Les variations du rayonnement solaire en surface (SSR) peuvent avoir un impact significatif sur divers aspects du système climatique, et notamment sur le développement socio-économique d’un pays. Pour identifier les impacts possibles du changement climatique sur le rayonnement solaire en surface à l'échelle régionale (~ 50 km) en Afrique australe jusqu'à la fin du 21ème siècle, on a analysé les données mensuelles produites dans le cadre du projet CORDEX-Afrique sur la période 1979-2099. Ces données sont issues des sorties de 5 modèles régionaux de climat (RCM) forcés par 10 modèles globaux de climat (GCM) CMIP5, pour deux scénarios d’émissions, RCP4.5 et RCP8.5, en Afrique australe (SA) et sur une partie du SWIO (0-40°S ; 0- 60°E). Pour contribuer au projet futur proposé qui vise à approfondir l'étude des changements de SSR à l'échelle locale (~ 1 km de résolution horizontale) à l'île de la Réunion et à l'île Maurice, situées dans le Sud-ouest de l'océan Indien (SWIO), près du bord d’Est du domaine CORDEX-Afrique, des simulations climatiques ont été réalisées sur trois fenêtres temporelles de 10 ans : a) le passé 1996-2005 ; et b) le futur 2046-2055 et 2090-2099, en utilisant la version 4 du RCM RegCM (RegCM4), forcé par : 1) les réanalyses climatiques ERA-Interim (ERAINT) du centre européen pour les prévisions météorologiques à moyen terme (ECMWF) pour simuler un passé récent seulement ; et 2) deux GCMs (HadGEM2-ES et GFDL-ESM2M) de l’exercice CMIP5 de simulations du climat passé et futur pour le scénario d’émissions RCP8.5 à l’échelle régionale de 50km en Afrique australe et dans le sud-ouest de l’océan Indien (0-40°S ; 0- 100°E). L’analyse de l’impact du changement climatique sur le SSR sur la base de ces simulations reste cependant limitée, à cause de leur couverture temporelle (3 périodes de 10 ans) et du nombre de modèles (2 GCMs, 1 RCM) et de scénarios (1 RCP) utilisés. Il ressort de l’analyse des simulations de l’ensemble CORDEX-Afrique que : 1) sur la période passée récente, les GCMs forceurs surestiment généralement SSR d'environ 1 W/m2 en été austral (DJF : Décembre-Janvier-Février), et de 7,5 W/m2 en hiver austral (JJA : Juin-Juillet-Août), tandis que les RCMs, forcés par ces GCMs, sous-estiment SSR d'environ -32 W/m2 et de -14 W/m2 en été et en hiver, respectivement. 2) Les projections multi-modèles de changement de SSR simulées par les RCMs et leurs GCMs forceurs sont assez cohérentes. Les GCMs prévoient, en moyenne multi-modèles, une augmentation statistiquement significative de SSR d'environ 8 W/m2 en 2099 selon le scénario RCP4.5 et de 12 W/m2 en 2099 selon le scénario RCP8.5 sur le Centre de l’Afrique australe (SA-C), et une diminution de SSR, avec un degré de confiance élevé, d'environ -5 W/m2 en 2099 selon le scénario RCP4.5 et de -10 W/m2 en 2099 selon le scénario RCP8.5, pendant la saison DJF, en Afrique équatoriale (EA-E). Dans ces deux régions, les RCMs produisent, en moyenne multi-modèles, des tendances similaires (avec un degré de confiance élevé) à celles des GCMs, mais sur des zones d’extension spatiale plus faible que celle des GCMs. Cependant, pour la saison JJA, une augmentation de SSR, d'amplitude similaire dans les simulations GCMs et RCMs (~5 W/m2 en 2099 selon le scénario RCP4.5 et 10 W/m2 selon le scénario RCP8.5), est attendue dans la région EA-E. 3). Une diminution significative de la nébulosité (environ -6% en 2099) est attendue sur le continent sud-africain pour les GCMs comme pour les RCMs. 4) Le scénario RCP8.5 produit des changements d’amplitude supérieure de 2.5W/m2 pour les GCMs forceurs et de 5W/m2 pour les RCMs en 2099 à celle pour le scénario RCP4.5. 