Geosciences Dept. Seminar: A full seasonal cycle of mixed layer instabilities from ocean gliders

Abstract: 

A year-long, submesoscale-resolving time series of near-surface buoyancy gradients, potential vorticity and instability characteristics, collected by ocean gliders, that provides insight into open-ocean (away from strong and persistent geostrophic fronts) submesoscale dynamics over a full annual cycle. The gliders continuously sampled a 225 km^2 region in the subtropical northeast Atlantic, measuring temperature, salinity and pressure along 292 short (~20km) hydrographic sections.

Glider observations show a seasonal cycle in near-surface stratification. Throughout the fall (September through November), the mixed layer deepens, predominantly through gravitational instability, indicating that surface cooling dominates submesoscale restratification processes. During winter (December through March), mixed layer depths are more variable, and estimates of the balanced Richardson number, which measures the relative importance of lateral and vertical buoyancy gradients, depict conditions favorable to symmetric instability. The importance of mixed layer instabilities on the restratification of the mixed layer, as compared with surface heating and cooling, shows that submesoscale processes can reverse the sign of an equivalent heat flux up to 25% of the time during winter. These results demonstrate that the open-ocean mixed layer hosts various forced and unforced instabilities, which become more prevalent during winter, and emphasize that accurate parameterizations of submesoscale processes are needed throughout the ocean.

Seminar Organizer: Prof. Eyal Haifetz