Topographic forcing of submesoscale instability in the Antarctic Circumpolar Current

TL;DR

This study uses a hindcast model to explore how topography influences submesoscale instabilities like symmetric and centrifugal instability in the Antarctic Circumpolar Current, revealing widespread subsurface mixing along continental margins.

Contribution

It demonstrates the role of topographic shearing in generating subsurface instabilities and persistent mixing in the ACC region, advancing understanding of submesoscale dynamics near complex topography.

Findings

Subsurface SI and CI are widespread along Antarctic continental margins.

Topographic shearing facilitates persistent mixing along specific banks.

Instabilities are linked to low potential vorticity and seamounts or slopes.

Abstract

Subpolar frontal zones are characterized by energetic storms, intense seasonal cycles, and close connectivity with surrounding continental shelf topography. At the same time, predicting the ocean state depends on appropriate partition of resolved and parameterized dynamics, the latter of which requires understanding the dynamical processes generating diffusivity throughout the water column. While submesoscale frontal instabilities are shown to produce turbulent kinetic energy (TKE) and mixing in the surface boundary layer (SBL) of the global ocean, their development in complex dynamical regimes (e.g., elevated preexisting turbulence, large ageostrophic shear, or in proximity to topographic boundaries) is less understood. This study investigates the development of submesoscale instabilities, i.e. symmetric instability (SI) and centrifugal instability (CI), near topographic boundaries using a hindcast model of the Drake Passage and Scotia Sea region. The model suggests subsurface SI and CI are widespread along the northern continental margins of the Antarctic Circumpolar Current (ACC) due to topographic shearing of the anticyclonic side of Polar Front jets. Forced instabilities may facilitate persistent mixing along Namuncur\'a - Burwood Bank, as well as in other southern (northern) hemisphere currents with low potential vorticity and a seamount or sloping topography on the left- (right-) downstream side.