  The processes and instabilities occurring at the ocean surface in the
northern Gulf of Mexico are investigated with a regional model at
submesoscale-permitting horizontal grid resolution (i.e., HR with dx = 1.6 km)
over a three-year period, from January 2010 to December 2012. A
mesoscale-resolving, lower resolution run (LR, with dx = 5 km) is also
considered for comparison. The HR run is obtained through two-way nesting
within the LR run. In HR quantities such local Rossby number, horizontal
divergence, vertical velocity, and strain rate are amplified in winter, when
the mixed layer is deepest, as found in other basins. In the model
configuration considered this amplification occurs in surface waters over the
continental slope and off-shore but not over the shelf. Submesoscale structures
consist of a mixture of fronts and eddies generated by frontogenesis and mixed
layer instabilities, with elevated conversion rates of available potential
energy (APE) into eddy kinetic energy (EKE). In all quantities a secondary
maximum emerges during the summer season, when the mixed layer depth is
shallowest, barely 15-20 m. The secondary peak extends to the coast and is due
to the intense lateral density gradients created by the fresh water inflow from
the Mississippi River system. Submesoscale structures in summer consist
predominately of fronts, as observed in the aftermath of the 2010 Deepwater
Horizon oil spill, and their secondary circulations are impeded due to the
limited depth of the mixed layer. Freshwater river input is key to the
submesoscale activity in summer but modulates it also in winter, as shown with
a sensitivity run in which the riverine inflow is absent. Implications for
transport studies in regions characterized by intense freshwater fluxes and for
submesoscale parameterizations are discussed.