5). Comme pour les sorties du modèle RegCM4, les structures des biais ou des changements de SSR issu des RCMs du programme CORDEX-Afrique sont globalement corrélées avec celles de couverture nuageuse totale des RCMs. L’analyse des sorties du modèle RegCM4 indique que : ..... / Changes in Surface Solar Radiation (SSR) have the potential to significantly impact diverse aspects of the climate system, and notably the socio-economic development of any nation. To identify the possible impacts of climate change on SSR at regional scales (~50 km) over Southern Africa and the South West Indian Ocean (SA-SWIO; 0-40°S ; 0- 100°E) up to the end of the 21st century, a slice downscaling experiment consisting of simulations covering three temporal windows: a) the present 1996-2005; b) the future 2046-2055 and 2090-2099 conducted with the Regional Climate Model (RCM) RegCM version 4, driven by the European Center for Medium-range Weather Forecasting (ECMWF) ERA-Interim reanalysis (ERAINT, only present) and 2 Global Climate Model (GCMs: HadGEM2-ES and GFDL-ESM2M) from the Coupled Model Intercomparison Project Phase 5 (CMIP5) under RCP8.5 scenario, are performed and evaluated. Since the slice simulation is of limited temporal coverage, number of regional and driven global models and climate change forcings, mainly because of the limit of available computational resources, the study towards a comprehensive knowledge of SSR changes in context of climate change is thus extended: an ensemble consisting of outputs from 20 regional climate downscaling realisations based on 5 RCMs that participated in the Coordinated Regional Downscaling Experiment (CORDEX) program (CORDEX-Africa) along with their 10 driving GCMs from CMIP5 covering southern Africa (0-40°S; 0- 100°E) during the period of 1990-2099 is analyzed under RCP4.5 and RCP8.5 up to 2099.The slice experiment indicates that 1) RegCM4 simulates present-day seasonal climatology, (surface air temperature, precipitation and SSR) quite well, but has a negative total cloud cover bias (about -20% in absolute percentage) when forced by the ERAINT and the two GCMs. 2) Internal variability of RegCM4-simulated annual means SSR (about 0.2 W/m2) is of one order smaller than the model bias compared with reference data. 3) RegCM4 simulates SSR changes in opposite signs when driven by the different GCMs under RCP8.5 scenario. 4) Electricity potential calculated using first-order estimation based on the RegCM simulations indicates a change less then 2% to 2099 with respect on present level.It is also found from the ensemble study that: 1) GCMs ensemble generally overestimates SSR by about 1 W/m2 in austral summer (December, January, and February, short as DJF) and 7.5 W/m2 in austral winter (June, July and August, short as JJA), while RCMs ensemble mean shows underestimations of SSR by about -32 W/m2 and -14 W/m2 in summer and winter seasons respectively when driven by GCMs. 2) Multi-model mean projections of SSR change patterns simulated by the GCMs and their embedded RCMs are fairly consistent. 3) GCMs project, in their multi-model means, a statistically significant increase of SSR of about 8 W/m2 in RCP4.5 and 12 W/m2 in RCP8.5 by 2099 over Centre Southern Africa (SA-C) and a highly confident decreasing SSR over Eastern Equatorial Africa (EA-E) of about -5 W/m2 in RCP4.5 and -10 W/m2 in RCP8.5 during the DJF season. RCMs simulate SSR change with statistical confidence over SA-C and EA-E area as well with a little spatial extension compared to GCMs. However, in the JJA season, an increase of SSR is found over EA-E of about 5 W/m2 by 2099 under RCP4.5 and 10 W/m2 under RCP8.5, of similar amplitudes in both the GCMs and RCMs simulations. 4) Significant cloudiness decrease (about -6 % to 2099) is found over continent of SA for GCMs and also shown in RCMs. 5) Larger SSR changes are found in the RCP8.5 scenario than in the RCP4.5 scenario in 2099, with about 2.5 W/m2 enhanced changes in GCMs and about 5 W/m2 in RCMs. 6) Either the biases or the changes pattern of SSR are overall correlated with the patterns of total cloud cover from RCMs in CORDEX-Africa program (for RegCM4 as well). The slice experiment indicates that ...

