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Habilitation à diriger des recherches

Ocean- atmosphere variability over the Indo- Pacific basin

Abstract : The tropical Indian and Pacific oceans share the largest span of warm water and deep atmospheric convection on our planet, which is also the ascending part of the Walker circulation, an essential component of the climate system. This region is also home to strong atmospheric and oceanic intraseasonal to interannual variability, with strong climatic consequences. The El Niño phenomenon, and, to a lesser extent, the Indian Ocean dipole (IOD) indeed have strong climatic consequences around these two basins, but also at global scale. Intraseasonal variability linked to the Madden-Julian oscillation (MJO) is also important, with its modulation of Indian and Australian monsoons and its potentially important role in triggering El Niño. In this document, I describe my research on intraseasonal to interannual oceanic and atmospheric variability in the Indian and Pacific basins. El Niño or the IOD are by essence coupled ocean-atmosphere phenomena: they result from positive feeadbacks arising from air-sea interactions (the "Bjerknes feedback"). At intraseasonal timescale, the coupling seems less important and only partially modifies the characteristic of a dominantly atmospheric or oceanic mode of variability. For example, Tropical Instability Waves in the eastern Pacific are the results of internal oceanic instabilities. However, they influence the stability of the atmospheric boundary layer and the surface windstress, and this feedback tends to reduce slightly their variability. On the other hand, the MJO is predominantly an atmospheric process, resulting from the coupling between atmospheric dynamics and deep atmospheric convection. But I will show that this phenomenon induce astrong oceanic response in the Indian ocean, both in terms of dynamics and surface temperature. The degree to which air-sea coupling influences properties of the MJO however remains an open question. I will also discuss interactions between those various modes of variability. I will for example illustrate how intraseasonal atmospheric variability can trigger an El Niño; how an El Niño can suppress tropical instability waves and be affected in return; how the IOD modulates the MJO activity and the interactions between the IOD and El Niño. I will then discuss a region of the Indian ocean which is emblematic of these scale interactions and in which I have developed past (Cirene cruises from 2005 and 2008) and hopefully future (TRIO cruise) observational programs. The 5°S-10°S band in the Indian Ocean has an elevated thermocline and shallow mixed layer due to climatological Ekman pumping, but high SST close to the threshold of deep atmospheric convection. These two factors increase air-sea coupling in this region, which has clear variability at synoptic (cyclones), intraseasonal (MJO) and interannual (IOD) timescales. This region has strong climatic consequences and impacts the quality of the following Indian monsoon, the number of cyclones in the Madagascar-La Réunion area and even atmospheric patterns over the Maritime continent and Northern Pacific ocean. I will conclude by presenting my future research plans and my projects in terms of cruises and observational networks.
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Habilitation à diriger des recherches
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Submitted on : Friday, November 23, 2012 - 3:41:52 PM
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Jérôme Vialard. Ocean- atmosphere variability over the Indo- Pacific basin. Ocean, Atmosphere. Université Pierre et Marie Curie - Paris VI, 2009. ⟨tel-00755963⟩

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