Quatre événements El Niño extrêmes ont eu lieu durant les cinq dernières décennies (1972/1973, 1982/1983, 1997/1998 et 2015/2016) et étaient caractérises comme forts dans la région Niño 3.4. Ils présentent des différences significatives dans leur évolution qui induisent des anomalies distinctes de précipitations le long du versant Pacifique Péruvien illustrant la non-linéarité de la téléconnexion ENSO sur les précipitations dans cette région. Les pluies extrêmes ont un impact néfaste sur la population et les secteurs productifs en raison des inondations et des glissements de terrain qui s'ensuivent. Néanmoins, à ce jour, les patrons de circulation climatique clé de leurs évolutions et magnitudes différentes sont encore très peu connus. Dans cette thèse, nous montrons que les différentes configurations de précipitations lors de ces événements sont associées au transport de l'humidité provenant de différentes sources d'humidité à grande échelle. Lors des événements 1983-1998 (2016), ils sont respectivement liés à une forte (faible à modérée) humidité provenant du réchauffement du Pacifique (également mais aussi provenant de L'Atlantique via l'Amazonie). La caractéristique de ces transports d'humidité est due à une réponse atmosphérique opposée entre les événements de 1983-1998 (qui sont similaires) et l'événement de 2016 qui présente des patterns de transport d'humidité déphasés. Ces précipitations sont liées à l'humidité provenant de ces sources et la circulation atmosphérique régionale des vents de niveau supérieur (100 à 300 hPa) influe sur la quantité d'humidité qui pénètre dans la région Nord-Centre du versant Pacifique Péruvien. L'interaction de la circulation à grande échelle et régionale et le transport de l'humidité du Pacifique est expliqué par le mode Ep qui est associé à des précipitations dans la région Centre-Nord. La forte dispersion des précipitations dans les régions montagneuses est expliquée par le mode Cp pendant les phénomènes El Niño modérés (extrêmes) et est liée au transport d'humidité de niveau moyen-bas (haut) de l'Amazonie (Pacifique) atteignant les hautes terres. / Four strong El Niño events took place within the last five decades (1972/1973, 1982/1983, 1997/1998 and 2015/2016) recorded as strong in the Niño 3.4 region. They can exhibit significant differences in their evolution associated with a distinct rainfall anomaly evolution along the PPB (Peruvian Pacific Basin), which illustrates the strong nonlinearity of the ENSO teleconnection on the rainfall in this area. These extreme rainfalls have harmful impacts on the population and productive sectors due to floods and landslides which are trigged by them. Yet the key climatic circulation pattern for their different evolution and magnitude are still unknown. Here we show that different rainfall patterns during these events are associated with moisture transport originated from different large-scale moisture sources. For example, in the 1983 -1998 (2016) events appear as related with strong (weak to moderate) moisture coming from the Pacific warming (also coming from Atlantic Ocean through the Amazon basin). Characteristic of these moisture transports is due to an atmospheric response opposite between the 1983-1998 events (that are similar) and 2016 event experiencing out-of-phase moisture transport patterns. Although these rainfalls are linked to the moisture arrival from those sources, the moisture amount entering the PPB can be also influenced by regional atmospheric circulation of upper level winds (100 to 300 hPa) leading to different enhanced moisture transport associated with different rainfall anomalies in the North-Centre PPB. The interplay of large-scale and regional circulation and Pacific moisture transport explains the Ep mode associated with rainfall in the north-Centre PPB. The high dispersion of rainfall in highlands (Cp mode) during the moderate (extremes) El Niño appears as linked to low-middle (high level) moisture transport from the Amazon (Pacific) reaching highlands.