Au sein de l’hémisphère sud, l’Afrique australe et le sud-ouest de l’océan Indien forment l’une des trois zones préférentielles de développement des interactions entre les tropiques et les moyennes latitudes. Il s’agit de la Zone de Convergence Sud-Indienne (ZCSI) où se forment, principalement en été austral, des systèmes synoptiques (entre 3 et 5 jours) caractérisés par des bandes nuageuses orientées nord-ouest/sud-est : les Talwegs Tropicaux-Tempérés (TTT). Cette recherche doctorale vise à améliorer les connaissances liées aux TTT, avec une étude sur la dynamique associée à ces systèmes et une analyse de leur évolution possible au cours du XXIème siècle. La première partie de l’étude s’attache à identifier les conditions favorables à la formation et au développement des TTT. Une classification en régimes de temps est utilisée pour identifier les TTT d’une part, et les perturbations des moyennes latitudes d’autre part, permettant de mieux documenter la variabilité spatio-temporelle des TTT et les conditions de leur développement. Ces événements sont responsables d’environ 20% des précipitations saisonnières sud-africaines. Cette contribution augmente selon un gradient ouest-est. La comparaison entre les deux classifications confirme que les perturbations des moyennes latitudes sont une condition nécessaire pour le développement de TTT, mais non suffisante. Dans les tropiques, des advections d’humidité depuis l’océan Atlantique tropical associées à un excès d'énergie statique humide sur le canal du Mozambique forment les conditions supplémentaires favorables à la convection atmosphérique profonde à proximité du continent. La seconde partie étudie comment les TTT, les précipitations et de manière plus générale le climat d’Afrique australe, pourraient évoluer au cours du XXIème siècle sous l’effet du forçage radiatif associé aux émissions de gaz à effet de serre (GES). Cette approche est multi-modèle (huit modèles climatiques sont retenus de l’exercice 5 du GIEC) et multi-trajectoire (RCP 8.5 et 2.6). Les huit modèles restituent convenablement les caractéristiques actuelles du climat d’Afrique australe ainsi que les bandes nuageuses associées aux TTT, tant en termes de variabilité spatiale que de fréquence d’occurrences. Sur le XXIème siècle, ils divergent sur l’évolution des précipitations saisonnières (NDJF). En revanche, ils convergent sur l’augmentation des quantités précipitées par jour de pluie sur le sud-est de l’Afrique australe. Ces changements ne sont pas à relier à une évolution spatio-temporelle des TTT, leur structure spatiale, leur fréquence d’occurrence et leur contribution aux précipitations restent stationnaires tout au long du siècle, mais à des événements pluviogènes extrêmes plus fréquents et plus intenses. / In the Southern Hemisphere, Southern Africa and the south-west Indian Ocean are one of the three preferred regions where interactions between the tropics and midlatitudes develop. This is the South Indian Convergence Zone (SICZ), where northwest-southeast oriented cloud bands form at the synoptic scale (between 3 and 5 days). These bands are mainly found during the austral summer and are commonly referred to as tropical temperate troughs (TTTs). This research aims at improving our knowledge related to TTTs, with a study on the dynamics associated with these systems, and an analysis of their possible evolution during the 21st century.The first part of this thesis aims at identifying favorable conditions for the formation and the development of TTTs. Weather regimes analysis is used to identify TTTs on the one hand and mid-latitude perturbations on the other hand, allowing us to better document the spatial and temporal variability of TTTs together with background climate conditions. The events identified account for 20% of seasonal rainfall on average. Their contribution increases according to a west to east gradient. The comparison between these two classifications, partitioned using a k-means clustering, first confirms that midlatitude perturbations are a necessary condition for TTT development, but they are not sufficient. An excess of moist static energy over the Mozambique Channel partly supplied by advections from remote regions (mostly the southern Atlantic basin and the south-west Indian Ocean) form additional conditions favoring deep atmospheric convection over and near the Southern Africa. The second part investigates possible changes in precipitation, TTTs and more generally climate over Southern Africa during the 21st century in response to radiative forcing associated with greenhouse gas emissions (GHG). A multi-model (height climate models taken on the IPCC Fifth Assessment Report) and multi-scenario (RCP 8.5 and 2.6) approach is chosen. All models are skillful to reproduce Southern Africa current climate characteristics and cloud bands associated with TTTs, both in terms of spatial variability and frequency of occurrences. During the 21th century, there is no consensus between the models on the future evolution of seasonal rainfall (NDJF). However, all simulate an increase in the amounts precipitated by rainy day over the south-east part of southern Africa. These changes are not related to an evolution of TTTs: their spatial patterns, frequency of occurrences and contribution to rainfall remain stationary throughout the 21st century, but they associated with extreme rainfall events that become more frequent and more intense.