On the importance of the atmospheric coupling to the small-scale ocean in the modulation of latent heat flux

In this study, we address the role of the ocean fine scales in north-west tropical Atlantic Ocean air-sea interactions. With this purpose, we use satellite observations of the ocean and the atmosphere, the ERA5 atmospheric reanalysis and a set of regional numerical simulations of the lower atmosphere. In particular, we focus on the coupling between the sea-surface temperature (SST) and the marine atmospheric boundary layer (MABL). We also evaluate the latent heat flux (LHF) sensitivity to SST. The results suggest that the SST-MABL coupling depends on the spatial scale of interest. At scales larger than the ocean mesoscale (larger than 150 km), negative correlations are observed between near-surface wind speed (U_10m) and SST and positive correlations between near-surface specific humidity (q_2m) and SST. However, when smaller scales (1 – 150km, i.e., encompassing the ocean mesoscale and a portion of the submesoscale) are considered, the U_10m-SST and q_2m-SST correlate inversely. This is interpreted in terms of an active ocean modifying the near-surface atmospheric state, driving convection, mixing and entrainment of air from the free troposphere into the MABL.

The estimated values of the ocean-atmosphere coupling at the ocean small-scale are then used to develop a linear and SST-based downscaling method aiming to include and further investigate the impact of these fine-scale SST features into an available low-resolution latent heat flux (LHF) data set. The results show that they induce a significant increase of LHF (30% - 40% per °C of SST). We identify two mechanisms causing such a large increase of LHF: (1) the thermodynamic contribution that only includes the increase in LHF with larger SSTs associated with the Clausius-Clapeyron dependence of saturating water vapor pressure on SST and (2) the dynamical contribution related to the change in vertical stratification of the MABL as a consequence of SST anomalies. Using different downscaling setups, we conclude that largest contribution comes from the dynamic mode (28% against 5% for the thermodynamic mode). To validate our approach and results, we have implemented a set of high-resolution WRF numerical simulations forced by high-resolution satellite SST that we have analyzed in terms of LHF using the same algorithm.

To provide further validation to our results we use the high spatio-temporal resolution of in-situ data collected during the EUREC⁴A-OA/ATOMIC campaigns that go beyond the coarse spatial grid of available satellite observations and include additional variables to the SST such as the impact of ocean currents and the local vertical stratification of the upper ocean.