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

Abstract To study submesoscale instabilities in the ocean mixed layer, this study uses the novel ICON-SMT configuration that exploits a telescoping grid refinement to achieve a horizontal resolution finer than 1 km over wide areas of the North Atlantic. The model’s ability to simulate mesoscale to submesoscale turbulence is validated by comparing spatial power spectra of sea surface temperature and height with satellite data and a 10 km eddy-resolving simulation. We find more realistic variability in the refined grid simulation compared to the coarser simulation over a wide range of scales, including the mesoscale eddy regime. Furthermore, the high-resolution permits submesoscale baroclinic instabilities at ocean fronts and we observe strong frontal overturning and re-stratification. Overturning rates are diagnosed from eddy buoyancy flux and mean front characteristics such as horizontal and vertical density gradients. To accurately capture the vertical extent of mixed-layer eddy instabilities, commonly used threshold algorithms for identifying the mixed-layer depth must be modified. We compare spatial and time filtering approaches for estimating submesoscale eddy fluxes and find qualitative similarity, although time filtering yields stronger fluxes. The diagnosed overturning rates are compared to two submesoscale baroclinic instability parameterizations. Both capture overturning magnitude at ocean fronts within an order of magnitude but overestimate it at eddy rims. Comparing submesoscale eddy fluxes in the entire study area shows two different regimes where the parameterizations slightly differ in the ability to capture the diagnosed eddy fluxes.