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21578号 / 理博第4485号 / 新制||理||1644(附属図書館) / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 余田 成男, 教授 秋友 和典, 准教授 石岡 圭一 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Sur les effets du changement climatique aux échelles globale et régionale. Il montre en particulierqu'aucun consensus ne peut être trouvé pour ce qui concerne l'évolution future de lapluviométrie -- et de la dynamique atmosphérique associée -- en région de mousson africaine.Ce mémoire revisite cette question à la lumière des nouvelles données disponibles et selon uneapproche évitant toute surreprésentation du nombre de simulations disponibles pour un type demodèle donné, tout en prenant en compte la diversité des modèles ainsi que leur évolution dansle temps : sorties de vingt modèles de circulation générale (MCGs) ayant participé aux exercicesCMIP3 (douze MCGs) et CMIP5 (huit MCGs) sous les scénarios d'émissions A1B et rcp4.5,respectivement. Les sorties sont analysées principalement sur deux fenêtres de quarante ans --périodes actuelle (1960-1999) et future (2031-2070) -- et les résultats discutés au regard de leurvraisemblance selon une approche permettant à la fois de quantifier les différences futur moinsactuel, de mesurer les significativités et les robustesses statistiques et d'associer une probabilitémesurant le consensus des modèles en fonction des échelles et des variables considérées.Les analyses menées sur CMIP3 et CMIP5 montrent qu'un consensus sur l'effet du changementclimatique en Afrique de l'Ouest peut être obtenu si l'on ne fait pas de l'ensemble de labande sahélienne une entité homogène et qu'on raisonne à des échelles spatiales inférieures. Lesrésultats révèlent une évolution contrastée entre le centre et l'ouest du Sahel avec, pour le futur(i) une hausse des précipitations au centre s'expliquant surtout par une plus grande convergencedes flux dans les basses couches, ainsi qu'une pénétration plus au nord de la mousson ;(ii) une baisse des précipitations à l'ouest s'expliquant par le renforcement de la circulation detype Walker, du Jet d'Est Africain (JEA) et de la subsidence dans les couches moyennes. Parailleurs, on peut s'attendre à une modification du cycle annuel moyen avec un retrait retardé dela mousson. Ce retard est notamment lié aux apports supplémentaires d'humidité depuis l'Atlantique,dus au renforcement des contrastes thermiques et d'humidité entre océan et continent,mais aussi et surtout aux apports tardifs d'humidité depuis la Méditerranée et au renforcementdes flux de nord en septembre et octobre en direction du Sahel

The strength of the Atlantic meridional overturning circulation (AMOC) is believed to be associated with changes in surface buoyancy in the subpolar North Atlantic, which naturally leads to a notion that the AMOC has been weakening under global warming. Yet, a variety source of observations and its assimilation into ocean circulation models have not supported such an AMOC decline so far. In this study, an aspect that has not been paid attention, regarding the maintenance of the AMOC strength, is explored: storm activity in the subpolar North Atlantic (NA). An analysis using reanalysis data shows that the wintertime turbulent heat flux variability in the LS deep convection region is largely controlled by a small number of extreme heat flux event days, suggesting a pivotal role of winter storms in prompting LS deep-water formation. A set of forced ocean-ice model simulations, in which synoptic winter storm activity associated with these event days is either suppressed or doubled over the subpolar NA, confirms the above analysis as the altered storm activity results in a substantial change in LS convection and the AMOC strength. These experiments also show an upward AMOC trend during the late twentieth century, the degree of which is to some extent related to the intensity of storm activity in the LS. The upward AMOC trend found in the first part of the dissertation opposes to a downward AMOC trend in the twentieth century coupled model simulations employing the identical ocean component. An analysis suggests that contrast to the ocean-ice model, storm activity in the LS convection region and associated heat flux decreases during the late twentieth century. Although there is also a buoyancy increase over the LS, the wintertime heat flux decrease appears to be a more dominant factor for a decrease in convection in the LS, as an increasing freshwater input from Arctic/Subarctic Ocean bypasses the interior LS along the western boundary current. Therefore, the downward AMOC trend in the coupled model can be linked ultimately to the decreasing storm activity over the LS. This study therefore suggests that storm activity over the major convection regions needs to be paid further attention in assessing AMOC variations, including long-term trend in response to a warming scenario, in future studies.

