Mesoscale and submesoscale variability in Drake Passage, Southern Ocean: observations from ocean gliders

The Southern Ocean is one of the most energetic regions of the global ocean. Its unique environment, characterized by strong, equivalent barotropic zonal flows and the outcropping of a broad range of density classes with outcrop, means that its mesoscale and submesoscale characteristics may be very different from other regions of the ocean. Observations of the Southern Ocean's subsurface velocity and density structure at mesoscale and submesoscale resolution are extremely limited. Here we present results deriving from observations collected by ocean gliders across two years. The first deployment, carried out during austral summer 2014-2015, used two gliders to focus on the interaction between the southern boundary of the Antarctic Circumpolar Current with the continental shelf and slope of the Antarctic Peninsula. Across three months of observations, we show that there are abrupt changes in mixed layer depths, eddy formation process and lateral surface buoyancy gradients occur across the Shackleton Fracture Zone (SFZ). We will also present recent observations from two ocean gliders deployed in southern Drake Passage in austral fall and winter, 2016 (May through August). These gliders completed multiple meridional transects across Drake Passage, including crossings of both the Polar Front and the Subantarctic Front. The gliders were flown in parallel for most of the mission with a spatial separation of approximately 10 km, permitting a three-dimensional calculation of buoyancy gradients in the upper 1000 m at submesoscale resolution. Preliminary results show that stratification and velocity structures indicative of submesoscale instabilities are strongly localized by the position of the ACC's frontal currents. Thin intrusions with vertical scales as small as 20 m as also pervasive in these frontal regions. This data set will be used to characterize the type and frequency of submesoscale instabilities that occur in winter when mixed layer depths are between 100 and 200 m deep in this region.