Submesoscale Dynamics Strengthen Mesoscale Agulhas Eddies

The comparison of satellite observations with two model simulations and the computation of scale kinetic energy fluxes highlight the importance of including submesoscale effects for a realistic representation of mesoscale Agulhas eddies. In the Agulhas ring path, horizontal-wavenumber spectra of sea-surface height as well as of sea-surface temperature computed from a submesoscale permitting simulation are shown to be in very good agreement with those from the observations. In contrast, a strongly eddying simulation that however does not simulate submesoscale flows shows too less power spectral densities on all scales and too steep spectral slopes. Results of an eddy-detection algorithm applied to the model outputs as well as to a gridded sea-surface height satellite product show that in particular strong cyclones are much better represented when submesoscale flows are permitted in the model. The improvements in the representation of the mesoscale dynamics are attributed to the almost full resolution of the open ocean inverse kinetic energy cascade and a better representation of the formation mechanisms of Agulhas cyclones near the Agulhas Bank. The winter-time surface scale kinetic energy fluxes computed with both a spectral and a coarse-graining approach show an inverse cascade in the Agulhas ring path down to scales of 10 km. The kinetic energy cascades upscale and leads on average to 7 times more kinetic energy flux into the mesoscales, when submesoscale dynamics are permitted by the model. Finally, we present indications for an upscale flux into the mesoscales due to the absorption of submesoscale mixed layer eddies.