Submesoscale circulation in the Northern Gulf of Mexico: Surface processes and the impact of the freshwater river input

TL;DR

This study uses high-resolution regional modeling to analyze submesoscale surface processes in the northern Gulf of Mexico, highlighting the influence of seasonal mixed layer depth and freshwater input on surface structures and dynamics.

Contribution

It provides new insights into the seasonal variability and freshwater impact on submesoscale surface circulation in the Gulf of Mexico using high-resolution modeling.

Findings

Submesoscale activity peaks in winter over the slope and offshore.

A secondary summer maximum is driven by freshwater-induced density gradients.

Freshwater input significantly influences submesoscale structures year-round.

Abstract

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.