variable, precipitation data, and a two-dimensional risk matrix to compound the two risk factors, over a time span of 14 years (2003-2016). However, the results indicate that it might be more beneficial to define dry spells in terms of crop water stress, instead of a precipitation threshold. This study also proposed a method for translating a cumulative distribution curve into a risk representation that is adapted for low-literacy users by combining numbers and text with graphics, color and voice descriptions. Ultimately, however, the usability of the tool cannot be determined solely through literature, but must involve the end-users in its design. / Småskaliga jordbrukare är goda entreprenörer som samlat på sig kunskaper och erfarenheter över flera generationer. Däremot är vissa metoder som jordbrukarna använder sig av idag för att förvalta sitt jordbruk inte anpassade till nutida klimatförändringar. För att jordbrukarna ska förstå riskerna som de åtar sig vid valet av dessa metoder försöker forskare vid TU Delft nu ta fram ett verktyg för att underlätta jordbrukares förmåga att ta självständiga men välgrundade beslut om sitt jordbruk. Verktyget är baserat på en socio-hydrologisk modell som är framtagen i Python och som förser specifika investerings- och inkomstdata för enskilda jordbrukare. Syftet med detta kandidatarbete är att bidra till verktyget genom att undersöka de hydroklimatiska risker som uppstår till följd av föränderliga och osäkra klimatologiska förhållanden för jordbrukare i delstaten Maharashtra, Indien. Två riskfaktorer karakteriserades baserat på en litteraturstudie om indiska jordbrukares riskuppfattningar: avvikelser i starten på den indiska sommarmonsunen och antal torrperioder under monsunsäsongen. Dessutom utfördes en känslighetsanalys för att undersöka om och hur den existerande modellens utdata av skörd påverkades av de valda riskfaktorerna. Monsunstarten och torrperioderna togs fram genom metoder som enbart använde historiska nederbördsdata över tidsperioden 2003-2016 och kombinerades sedan med hjälp av en tvådimensionell riskmatris. Resultaten visade att det fanns anledning att ifrågasätta hur torrperioderna definierades och att det kan vara mer fördelaktigt att undersöka vattenbrist för grödan, snarare än att enbart förlita sig på nederbördsdata. Vidare föreslog denna studie en metod för att översätta en kumulativ fördelningsfunktion till en grafisk riskframställning som är anpassad till användare med låg läskunnighet genom att kombinera siffror med text, grafik, färg och ljudförklaringar. I slutändan kan dock inte användbarheten av verktyget enbart avgöras utifrån litteratur, utan måste även inkludera återkoppling från slutanvändarna.

In the recent scenario of climate change, the natural variability and uncertainty associated with the hydrologic variables is of great concern to the community. This thesis opens up a new area of multi-disciplinary research. It is a promising field of research in hydrology and water resources that uses the information from the field of atmospheric science. A new way to identify and capture the variability and uncertainty associated with the hydrologic variables is established through this thesis. Assessment of hydroclimatic teleconnection for Indian subcontinent and its use in basin-scale hydrologic time series analysis and forecasting is the broad aim of this PhD thesis. The initial part of the thesis is devoted to investigate and establish the dependence of Indian summer monsoon rainfall (ISMR) on large-scale Oceanic-atmospheric circulation phenomena from tropical Pacific Ocean and Indian Ocean regions. El Niño-Southern Oscillation (ENSO) is the well established coupled Ocean-atmosphere mode of tropical Pacific Ocean whereas Indian Ocean Dipole (IOD) mode is the recently identified coupled Ocean-atmosphere mode of tropical Indian Ocean. Equatorial Indian Ocean Oscillation (EQUINOO) is known as the atmospheric component of IOD mode. The potential of ENSO and EQUINOO for predicting ISMR is investigated by Bayesian dynamic linear model (BDLM). A major advantage of this method is that, it is able to capture the dynamic nature of the cause-effect relationship between large-scale circulation information and hydrologic variables, which is quite expected in the climate change scenario. Another new method, proposed to capture the dependence between the teleconnected hydroclimatic variables is based on the theory of copula, which itself is quite new to the field of hydrology. The dependence of ISMR on ENSO and EQUINOO is captured and investigated for its potential use to predict the monthly variation of ISMR using the proposed method. The association of monthly variation of ISMR with the combined information of ENSO and EQUINOO, denoted by monthly composite index (MCI), is also investigated and established. The spatial variability of such association is also investigated. It is observed that MCI is significantly associated with monthly rainfall variation all over India, except over North-East (NE) India, where it is poor. Having established the hydroclimatic teleconnection at a comparatively larger scale, the hydroclimatic teleconnection for basin-scale hydrologic variables is then investigated and established. The association of large-scale atmospheric circulation with inflow during monsoon season into Hirakud reservoir, located in the state of Orissa in India, has been investigated. The strong predictive potential of the composite index of ENSO and EQUINOO is established for extreme inflow conditions. So the methodology of inflow prediction using the information of hydroclimatic teleconnection would be very suitable even for ungauged or poorly gauged watersheds as this approach does not use any information about the rainfall in the catchment. Recognizing the basin-scale hydroclimatic association with both ENSO and EQUINOO at seasonal scale, the information of hydroclimatic teleconnection is used for streamflow forecasting for the Mahanadi River basin in the state of Orissa, India, both at seasonal and monthly scale. It is established that the basin-scale streamflow is influenced by the large-scale atmospheric circulation phenomena. Information of streamflow from previous month(s) alone, as used in most of the traditional modeling approaches, is shown to be inadequate. It is successfully established that incorporation of large-scale atmospheric circulation information significantly improves the performance of prediction at monthly scale. Again, the prevailing conditions/characteristics of watershed are also important. Thus, consideration of both the information of previous streamflow and large-scale atmospheric circulations are important for basin-scale streamflow prediction at monthly time-scale. Adopting the developed approach of using the information of hydroclimatic teleconnection, hydrologic variables can be predicted with better accuracy which will be a very useful input for better management of water resources.