Submitted by Fabio Sobreira Campos da Costa (fabio.sobreira@ufpe.br) on 2016-03-18T13:04:49Z No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese_Gbekpo_Aubains_2015.pdf: 10869276 bytes, checksum: 0b22521ef971c75f3112dcbc74fcbb7a (MD5) / Made available in DSpace on 2016-03-18T13:04:50Z (GMT). No. of bitstreams: 2 license_rdf: 1232 bytes, checksum: 66e71c371cc565284e70f40736c94386 (MD5) Tese_Gbekpo_Aubains_2015.pdf: 10869276 bytes, checksum: 0b22521ef971c75f3112dcbc74fcbb7a (MD5) Previous issue date: 2015-04-08 / As interações do sistema oceano-atmosfera no Atlântico tropical e suas contribuições à grande variabilidade da precipitação ao longo da costa do nordeste do Brasil (NEB) foram investigadas para os anos de 1974-2008. Os núcleos das estações chuvosas de Março-Abril e de Junho-Julho foram identificados para a parte norte do Nordeste do Brasil (NNEB) e a parte leste do Nordeste do Brasil (ENEB), respectivamente. As regressões lineares defasadas entre as anomalias da Temperatura da Superfície do Mar (TSM), da Pseudo tensão de cisalhamento de vento (PWS), do Fluxo de calor latente (LHF), da Umidade especifica do ar, e as anomalias (positivas e negativas) de precipitação forte no NNEB e no ENEB mostram que a variabilidade da precipitação dessas regiões é diferentemente influenciada pela dinâmica do Atlântico tropical. Quando a zona de convergência intertropical (ZCIT) é anormalmente deslocada para o sul alguns meses antes da estação chuvosa do NNEB, a fase negativa do Modo Meridional do Atlântico (AMM) (fortalecimento dos ventos alísios do nordeste, relaxamento dos ventos alísios do sudeste, maior evaporação no hemisfério norte, menor evaporação no hemisfério sul, TSM mais fria no hemisfério norte, e TSM mais quente no hemisfério Sul), aumenta a precipitação durante a estação chuvosa. O efeito oposto ocorre durante a fase positiva do AMM. Além disso, o estudo mostra a grande influência e um efeito preditivo da região Noroeste do Atlântico Equatorial noroeste (NWEA) sobre a precipitação do NNEB. Com relação ao estado subsuperficial do oceano, os resultados indicam que uma camada de barreira mais fina na NWEA de Novembro-Dezembro até Março-Abril é associada ao resfriamento progressivo da TSM, ao reforço do componente meridional do vento nordeste e precipitações intensas sobre o NNEB. Já a influência da dinâmica do Atlântico tropical sobre a variabilidade da precipitação no ENEB em Junho-Julho indica uma propagação para noroeste de uma área de forte correlação positiva de TSM e de Umidade específica do ar, deslocando-se da parte sudeste do Atlântico tropical (de Fevereiro-Março) para a região da Piscina Quente do Atlântico Sudoeste (SAWP), situada ao largo do Brasil (Junho-Julho). A área de propagação das anomalias, observada segue globalmente o caminho do ramo sul da Corrente Sul Equatorial (sSEC), que é responsável pelo transporte de calor oceânico de leste para oeste no Atlântico tropical sul. O deslocamento da fase mais quente da advecção horizontal de calor oceânico, na camada de mistura, de leste da bacia (entre 5º-15ºS) para a costa do Brasil em Junho-Agosto corrobora a influência da sSEC sobre o núcleo da chuva do ENEB. Uma aceleração dos ventos alísios de sudeste, associada a uma convergência da anomalia do vento sobre a SAWP, produz excesso de umidade do ar sobre a região e provoca mais precipitação sobre ENEB. O efeito oposto ocorre para os episódios menos chuvosos. De acordo com o estudo, a SAWP se mostra como uma área de potencial para o estabelecimento de um índice de previsão de chuvas no ENEB. / Tropical Atlantic Ocean-atmosphere interactions and their contributions to strong variability of rainfall along the Northeast Brazilian coast (NEB) were investigated for the years 1974-2008. The core rainy seasons of March-April and June-July were identified for northern Northeast Brazil (NNEB) and eastern Northeast Brazil (ENEB), respectively. Lagged linear regressions between sea surface temperature (SST), pseudo wind stress (PWS), latent heat flux (LHF) and air specific humidity anomalies over the entire tropical Atlantic and strong rainfall anomalies in NNEB and ENEB show that the rainfall variability of these regions is differentially influenced by the dynamics of the tropical Atlantic. When the intertropical convergence zone (ITCZ) is abnormally displaced southward a few months prior to the NNEB rainy season, the associated meridional mode (strengthening of the northeast trade winds, relaxation of the southeast trade winds, strong evaporation in the north, weak evaporation in the south, colder SST in the North, and warmer SST in the South) increases precipitation during the rainy season. The opposite effect occurs during the positive phase of the dipole. Additionally, this study shows