Submitted by Automa??o e Estat?stica (sst@bczm.ufrn.br) on 2016-10-11T22:53:53Z No. of bitstreams: 1 JoseUelitonPinheiro_TESE.pdf: 3704869 bytes, checksum: 22878ed4bfe495c030dc94c7fe144d8b (MD5) / Approved for entry into archive by Arlan Eloi Leite Silva (eloihistoriador@yahoo.com.br) on 2016-10-18T00:13:18Z (GMT) No. of bitstreams: 1 JoseUelitonPinheiro_TESE.pdf: 3704869 bytes, checksum: 22878ed4bfe495c030dc94c7fe144d8b (MD5) / Made available in DSpace on 2016-10-18T00:13:18Z (GMT). No. of bitstreams: 1 JoseUelitonPinheiro_TESE.pdf: 3704869 bytes, checksum: 22878ed4bfe495c030dc94c7fe144d8b (MD5) Previous issue date: 2015-03-18 / Neste trabalho foi estudado a sa?da de modelos de mudan?as clim?ticas do IPCC/AR5/CMIP5 que melhor expressam a atua??o dos V?rtices Cicl?nicos em Altos N?veis (VCANs) no Nordeste Brasileiro (NEB), no clima atual e cen?rios futuros, como tamb?m sua influ?ncia na precipita??o. Para alcan?ar os objetivos propostos na Tese foram feitos quatro ESTUDOS EXPERIMENTAIS. O primeiro, denominado de Piloto, onde estudou-se 13 modelos de mudan?as clim?ticas para avaliar e selecionar o modelo que melhor expressava a atua??o dos VCANs no NEB. O segundo onde, uma vez selecionado o melhor modelo, o MIROC4h, avaliou a efici?ncia desse modelo comparando com dados de rean?lise para um per?odo de 31 anos (1975-2005). O terceiro onde foram analisados os cen?rios clim?ticos futuros do MIROC4h para o per?odo de 21 anos (2015-2035). E o quarto onde foi analisado a contribui??o dos VCANs na precipita??o sobre o NEB atrav?s dos dados de rean?lise do NCEP/NCAR/DOE. Foram utilizadas duas abordagens estat?sticas comparativas nos Estudos Experimentais 1 e 2, a primeira utilizando o N?mero de Dias de Atua??o de VCANs (N.D.A.VCANs) mensais e a segunda usando o comparativo da atua??o de VCANs di?rio, com a utiliza??o dos ?ndices estat?sticos: correla??es de Pearson, Kendall e Spearman, Raiz quadrada do erro quadr?tico m?dio (RMSE), Raiz quadrada do erro quadr?tico m?dio normalizada (NRMSE), Nash-Sutcliffe (NSE), Kling-Gupta (KGE), ?ndice de Concord?ncia de Willmott (d), ?ndice de Propor??o Correta (PC), ?ndice de Sucesso Cr?tico (ISC), Probabilidade de Detec??o (POD), Taxa de alarme Falso (TAF) e Taxa de Tend?ncia (VI?S). E nos Experimentos 3 e 4 foram calculados desvios e m?dias. Os resultados mostraram a viabilidade na representa??o dos VCANs nos modelos de mudan?as clim?ticas do CMIP5, seja para o clima atual como nos cen?rios futuros. Com rela??o a contribui??o dos VCANs para a precipita??o do NEB estes apresentam percentuais que variam de 47,88% (LNE) a 49,89%(NNE) para o per?odo de outubro a mar?o. Sendo que Cear? (49,89%), Piau? (49,49%) e Maranh?o (47,88%) s?o os Estados onde os VCANs induzem mais precipita??o e Alagoas (41,93%) e Sergipe (38,03%) os Estados onde os VCANs induzem menos precipita??o. A proje??o de cen?rio futuro para os VCANs revelaram um desvio negativo 8,97% na ocorr?ncia deste fen?meno no NEB e ?reas adjacentes para o per?odo de 2015 a 2035. O que poder? impactar em -4,08% a precipita??o do NEB neste per?odo. / In this work, was analysed the climate change models's output from IPCC/AR5/CMIP5 that best express the performance of Upper Tropospheric Cyclonic Vortices (UTCV) in Northeast Brazil (NEB), its influence on rainfall and the possibility of projection of future climate scenarios. To achieve the proposed objectives in the thesis were made four EXPERIMENTAL STUDIES.The first, called Pilot, which were evaluated 13 models of climate change and selected the model that best expressed the actions of UTCV in the NEB. The second that evaluated the efficiency of the best model (MIROC4h), comparing it with reanalysis data for a period of 31 years (1975-2005). The third where the future climate scenarios from MIROC4h were analyzed for the period of 21 years (2015-2035). And the fourth where it was analyzed the contribution of UTCV in precipitation over the NEB through the reanalysis data from NCEP/NCAR/DOE. Two approaches comparative statistics were used in the Experimental Studies 1 and 2 with the use of