Sarah T. Gille (Scripps Institution of Oceanography, University of California, San Diego, USA); Teresa K. Chereskin (Scripps Institution of Oceanography, University of California, San Diego, USA); Dimitris Menemenlis (Jet Propulsion Laboratory, CalTech/NASA, United States)

Satellite altimetry has played a central role in the “mesoscale revolution” that provided fundamental new knowledge about the variability and dynamics of the ocean at the energy-containing, mesoscales (100 − 300 km). Much less is known about the submesoscales, flows with length-scales smaller than the mesoscales (< 100 km). While the mesoscales are dominantly in geostrophic balance, the prevalence of geostrophic motions at submesoscales is not guaranteed. In preparation for the upcoming Surface Water and Ocean Topography (SWOT) mission, which has the potential to revolutionize the youthful field of submesoscale oceanography, we investigate the dynamics that dominate the SWOT-relevant submesoscales (roughly 10-100 km) and annual variability. The region of focus is the Kuroshio Extension, where previous studies indicate strong seasonal cycle.

Using the output of two new high-resolution numerical simulations with embedded tides, we show that, in the Kuroshio Extension, there exists a strong seasonal modulation of the submesoscales dynamics. This seasonality at submesoscales is strongly highlighted in second-order statistics of the lateral surface velocity gradient tensor (see attached figure). In late winter/early spring, submesoscale turbulence dominates the near-surface variability. A significant fraction of the wintertime surface kinetic energy (KE) and sea-surface height (SSH) variance is accounted for by geostrophic flows. Summertime re-stratification weakens submesoscale turbulence and enhances inertia-gravity waves near the surface. Hence, ageostrophic flows account for most of the submesoscale surface KE and SSH variance in late summer/early fall. In other words, most of the summertime Kuroshio Extension submesoscale SSH variability is due to inertia-gravity waves. In the context of SWOT, this means that if one is interested only in the geostrophic flow, so that surface velocity can be diagnosed from SSH, then the noise-to-signal ratio will have a strong seasonality. More broadly, we conjecture that upper-ocean submesoscale stirring and mixing undergo a vigorous seasonal cycle in high eddy kinetic energy regions.