Reproducibility and variability of submesoscale frontal eddies on a broad, low-energy shelf of freshwater influence

Abstract

A multi-decadal simulation of ocean circulation in the northern Gulf of Mexico produces strong submesoscale instabilities in the Mississippi/Atchafalaya plume fronts. The model skill in reproducing these submesoscale frontal eddies over the Texas-Louisiana shelf is assessed using simulated and observed salinity and velocity fields as a way to investigate simulation accuracy and quantify the variability of frontal eddies. The model successfully reproduces mean salinity structure observed in multi-year densely sampled CTD profiles. Variability associated with submesoscale eddies is the largest source of error in predicted salinity. On the other hand, the model is statistically able to reproduce the magnitude and characteristics of frontal eddies; metrics for eddy kinetic energy are similar between the observations and simulation, and observed horizontal salinity gradients have similar occurrence rates in the model when sampled in a manner similar to the observations. Seasonal and inter-annual variability of frontal eddies is associated with the volume of freshwater onto the shelf and wind stress. Wind stress, the highest in winter and lowest in summer, contributes to the suppression of baroclinic instability during non-summer seasons. River streamflow, highest in spring, creates strong horizontal and vertical density gradients. These strong horizontal density gradients, along with weak seasonal upwelling-favorable winds that tend to broaden the plume, are the primary factors in exciting submesoscale instabilities during summer on the Texas-Louisiana shelf. At decadal scales, streamflow, EKE, and salinity gradients have a positive correlation suggesting that long-term variability of frontal eddies may be influenced remotely by inter-annual variability in the Mississippi River outflow.