Year-long observations of open-ocean submesoscale motions from ocean gliders

Abstract:

Submesoscale processes may influence the depth and stratification of the ocean surface boundary layer. Yet the prevalence of these motions throughout the ocean and the conditions that trigger them have been difficult to ascertain. Previous submesoscale-focused observational programs have considered regions with strong frontal currents, such as the Gulf Stream, where conditions are favorable for submesoscale instabilities. Here we present a times series of hydrographic observations, obtained at submesoscale resolution, from a region characterized by a weak mean flow. As part of the Ocean Surface Mixing, Ocean Submesoscale Interaction Study (OSMOSIS), glider pairs occupied a 20 km by 20 km region over the Porcupine Abyssal Plain in the northeast Atlantic from September 2012 to September 2013. This data set provides a unique opportunity to study the physical processes that contribute to upper ocean mixed-layer variability over a full seasonal cycle.

We analyze roughly 300 sections of buoyancy and geostrophic velocity to approximate potential vorticity, PV. Conditions indicative of symmetric instability (SI), q < 0 with lateral buoyancy gradients dominant over vertical buoyancy gradients, are rampant in late autumn and in winter. During this period mixed layer depths are variable, but observations of SI-favorable conditions are often immediately followed by a shoaling of the mixed layer. Occurrences of negative PV stop abruptly in the spring when the mixed layer shoals to ~50 m. We analyze the evolving buoyancy structure with respect to frontogenesis diagnosed from sea surface temperature, mesoscale activity diagnosed from sea surface height and wind forcing diagnosed from reanalysis. Preliminary results suggest that SI events occur during transitions from vortex-dominated to strain-dominated regimes and are enhanced by down-front winds. We also discuss the analysis of a 1/48-degree (~1.5 km-resolution) ocean model in the same region, where the frequency of SI events are similar to the glider observations.