Submesoscale coherent structures and SST gradient in ocean model simulations

Analyzing a suite of high-resolution ocean model simulations, it is found that the strength of the simulated surface temperature gradient is strongly influenced by atmospheric forcing. Within the mesoscale-submesoscale (5 km - 300 m) range in model resolution, imposing a fine-resolution forcing is more efficient than increasing the model resolution in enhancing the simulated SST gradient. The probability distribution of the magnitude of SST gradient is approximately log-normal in all cases. The submesoscale-resolving simulations produced ubiquitous, spatially stretched, coherent structures with sharp velocity gradients that are dominated by shear straining and deformation and accompanied by sharp SST gradients. They are identified as temperature fronts. This is in contrast to a simulation with a coarser resolution (L > 10 km) in which the mesoscale structures are dominated by eddies with strong vorticity. Cyclone-anticyclone asymmetry becomes evident for the front-like structures in the submesoscale-resolving simulations. The strongest structures are overwhelmingly cyclonic, likely due to inertial instability of strongly anticyclonic structures for which Rossby number exceeds unity.