statistical indices: Pearson , Kendall and Spearman's correlations, mean square error of the square root (RMSE), square of the normalized root mean square error (NRMSE), Nash-Sutcliffe (NSE), Kling-Gupta (KGE), Willmott Index (d) Proportion Correct Index (PCI), Critical Success Index (CSI), Probability of Detection (POD), False Alarm Rate (TAF ) and trend rate (BIAS). And in Experiments 3 and 4 were calculated deviations and averages. The results show that the representation of UTCV in climate change models of CMIP5 is feasible, either for the past climate and for the projection of future scenarios. Regarding the contribution of UTCV to precipitate the NEB these present percentages ranging from 47.88% (LNE) to 49.89% (NNE) for the period from October to March. Cear? (49.89%), Piau? (49.49%) and Maranh?o (47.88%) are the states where UTCV induce more rainfall and Alagoas (41.93%) and Sergipe (38.03%) are the states where UTCV induce less precipitation. The projection of future scenario for UTCV revealed a 8.97% negative difference in the occurrence of this phenomenon in the NEB and surrounding areas for the period 2015 to 2035. What may impact -4.08% the precipitation of the NEB.

Uncertainty in rainfall forecasts affects the level of quality and assurance for decisions made to manage water resource-based systems. However, eliminating uncertainty in a complete manner could be difficult, decision-makers thus are challenged to make decisions in the light of uncertainty. This study provides statistical models as an approach to cope with uncertainty, including: a) a statistical method relying on a Gaussian mixture (GM) model to assist in better characterize uncertainty in climate model projections and evaluate their performance in matching observations; b) a stochastic model that incorporates the El Niño–Southern Oscillation (ENSO) cycle to narrow uncertainty in seasonal rainfall forecasts; and c) a statistical approach to determine to what extent drought events forecasted using ENSO information could be utilized in the water resources decision-making process. This study also investigates the relationship between calibration and lead time on the ability to narrow the interannual uncertainty of forecasts and the associated usefulness for decision making. These objectives are demonstrated for the northwest region of Costa Rica as a case study of a developing country in Central America. This region of Costa Rica is under an increasing risk of future water shortages due to climate change, increased demand, and high variability in the bimodal cycle of seasonal rainfall. First, the GM model is shown to be a suitable approach to compare and characterize long-term projections of climate models. The GM representation of seasonal cycles is then employed to construct detailed comparison tests for climate models with respect to observed rainfall data. Three verification metrics demonstrate that an acceptable degree of predictability can be obtained by incorporating ENSO information in reducing error and interannual variability in the forecast of seasonal rainfall. The predictability of multicategory rainfall forecasts in the late portion of the wet season surpasses that in the early portion of the wet season. Later, the value of drought forecast information for coping with uncertainty in making decisions on water management is determined by quantifying the reduction in expected losses relative to a perfect forecast. Both the discrimination ability and the relative economic value of drought-event forecasts are improved by the proposed forecast method, especially after calibration. Positive relative economic value is found only for a range of scenarios of the cost-loss ratio, which indicates that the proposed forecast could be used for specific cases. Otherwise, taking actions (no-actions) is preferred as the cost-loss ratio approaches zero (one). Overall, the approach of incorporating ENSO information into seasonal rainfall forecasts would provide useful value to the decision-making process - in particular at lead times of one year ahead.

