Do Submesoscales Affect the Large-Scale Structure of the Thermocline?

Anirban Sinha, California Institute of Technology, Environmental Science and Engineering, Pasadena, CA, United States, Jörn Callies, California Institute of Technology, Pasadena, CA, United States and Dimitris Menemenlis, NASA Jet Propulsion Laboratory, Pasadena, United States
Submesoscale baroclinic instabilities have been shown to play a major role in restratifying the surface mixed layer and in driving seasonality in submesoscale turbulent kinetic energy in the mixed layer. But an understanding of whether and how the vertical fluxes generated by these submesoscale turbulent motions affect the permanent thermocline below is still lacking. Here we address this potential impact on the large-scale circulation and stratification of the upper ocean.

We quantify the impact of submesoscale flows in the North Atlantic subtropical gyre with a series of numerical simulations with a range of horizontal grid spacings: 16 km, 8 km, 4 km, and 2 km. The simulations are forced by realistic atmospheric fields from reanalysis, solar and lunar tidal potentials, and boundary conditions of hydrography and currents estimated from observations. All simulations are integrated over 50 years to allow the thermocline to equilibrate.
The equilibrium stratification of the thermocline changes drastically as we refine the grid spacing from 16 km to 8 km, i.e. as we start fully resolving mesoscale eddies. The stratification remains largely unchanged, however, between the 8 km, 4 km, and 2 km resolution runs. The structure of the thermocline is robust, even though the submesoscale vertical buoyancy fluxes keep increasing as mixed-layer instabilities are increasingly well resolved. We present an energetic argument with which this robustness of the thermocline structure can be explained.