The Role of Sub-mesoscale Processes in the Vorticity Balance Derived by HFDR

High Frequency Doppler Radar (HFDR) ocean surface currents are used to calculate the near-surface vorticity balance at 1.5 km spatial resolution in areas where the vorticity forcing signal is known to be strong: the lee of the mountainous islands (Hawaii and Philippines). The balance is separated into low-frequency quasi-geostrophic motions and shorter time and length scale sub-mesoscale motions, which induce turbulent stresses on the former. The vorticity equation is derived in terms of HFDR measurable quantities, with the exception of the forcing term which is obtained by a combination of an atmospheric and an ocean general circulation model. Results show that although the low-frequency balance is dominated by the advective and vertical stretching terms, the eddy fluxes induced by sub-mesoscale processes are an important sink of vorticity, while the combination of wind stress curl and a variable mixed layer depth can, at times, be an important source. Preliminary results of the energy equation will also be presented to complement the analysis.