The lower atmospheric response to SST sub-mesoscale variability: low level clouds and surface fluxes

Many processes happen at the air-sea interface. Ocean-atmosphere exchange energy (heat, moisture and momentum), mass (precipitation, evaporation).

Ocean sea surface thermal structures affect the stability of the air-column and its properties through dynamical and thermodynamical processes within the marine atmospheric boundary layer (MABL). Such processes severely impact on sensitivity of surface fluxes, wind and cloud cover to fine scale variations of sea-surface temperature (SST). Moreover, nonlinearities in the fluxes of momentum, heat, freshwater and gases make the small scales important.

This study addresses the role of the ocean fine scales in north-west tropical Atlantic Ocean air-sea interactions, with the goal of evaluating the MABL statistical response to SST variability using cloud-resolving regional numerical simulations of the atmosphere and ocean. This is to quantify the physical process that can explain the modulation of low-level clouds and surface fluxes by SST variability.

Recent field campaign (EURECA and ATOMIC) have collected a wealth of data to study the link between circulation, clouds and air-sea coupling; deSzoeke et al. (2021) and Acquistapace et al. (2022) show that the increased MABL turbulence over warm SST is linked to enhanced low-level cloudiness because the MABL moisture is exported above the lifting condensation level (LCL). They do so in single case studies from in situ (ship) data.

Using numerical simulation of such region, we can extend the analysis of the entire EUREC4A region. We found that SST at the sub-mesoscale affects the air column stability and, therefore, the entrainment of dry and outer-MABL air. The main effects of such entrainment are visible in the dynamics of the mixed layer, both in terms of relative humidity (RH) response and in the vertical exchange of horizontal momentum. Changes in RH are reflected on surface fluxes and low-level clouds.