strong influence and predictive effect of the Northwestern Equatorial Atlantic (NWEA) on the NNEB rainfall. Thinner barrier layer in the NWEA from November-December to March-April is associated with progressive cooling of SST, strengthening of meridional component of the northeasterly wind and intense precipitations over the NNEB. The dynamical influence of the tropical Atlantic on the June-July ENEB rainfall variability shows a northwestward-propagating area of strong, positively correlated SST and air specific humidity from the southeastern tropical Atlantic (February-March) to the Southwestern Atlantic Warm Pool (SAWP) offshore of Brazil (June-July). The northwestward-propagating area, observed from February-March to June-July, follows the same pathway of the southern branch of south equatorial current (sSEC), which is responsible of the oceanic heat transport from east to west in the southern tropical Atlantic. The displacement of the warmest phase of horizontal advection of the oceanic mixed layer heat from the eastern part (between 5-15ºS) to the Brazilian coast in June-August confirms this influence of the sSEC on core rainy season in the ENEB. Furthermore, according to our study, the SAWP could be used as index of rainfall prediction in ENEB. An early acceleration of the southeasterly trade winds, associated with a strong convergence of the wind anomaly over the SAWP, produces excessive humidity over the region and causes more precipitation over ENEB. The opposite effect occurs for less rainy episodes.

Issues arising from climate change and long-term natural climate variability have become the focus of much recent research. In this study, we specifically explore the impacts of long-term climate variability and climate changes upon flood frequencies. The analyses of the flood frequencies are carried out in a comparative manner in catchments located in semiarid-temperate and tropical landscapes in Australia, namely Perth, Newcastle and Darwin, using a process-based derived flood frequency approach. The derived flood frequency analyses are carried out using deterministic rainfall-runoff models that capture the intrinsic water balance variability in the study catchments, and driven by temporal rainfall event sequences that are generated by a stochastic rainfall model that incorporates temporal variabilities over a multiplicity of time scales, ranging from within-event, between-event to seasonal, multi-annual and multi-decadal time scales. Six climate scenarios are considered for Newcastle, that combine the ENSO (El Niño Southern Oscillation) and IPO (Inter-decadal Pacific Oscillation) modes of variability, and six different climate scenarios are considered for Perth and Darwin that combine these different ENSO modes and step changes in climate (upwards or downwards) that occurred in 1970 in both regions, which were identified through statistical analysis. The results of the analyses showed that La Niña years cause higher annual maximum floods compared to El Niño and Neutral years in all three catchments. The impact of ENSO on annual maximum floods in the Newcastle catchment is enhanced when the IPO is negative and for Perth, the impact of ENSO weakens in the post-1970 period, while it strengthens in Darwin in the same period. In addition, the results of sensitivity and scenario analyses with the derived flood frequency model explored the change of dominant runoff generation processes contributing to floods in each of the study catchments. These analyses highlighted a switch from subsurface stormflow to saturation excess runoff with a change of return period, which was much more pronounced in Perth and Darwin, and not so in Newcastle. In Perth and Darwin this switch was caused by the interactions between the out-of-phase seasonal variabilities of rainfall and potential evaporation, whereas the seasonality was much weaker in Newcastle. On the other hand, the combination of higher rainfall intensities and shallower soil depths led to saturation excess runoff being the dominant mechanism in Newcastle across the full range of return periods. Consequently, within-storm rainfall intensity patterns were important in Newcastle in all major flood producing events (all return periods), where they were only important in Perth and Darwin for floods of high return periods, which occur during wet months in wet years, when saturation excess runoff was the dominant mechanism. Additionally, due to the possibility of a change of process from subsurface stormflow to saturation excess when conditions suited this switch, the estimates of flood frequency are highly uncertain especially at high return periods (in Darwin and Perth) and much less in Newcastle (when no process change was involved).

No description available.