Abstract:

The East Mediterranean Sea (EMS) circulation has previously been characterized as dominated by gyres, mesoscale eddies, and disjoint boundary currents. We develop nested high resolution EMS numerical simulations to examine circulation variability spectrum with emphasis on the yet unexplored EMS submesoscale. It is identified in model and altimetry data that there is a continuous cyclonic boundary current (BC) encircling the Levantine basin, rather than several disjoint currents. This EMS BC advects eddy chains downstream and is identified as a principle source of regional mesoscale and submesoscale variability. During the seasonal fall to winter mixed layer deepening, energetic submesoscale (O(10 km)) eddies, fronts, and filaments emerge throughout the basin, characterized by O(1) Rossby numbers. 

We identify the EMS submesoscale temporal scales as ~1-5 days through spatio-temoral spectral analysis. Mooring data confirms the EMS winter energization of submesoscales with the model-identified times scales. The submesoscale is associated with a ~ k^-2 kinetic energy (KE) wavenumber (k) spectral slope, shallower than the quasigeostrophic-like  ~ k^-3 slope diagnosed at summer. The shallowness of the winter spectrum is shown to be due to divergent subinertial motions, consistent with the Boyd 1992 theoretical model rather than surface quasigeostrophy. Using coarse graining, we diagnose a seasonal inverse (forward) KE cascade above (below) 30 km scales due to rotational (divergent) motions, and show that these commence after completion of the fall submesosacle energization. We also show via coarse graining that at scales larger than several 100 kms, the spectrum becomes near constant and a weak forward cascade occurs (from gyre scales) to mesoscales.