Air-sea interactions at small-scales (from O(10−) to O(100km)). Why does it matter ? Implications for regional atmospheric dynamics and associated rainfall.

– Fabien Desbiolles, CIMA, Université de Milan –

Résumé :

The climate modeling community encounters numerous challenges in accurately representing vital regional-to-mesoscale circulation processes. This deficiency often leads to an inadequate portrayal of critical features such as regional cloud cover and rainfall. The intricate and nonlinear dynamics of regional atmospheric systems are driven by a multitude of processes spanning from large to local scales and vice versa. Specifically, the local thermodynamic imbalance between oceanic and atmospheric fluids, coupled with the highly variable exchange of heat and momentum at the interface, plays a pivotal role in shaping these dynamics. Mesoscale eddies and fronts, typically on the order of O(10−100km), are widespread in the ocean and significantly impact air column stability, dynamic adjustments, convective processes, low-level cloud formations, and secondary marine atmospheric boundary layer (MABL) circulations. These mesoscale phenomena drive variability in air-sea flux, which fluctuate over several orders of magnitude. This presentation aims to provide a comprehensive overview of thermodynamic processes occurring between the low-level atmosphere and the upper ocean. For instance, we will delve into the two primary thermal feedback mechanisms known as Downward Momentum Mixing (DMM) and Pressure Adjustment (PA), both of which influence MABL thermodynamics. The DMM-induced response involves turbulence and large eddies in the lower troposphere, influenced by atmospheric stability and modified by local sea surface temperature (SST). This leads to increased vertical mixing and stronger surface winds over warmer waters. On the other hand, the PA-induced response involves the thermodynamic adjustment of air density to underlying SST structures, resulting in changes in atmospheric pressure that drive secondary circulations. Our goal is to untangle the effects of these processes, investigate the environmental conditions favoring them, and study their impacts on larger-scale variability and associated rainfall. Additionally, we will discuss the influence of aerosols on both low-level and precipitating clouds in this context. From local to remote responses, spanning small to regional/global scales, and occurring on subdaily to seasonal timescales, mesoscale air-sea interactions and their thermal and dynamical feedbacks are integral components of climate variability.

Lien Zoom :

https://ird-fr.zoom.us/j/98140994447?pwd=bm9vT2FjMVhlUEhPeEVRazNSVkFodz09