Overturning instability in forced ageostrophic oceanic flows

TL;DR

This paper develops new criteria for overturning instability in forced ageostrophic oceanic flows, highlighting the importance of ageostrophic shear effects in subpolar regions with complex topography.

Contribution

It introduces a framework that accounts for ageostrophic shear effects, deviating from traditional geostrophic PV criteria, and demonstrates its application in realistic ocean models.

Findings

Ageostrophic shear increases overturning instability by up to 20%.

Traditional geostrophic criteria may underestimate instability in forced regimes.

Layer-resolved measures are necessary for accurate instability diagnostics.

Abstract

The subpolar oceans are characterized by intense storm forcing and complex littoral topography. Submesoscale frontal instabilities are significant sources of turbulent kinetic energy (TKE) in these regions. However, criteria for identifying and parameterizing these instabilities in regional models have predominantly relied on a geostrophic framework that neglects generalized ageostrophic shear. We derive criteria for overturning instability that account for stabilizing and destabilizing effects of ageostrophic shear on mechanically forced boundaries, deviating from the geostrophically derived potential vorticity (PV) criterion, $qf < 0$. Ageostrophic forcing modifies stability from that implied by the vertical PV structure underlying bulk surface boundary layer diagnostics, which may limit the applicability of such bulk criteria in strongly forced regimes and motivate the need for layer-resolved measures. We demonstrate their application using a feature model of a wind-forced jet, as well as a 1-km Regional Ocean Modeling System (ROMS) hindcast of the high North Atlantic, and assess the importance of forced ageostrophic overturning instability (AOI) in intense frontal zones. In the feature model, ageostrophic shear increases overturning instability by up to 20%, compared to a strictly geostrophic framework.