The work presented in the thesis focuses on systematically documenting the multi scale nature of the temporal persistence and spatial coherence of tropical rainfall. There are three parts to the thesis: The first two parts utilize satellite-retrieved rainfall at multiple observational resolutions to characterize the space-time organization of rain; the third part assesses the ability of state-of-the-art coupled models to reproduce some of the observed features. In the first part of the study, which focuses on the temporal persistence of rain, we analyze the Tropical Rainfall Measurement Mission (TRMM) satellite-based observations to compare and contrast wet and dry spell characteristics over the tropics (30 S-30 N). Defining a wet (dry) spell as the number of consecutive rainy (nonrainy) days, we find that the distributions of wet spells (independent of spatial resolution) exhibit universality in the following sense. While both ocean and land regions with high seasonal rainfall accumulation (humid regions) show a predominance of 2-4 day wet spells, those regions with low seasonal rainfall accumulation (arid regions) exhibit a wet spell duration distribution that is essentially exponential in nature, with a peak at 1 day. The behaviour that we observed for wet spells is reversed for dry spell distributions. The total rainfall accumulated in each wet spell has also been analyzed, and we find that the major contribution to seasonal rainfall for arid regions comes from very short length wet spells; however, for humid regions, this contribution comes from wet spells of duration as long as 30 days. An exhaustive sensitivity study of factors that can potentially affect the wet and dry spell characteristics (e.g., resolution) shows that our findings are robust. We also explore the role of chance in determining the 2-4 day mode, as well as the inuence of organized convection in separating reality from chance. The second part deals with the spatial coherence of tropical rain. We take two different approaches, namely, a global and local view. The global view attempts to quantify the con-ventional view of rain, i.e., the dominance of the intertropical convergence zone (ITCZ), while the local view tries to answer the question: if it rains, how far is the influence felt in zonal and meridional directions? In both approaches, the classical e-folding length for spatial decorrelation is used as a measure of spatial coherence. The major finding in the global view approach is that, at short timescales of accumulation (daily to pentad to even monthly), rain over the Equator shows the most dominant zonal scale. It is only at larger timescales of accumulation (seasonal or annual) that the dominance of ITCZ around 7 N is evident. In addition, we also find a semi-log linearity between the spatial scales, seen from afar, and timescale of accumulation, with a break in linearity around typical synoptic timescales of 5-10 days. The local view quantifies the dominance of the zonal scale in the tropical ocean convergence zones, with an anisotropy value (ratio of zonal to meridional scales) of 3-4. Over land, on the other hand, the zonal and meridional scales are comparable in magnitude, suggesting that rain tends to be mostly isotropic over continental regions. This latter finding holds true, irrespective of the spatial and temporal resolutions at which rain is observed. Interestingly, the anisotropy over ocean, while invariant with spatial resolution, is found to be a function of temporal resolution: from a value of 3-4 at daily timescale, it decreases to around 1.5 at 3-hourly resolution, suggesting that perhaps rain fundamentally might be isotropic in nature at an event scale. The final part analyses a few models from the suite of Coupled Model Intercomparison Project (CMIP5) models, to evaluate their ability to reproduce some of these aforementioned features. For all the strong biases that models are known to have, some of the observed features are captured well by the models. Specifically, on the temporal persistence front, the observed 2-4 day mode of wet (dry) spells of rain over humid (arid) regions is also seen in models. The overestimation of longer duration wet spells appears to be the primary cause of a positive bias in the number of rainy days from the models. In general, the tendency of models to not stop raining results in lower and higher number of shorter and longer duration wet spells, respectively, and consequently an overall reduction in dry spells of all durations. On the spatial coherence front, the main finding from the global view approach is that the observed semi-log linearity of the zonal spatial scale of rainfall as a function of timescale of accumulation is strikingly well-reproduced by the models. Even more remarkable is that the models are able to mimic the break in this linearity around 5 days (typical synoptic scale). What the models fail to do prominently is the transition of the dominance of equatorial rain at smaller timescales of accumulation to the dominance of ITCZ at around 7 N at higher timescales of accumulation. Based on the local view approach, we find that, in general, even though the zonal and meridional scales are overestimated, the observed isotropy of continental rain is captured very well by the models. Over the oceans, the success is less prominent, especially with the core of the ITCZ showing much larger ratios than those observed. Thus, the models seem to be able to reproduce the anisotropy for the wrong reasons, and the proposed anisotropy ratio could be a useful metric in further diagnosis of climate models.

