Influence of the sea surface temperature decadal variability on tropical precipitationWest African and South American Monsoon

Zusammenfassung

Precipitation regimes in the Sahel, the Amazonia and the Northeast of Brazil have undergone changes over time with important consequences, being a major topic of study. At decadal-to-multidecadal time scales, these changes have been mainly associated with the global sea surface temperature (SST) variability. Particularly, the Sahel precipitation has been associated with the global warming (GW), the Atlantic Multidecadal Variability (AMV) and the Interdecadal Pacific Oscillation (IPO) modes of SST variability. The Amazonia and Northeast rainfall changes have been related to the Pacific and the Atlantic SST variability at decadal time scales, which is led by the AMV and IPO. Climate study through Global Circulation Models (GCMs) is crucial to understand climate change and its effects. So, in the first part of this Thesis a multi-model analysis is done addressing the influence of the main decadal-to-multidecadal modes of SST variability on Sahel, Amazon and Northeast rainfall using different GCMs simulations (historical, piControl and RCP8.5) from the 5th phase of the Coupled Model Intercomparison Project (CMIP5). A few studies suggest that the Sahel experienced a long wet period throughout the late-19th century. This motivates the second part of this Thesis, which seeks to reproduce this period with the LMDZ atmospheric GCM (AGCM) forced with observed SST since 1854. This Thesis aims to achieve a better understanding of the SST decadal-to-multidecadal variability on rainfall in the Sahel, Amazonia and Northeast regions. For that purpose, a multi-model analysis is done aiming to characterize the main modes of SST variability (GW, AMV and IPO), assess their impacts on precipitation and the causes of such links. Other goals are to seek the future evolution of these links, discuss the role of the radiative forcing on the AMV and IPO and assess the contribution of the SST modes to the total decadal-to-multidecadal rainfall variance. A final objective is to find out whether the long rainy period of the late-19th century can be reproduced with an AGCM forced with observed SST and the factors that caused it. The first part of results reveals that CMIP5 models, on average, can reproduce the principal observed GW, AMV and IPO features and their impacts. The main conclusions obtained are: - The GW has been prone to aerosol changes in the recent past, inducing inter-model differences but do not affect the way models reproduce, on average, the rainfall response: a drying in the Sahel and more precipitation in Amazonia and Northeast. - In positive AMV phases, the Sahel and Amazon precipitation are enhanced and reduced in the Northeast (the opposite during negative phases) trough anomalous shifts of the Intertropical Convergence Zone. - The IPO has negative impact on rainfall in the three regions through Walker circulation anomalies from the tropical Pacific to West Africa and northern South America. - The aerosols induce inter-model uncertainties as to the simulated AMV but does not affect the IPO, suggesting that the AMV may have a component of external forcing while the IPO is dominated by internal variability. - The RCP8.5 future projections reveal a different GW pattern and impacts on rainfall to the historical simulations, but similar AMV and IPO behavior. - A multi-linear regression analysis between the GW, AMV and IPO indices and the precipitation index of each region show that CMIP5 models do not reproduce the observed contribution of each SST mode to the total decadal-to-multidecadal rainfall variability. In a second part of results it is shown that the LMDZ model reproduces a long Sahel rainy period in the late-19th century in response to observed SST forcing since 1854. A set of sensitivity experiments shows that the Atlantic SST plays a dominant role inducing such a precipitation enhancement through enhanced convection over the Sahel and more moisture supply from the tropical Atlantic.