The seasonality of submesoscale-induced mixing across the mixed layer in the Gulf of Mexico

Submesoscale circulations associated with vorticy structures of kilometer size are characterized by vertical velocities as large as 100 m/day. The intensity of submesoscale motions is primarily modulated by oceanic frontogenesis and mixed-layer instabilities and influences significantly the transport of biological tracers through the water column. To evaluate the role of submesoscale motions on the upper ocean mixing and its seasonality, the regional ocean model system is configured at 2 horizontal grid resolutions, 1 km and 5 km, in the northern Gulf of Mexico. About 25000 3D Lagrangian tracers are then deployed at different depths (5m, 50 m and 100 m) and in different seasons and tracked for about 30 days. Results indicate that following the seasonal cycle of submesoscale circulations and of mixed-layer depth, vertical mixing is strong in winter, and suppressed during summertime. Although model resolution has little effect on lateral transport, vertical dispersion increases significantly with increased grid resolution. Tracers under the influence of submesoscale motions are upwelled much faster. Considering the time scale at which nutrient uptake by phytoplankton takes place, nutrient supply by submesoscale circulations is likely to be a major player in the Gulf of Mexico.