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As a result of increase in global average surface temperature, abnormalities in different hydroclimatic components such as evapotranspiration, stream flow and precipitation have been experienced. So investigation has to be carried out to assess the hidden abnormality subsisting in the hydroclimatological time series in the form of trend. This thesis broadly consists of following four parts. The first part comprises of a detailed review of various trend detection approaches. Approaches incorporating the effect of serial correlation for trend detection and interesting developments concerning various non parametric approaches are focused explicitly. Recent trends in annual, monthly, and seasonl (winter, pre-monsoon, monsoon and post-monsoon) Tmax and Tmin have been analyzed considering three time slots viz. 1901-2003, 1948-2003 and 1970-2003. For this purpose, time series of Tmax and Tmin of India as a whole and for seven homogeneous regions, viz. Western Himalaya (WH), Northwest (NW), Northeast (NE), North Central (NC), East coast (EC), West coast (WC) and Interior Peninsula (IP) were originally considered. During the last three decades significant upward trend in Tmin is found to be present in all regions considered either at annual or seasonal level. Sequential Mann Kendall test revealed that most of the significant upward trends both in Tmax and Tmin began after 1970. The second part discusses about numerous climate models from both Coupled Model Inter comparison Project-5 and 3 (i.e. CMIP5, CMIP3) and their skills in simulating Indian climate and assessing their performance using various evaluation measures. Performances of climate models were evaluated for whole of India and over all the individual grid points covering India. The newly defined metric symbolized as Skill_All is an intersection of the three metrics i.e. Skill_r, Skill_s and Skill_rmse, is used for overall model evaluation analysis. A notable enhancement of Skill_All for CMIP5 over CMIP3 was found. After overall model evaluation study, Compromise Programming, a distance based decision making technique, was employed to rank the GCMs gridwise. Entropy method was employed to obtain weights of the chosen indicators. Group decision making methodology was used to arrive at a consensus based on the ranking pattern obtained by individual grid points. In the third part, a detailed detection and attribution (D&A) analysis is performed to determine the causes of changes in seasonal Tmax and Tmin during the period 1950-2005. This formal D&A exercise helps in providing better insight (than trend detection analysis) into the nature of the observed seasonal temperature changes. It was noticed that the emergence of observed trend was more pronounced in Tmin compared to Tmax. Although observed changes were not solely associated with one specific causative factor, most of the changes in Tmin are above the bounds of natural internal climate variability. Finally in the fourth part, to understand the climate change impact on the hydrological cycle, a spatiotemporal change detection study of potential evapotranspiration (PET) along with Tmax and Tmin over India has been performed. Climatology patterns for PET confirmed a greater PET rate during the month of March, April, May and June. A significant increasing trend in both Tmax and Tmin (Tmin being more) was observed in more number of grid points compared to PET. Significant positive trends in Tmax, Tmin and PET were observed over most of the grid points in the IP region. Heterogeneities existed in the spatiotemporal variability of PET over all India. This spatio-temporal change detection study would be helpful for present and future water resources management.