A Seasonal Cycle of Submesoscale Open-Ocean Motions from Gliders

The importance of submesoscale instabilities, particularly mixed layer instability and symmetric instability (SI), on upper ocean mixing has been well documented in strong, persistent fronts such as the Kuroshio and Gulf Stream western boundary currents. The prevalence and importance of these dynamics on near-surface stratification in the open ocean, where submesoscale fronts are more often associated with coherent mesoscale eddies, is more complex and poorly constrained. We present a unique time series of near-surface lateral buoyancy distributions, potential vorticity and submesoscale instability characteristics from pairs of ocean gliders deployed through a full annual cycle. The ocean gliders continuously sampled a 400 square kilometer region of the northeast Atlantic ocean collecting measurements of temperature, salinity, dissolved oxygen, fluorescence and optical backscatter.

The glider observations show a distinct seasonal trend in near-surface mixing. Throughout the late summer and fall, the mixed layer gradually deepens predominantly through gravitational instability, suggesting that surface cooling dominates submesoscale re-stratification processes. During winter months, mixed layer depths are considerably more intermittent. During this period, estimations of the Richardson angle (a measure of the ratio of lateral and vertical buoyancy gradients) show persistent occurrences of SI. These events coincide with observations of the injection of near surface properties such as salinity and fluorescence, into the interior, consistent with strong mixing along isopycnal surfaces. A parameterization for the rate of kinetic energy extracted by SI is used to estimate a turbulent dissipation contribution.