Slope currents in the Northeastern Gulf of Mexico

Cathrine Hancock, Florida State University, Geophysical Fluid Dynamics Institute, Tallahassee, FL, United States, Kevin Speer, Florida State University, Geophysical Fluid Dynamics Institute(GFDI), Department of Scientific Computing(DSC), Tallahassee, United States, Joao Marcos Souza, New Zealand Meteorological Service (MetService), MetOcean Division, Raglan, New Zealand and Steven L Morey, Florida Agricultural and Mechanical University, Distinguished Research Scientists, NOAA Center for Coastal and Marine Ecosystems, and Professor, Tallahassee, United States
Using RAFOS float data, mooring data and output from a 5-year ROMS simulation, we investigate the subsurface flow patterns in the northeastern Gulf of Mexico (GoM). A combination of along-slope flows and eddy activity sets the basic flow pattern near 400m depth in the northeastern GoM. Years with persistent eastward or westward along-slope flow through De Soto Canyon seed exit pathways into the Atlantic or western GoM generating increased lagrangian dispersion from the eastern GoM. Though the Loop Current is responsible for much of the eddy activity in the northeastern GoM, certain eastward flow patterns along the Louisiana-Mississippi slope can produce both cyclonic and anticyclonic stationary eddies in De Soto Canyon. This is due to the 90˚ bend in bathymetry within the canyon, created by the junction of the Mississippi-Alabama shelf with the West Florida Shelf. As eddy activity in the canyon and along its flanks increases, particles spread more uniformly throughout the northeastern GoM. Despite this increased eddy-driven horizontal dispersion, particles still tend to remain within the eastern GoM. We also observe an increase in particle recirculation in the De Soto Canyon area, whenever eddy activity is high. The canyon is a natural pathway for particle transport across the slope, and eddy activity in the canyon allows for enhanced exchange of water masses between the deeper ocean and shelf waters.
Large interannual variability is observed in along slope flow direction and eddy activity. During the period 2010-2014, there was no discernable seasonal cycle associated with the prevalence of the different flow patterns. Since particle transport between the deeper ocean and shelf is heavily reliant on eddy activity, years with predominantly westward flow will experience a reduction in such transport. This will affect nutrient and contaminant transports between the deep ocean and the slope.