Anticyclone SST anomaly evolution driven by increased vertical mixing in anticyclones

– Alexandre Barboni, Post-doctorant, SHOM –

Résumé :

Mesoscale eddies are ubiquitous turbulent structures in the ocean. Most studies on the mesoscale use a composite approach, averaging eddies in time and space. Recently several studies developed a Lagrangian approach following individual eddies in time. This approach revealed a complex seasonal evolution where anticyclones shift from warm sea surface temperature (SST) anomalies in winter to cold ones in summer. Mesoscale seasonal evolution is assessed here in a high-resolution numerical model with realistic background stratification and fluxes. Our sensitivity analysis shows that the eddy SST anomaly can be accurately reproduced only if the vertical resolution is high enough (~4m in near surface) and if the atmospheric forcing contains high-frequency. In summer with this configuration, the vertical mixing parameterized by the k-ε closure scheme is 3 times higher inside the eddy than outside the eddy, and leads to an anticyclonic cold core SST anomaly. This differential mixing is explained by near-inertial waves ( ω ~ f ), triggered by the high-frequency atmospheric forcing. Near-inertial waves propagate more energy inside the eddy because of the lower effective Coriolis parameter f in the anticyclone core. On the other hand, eddy MLD anomaly appears more sensitive to horizontal resolution, and requires SST retroaction on air-sea fluxes. At present stage, global operational models do not have the resolution to capture these phenomena. According to this study vertical grid step about 4m in the upper thermocline would then be necessary to accurately reproduce mesoscale temporal evolution. At last a comparison with observation is done from a recent observations in the Eastern Mediterranean and Arabian seas.