Ocean-atmosphere interactions in the tropical Atlantic seasonal cycle and multidecadal variability of ENSO

Abstract

The interaction between the ocean and atmosphere drives changes in the climate system in a wide variety of timescales. The strong annual cycle in the equatorial Atlantic, especially over the east, has been object of extensive research but the role of ocean-atmosphere interactions in driving the seasonal cycle remains to be fully understood in this region. The west African monsoon and the Atlantic cold tongue are the main phenomena controlling the seasonal variability in the equatorial Atlantic and a better understanding of their interaction is crucial for a complete comprehension of the dynamics of the annual cycle. Ocean atmosphere interactions are the main driver of ENSO, which is the leading mode of ocean-atmosphere variability at interannual timescales in the tropics. ENSO properties have experienced large changes in the last few decades but the drivers behind those changes are still in debate. The three studies presented in this thesis are based in climate model simulations. In the first and second papers the atmosphere and ocean components of NorESM model are used to investigate the dynamics of the seasonal cycle in the equatorial Atlantic. The third paper focuses on the identification of multidecadal modulation of ENSO properties by means of a strongly simplified model: the conceptual recharge oscillator model. The first part of this thesis presents an in-depth study of the mechanisms of the seasonal cycle in the equatorial Atlantic with special focus on the quantification of the role of the coupling between the ocean and the atmosphere. My results show that thermodynamic coupling is the main driver of the seasonal cycle in the western equatorial Atlantic and indicate that the dynamical Bjerknes feedback plays a secondary role. In the east, ocean dynamics and the monsoon are the main drivers of the seasonal cycle in the ocean and atmosphere, respectively, with ocean-atmosphere interactions contributing to the amplification of the annual cycle. In the second part of this thesis, I study the changes in observed ENSO properties at multidecadal timescales. The large observed changes in ENSO in the recent decades are reproduced with a conceptual model based on the recharge and discharge of the Pacific equatorial upper ocean heat content. This indicates that dynamic coupling is the main driver of ENSO in the last decades with the thermocline feedback being the mechanism responsible of the amplification of the SST anomalies in the eastern equatorial Pacific

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