How Atmospheric Forcing Frequency, Horizontal and Vertical Grid Resolutions Impact Mesoscale Eddy Evolution in a Numerical Model

Seasonal evolution of both surface signature and subsurface structure of a Mediterranean mesoscale anticyclones is assessed using the Coastal and Regional Ocean Community high-resolution numerical model with realistic background stratification and fluxes. In good agreement with remote-sensing and in-situ observations, our numerical simulations capture the seasonal cycle of the anomalies induced by the anticyclone, both in the sea surface temperature (SST) and in the mixed layer depth (MLD). The eddy signature on the SST shifts from warm-core in winter to cold-core in summer, while the MLD deepens significantly in the core of the anticyclone in late winter. Our sensitivity analysis shows that the eddy SST anomaly can be accurately reproduced only if the vertical resolution is high enough (∼4 m 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 three 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, triggered by the high-frequency atmospheric forcing. Near-inertial waves propagate more energy inside the eddy because of the lower effective Coriolis parameter 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. These results detail the need of high frequency forcing, high vertical and horizontal resolutions to accurately reproduce the evolution of a mesoscale eddy. Key Points Enhanced mixing in anticyclones explain inverse eddy sea surface temperature (SST) signature Vertical resolution is crucial to model eddy core mixing triggered by near-inertial waves Mixed layer anomaly is mainly driven by SST retroaction on air-sea fluxes Plain Language Summary Mesoscale eddies are turbulent structures present in every regions of the world ocean, and accounting for a significant part of its kinetic energy budget. These structures can be tracked in time and recently revealed a seasonal cycle from in situ data. An anticyclone (clockwise rotating eddy in the northern hemisphere) is observed in the Mediterranean to be predominantly warm at the surface and to deepen the mixed layer in winter, but shifts to a cold-core summer signature. This seasonal signal is not yet understood and studied in ocean models. In this study we assess the realism of an anticyclone seasonal evolution in high resolution numerical simulations. Eddy surface temperature seasonal shift is retrieved and is linked to an increased mixing at the eddy core spontaneously appearing at high vertical resolution (vertical grid size smaller than 4 m) in the presence of high frequency atmospheric forcing. This increased mixed is due to the preferred propagation of near-inertial waves in the anticyclone due to its negative relative vorticity. Eddy-induced mixed layer depth anomalies also appear to be triggered by sea surface temperature retroaction on air-sea fluxes. These results suggest that present-day operational ocean forecast models are too coarse to accurately retrieve mesoscale evolution.