PNNL

Abstract

Submesoscale processes are important contributors to the global heat budget and generally support upward heat transport. However, in salinity-stratified regions, such as the northern Gulf of Mexico with its influx of freshwater from the Mississippi-Atchafalaya river system, temperature can act as a passive tracer and submesoscale processes can contribute to downward heat transport. Oceanic heat content is an important factor in many environmental risks the region faces, for example, hurricane intensification, and marine heatwaves. Thus it is important to understand the submesoscale processes that contribute to the heat budget. During the 2022 field campaign of the Submesoscales Under Near-Resonant Inertial Shear Experiment (SUNRISE) in the northern Gulf of Mexico, a sampling plan was developed to study such submesoscale processes in high resolution. Over 31 hours, four assets (two research ships and two remotely controlled, uncrewed boats) drove in parallel across a dense filament, capturing its evolution in time and space. The observations show that surface waters, warmed by daytime solar radiation, were subducted and that the associated overturning circulation transported heat below the surface layer where it was later irreversibly mixed away. The estimated downward heat flux was as strong as the concurrent net air-sea heat flux into the ocean. The filament was then observed to rapidly collapse which we attribute to boundary layer turbulence and the breakdown of geostrophic balance. The collapsing fronts display behaviors indicative of gravity currents. These observations highlight how in salinity-stratified regions, restratification can be associated with downward heat flux and submesoscale dynamics can play an important role in the oceanic heat budget.