Overturning of mixed layer eddies in a submesoscale resolving simulation of the North Atlantic

Submesoscale Instabilities in the mixed layer can lead to ocean restratification and thus affect ocean-atmosphere feedbacks. In this study, a novel configuration of the ICON ocean model is applied which makes use of telescoping grid refinement such that a horizontal resolution finer than 600m is achieved over wide areas of the North Atlantic. The ability of the model to simulate mesoscale to submesoscale turbulence is validated by comparing spatial power spectra of sea surface temperature and sea surface hight with those of satellite products and a ICON simulation of 10km horizontal resolution (often referred to as "mesoscale eddy resolving").

We find more realistic variability in the simulation with the refined grid compared with coarser simulation over a wide range of scales that even includes the mesoscale eddy regime. Moreover, the high-resolution permits submesoscale baroclinic and symmetric instabilities. At single fronts, we observe strong overturning re-stratifying the fronts. Overturning rates are diagnosed regarding mean characteristics of the fronts like mean horizontal and vertical density gradients and mixed layer depth. Finally, the diagnosed overturning rates are compared to recent parameterizations introduced by Stone (1971) and Fox-Kemper (2008). It turns out that both parameterizations are roughly able to capture the bulk overturning along strong fronts but have problems in non-frontal regions.