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

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.

Persistent Lagrangian Transport Patterns in the Northwestern Gulf of Mexico

“…Submesoscale processes over the NWGoM have been shown to exhibit a primary energetic peak in the winter and a secondary energetic peak in the summer (Luo et al 2016).…”

Loop Current Spin-off Eddies, Slope Currents and Dispersal of Reef Fish Larvae from The Flower Gardens National Marine Sanctuary and The Florida Middle Grounds

“…However, ambient slope currents (MKE) were averaged over a four month period, blurring larval entrainment and cross isobath transport on short time scales. Submesoscale processes (Luo et al 2016) may be important at the shelf break, but will not be captured within the scales of this study.…”

The influence of mesoscale and submesoscale circulation on sinking particles in the northern Gulf of Mexico

“…For example, it has been shown that submesoscale eddies have been shown to contribute to the onset of plankton blooms by restratifying the mixed layer in early spring (Mahadevan et al, 2012), submesoscale filaments around the eddy perimeter subduct large amounts of POC during spring blooms at high latitudes (Omand et al, 2015), and submesoscale fronts can converge effectively and quickly any floating material into 'lines' a few meters in width and up to several kilometers in length (Zhong et al, 2012;Mahadevan, 2016). Submesoscale fronts are particularly abundant in the northern Gulf of Mexico (GoM) where surface density gradients are fueled by the energetic mesoscale field and also by the abundant riverine input (Zhong et al, 2012;Luo et al, 2016;Barkan et al, 2017a).…”

“…In between mesoscale structures, a large number of submesoscale circulations such as vorticity filaments and small vortices, on a spatial scale of 1-10 km and with a lifespan of days, can be seen, particularly in September. Submesoscale processes extract available potential energy from the mixed layer (Molemaker et al, 2005;McWilliams, 2008) through processes such as frontogenesis and mixed layer instabilities (Thomas and Ferrari, 2008;Luo et al, 2016;Barkan et al, 2017a). Submesoscale features are stronger and more numerous when more available potential energy can be extracted.…”