Atmospheric Response to Mesoscale Sea Surface Temperature Anomalies: Assessment of Mechanisms and Coupling Strength in a High-Resolution Coupled Model over the South Atlantic*

Many aspects of the coupling between the ocean and atmosphere at the mesoscale (on the order of 20–100 km) remain unknown. While recent observations from the Southern Ocean revealed that circular fronts associated with oceanic mesoscale eddies leave a distinct imprint on the overlying wind, cloud coverage, and rain, the mechanisms responsible for explaining these atmospheric changes are not well established. Here the atmospheric response above mesoscale ocean eddies is investigated utilizing a newly developed coupled atmosphere–ocean regional model [Consortium for Small-Scale Modeling–Regional Ocean Modelling System (COSMO-ROMS)] configured at a horizontal resolution of ~10 km for the South Atlantic and run for a 3-month period during austral winter of 2004. The model-simulated changes in surface wind, cloud fraction, and rain above the oceanic eddies are very consistent with the relationships inferred from satellite observations for the same region and time. From diagnosing the model’s momentum balance, it is shown that the atmospheric imprint of the oceanic eddies are driven by the modification of vertical mixing in the atmospheric boundary layer, rather than secondary flows driven by horizontal pressure gradients. This is largely due to the very limited ability of the atmosphere to adjust its temperature over the time scale it takes for an air parcel to pass over these mesoscale oceanic features. This results in locally enhanced vertical gradients between the ocean surface and the overlying air and thus a rapid change in turbulent mixing in the atmospheric boundary layer and an associated change in the vertical momentum flux.