Mesoscale and Submesoscale air-sea interactions in the lower North Atlantic trades

In the past decades, numerous studies have shown that oceanic mesoscale activity, over scales of O(50–250) km, has a strong influence on the atmosphere through both the Thermal FeedBack (TFB) and the Current FeedBack (CFB). However, at the submesoscale, over scales of O(1–10) km, both TFB and CFB are not well understood, mainly due to technical barriers (observation and simulation). Here, a realistic high-resolution coupled oceanic model (dx = 1 km), including tidal forcing and river discharge, and atmospheric (dx = 2 km) model in the lower North Atlantic trades region over a period of 1-year (from July 2019 to June 2020) is used to assess the atmospheric response to submesoscale processes. We used classic coupling coefficients between the ocean and the atmosphere to quantify spatial and temporal variabilities of the TFB and CFB coupling.

Our results show that, similar to oceanic mesoscale activity, at the submesoscale, both TFB and CFB have an imprint on the low-level wind, surface stress and turbulent heat fluxes. On the one hand, the linear relationship between surface wind (stress) curl and surface current vorticity existing at the mesoscale regime is also supported at the submesoscale. At submesoscale, CFB, as at the mesoscale, is acting as a sink of energy from the ocean to the atmosphere, acting as an submesoscale eddy killer. Furthermore, the magnitude of surface stress curl introduced by submesoscale processes is greater by ~17 % than that presented by mesoscale processes, which is explained by a reduction of the wind response by ~55 %. On the other hand, the linear relationship between wind stress magnitude, or latent heat flux, and sea surface temperature (SST) anomalies, widely present at the mesoscale, is also found at the submesoscale. Similar results are found when considering wind stress curl/divergence and crosswind/downwind SST gradients coupling coefficients. However, the magnitude of the corresponding TFB coupling at submesoscale is reduced by ~30 % with respect to those at mesoscale.

Overall, our results emphasize the significant impact of both oceanic currents and strong SST fronts on the local wind/stress and latent heat flux response at the submesoscale regime.