Classifying Submesoscale Instabilities in the Antarctic Circumpolar Current using Seagliders

Submesoscale motions influence vertical velocities in the upper ocean and may generate flows that penetrate the strong, persistent buoyancy gradients found at the base of the mixed layer. These strong velocities are not currently captured by large-scale ocean models. In addition, submesoscale instabilities have a large impact on the energetics of the upper ocean in strong fontal regions, such as western boundary currents. However, despite its importance in setting global transport and vertical exchanges of nutrients and gases, studies of submesoscale motions in the Antarctic Circumpolar Current (ACC) have been limited due to a lack of observations. 

During the ChinStrAP (Changes in Stratification at the Antarctic Peninsula) project, two Seagliders were deployed northwest of the Antarctic Peninsula over a period of four months from December 2014 to April 2015, providing an unprecedented data set to investigate submesoscale phenomena. Over 40 distinct hydrographic sections were collected across the continental shelf break, comprising more than 1400 individual profiles. Temperature, salinity, and pressure data were collected at a horizontal resolution of less than 5 km. The data enable calculations of lateral buoyancy gradients and potential vorticity over an entire austral summer season and into the early fall. From these, the balanced Richardson angle (the ratio of the vertical to lateral buoyancy gradients) is used to classify submesoscale instabilities (PV < 0), including instances of gravitational, symmetric, and centrifugal instability. In addition, atmospheric reanalysis data is analyzed to examine changes in mixed layer depths following synoptic storms. The results emphasize the significant role that submesoscale motions play in modulating the near-surface stratification at a key location for the ventilation of deep density classes.