Role of mesoscale Air-sea interactions in shaping low-level atmospheric dynamics: From local and rapid to regional and seasonal scales

– Fabien DESBIOLLES – Department of Earth and Environmental Sciences, Universit`a di Milano, and Osservatorio Geofisico Sperimentale, Trieste, Italy –

Abstract : One of the larger challenges facing the climate modeling community is that climate models appear to poorly represent some of the key regional circulation processes and as a result, struggle to produce reasonable simulations of regional rainfall patterns. The difficulty is partly due to the regional climate dynamics being complex and strongly modulated by processes operating on the global scale down to regional and local scales. We will focus on two different areas, Southern Africa and the Mediterranean Sea.

We demonstrate here that the regional climate and local rainfall pattern is sensitive to the mesoscale air-sea interactions, and, more specifically to the so-called ocean thermal feedbacks. Two main mechanisms have been proposed to describe the MABL response to small-scale spatial and temporal variability of SST. The socalled Downward Momentum (DM) flux mixing mechanism and the Pressure Adjustment (PA) mechanism. The DM mechanism brings into play the turbulence and large eddies with the MABL stimulated by the turbulent fluctuations of momentum, temperature and moisture; theirself driven by SST gradient. The PA mechanism, initially proposed in a context of tropical dynamics, causes wind divergence in the MABL due to surface atmospheric pressure forced by temperature patterns. The above two mechanisms operate from meso- to submesoscales but it remains unclear the favorable conditions which makes one process prevailing on the other-one.

Over Southern Africa, SST gradients, notably over the great Agulhas system system, influence the vertical air column up to the troposphere, and mesoscale ocean patterns significantly modify incoming landward moisture fluxes. Furthermore, the seasonal effect of air-sea interactions on the low-level atmosphere tends to strengthen the Angola Low pressure system during austral summer. The Low-pressure system, associated with the confluence zone of low-level moisture fluxes originating from tropical and sub-tropical western Indian and tropical Atlantic Oceans, has been diagnosed as a key modulated of moisture distribution in the region on daily to seasonal time scales.

Over the Mediterranean, we first show that the Downward Momentum (DM) mixing mechanism is prevailing at daily time scale notably due to the stable nature of the air-column over the Mediterranean. Autumn season is the period during which the DM mechanism is more determining for the surface wind speed. We then show the corresponding vertical structure of the MABL over Sea Surface Temperature patterns, with a significant imprint until 925 hPa. Finally we show that the downwind SST gradient modifies significantly cloud cover, reducing when the flows corresponds to warm-to-cold SST patterns, increasing for a cold-to-warm scenario. Rainfall events are significantly more (less) frequent over extreme negative (positive) Downwind SST gradients.

I will end up that presentation with some perspectives about DM and PA responsible of thermal feedbacks and their potential role in different atmospheric dynamics background. An introduction of two international projects, EUREC4A and HARMONY, will help us to narrow down specific questions.