Submesoscale dynamics of dense overflows. Part I: Potential vorticity fluxes and symmetric instability

Victoria Whitley , Jacob O. Wenegrat, Leif Thomas

Dense overflows descending from continental shelves are a key component of the global overturning circulation, yet the submesoscale processes that shape their potential vorticity (PV) structure and drive cross-shelf exchange remain poorly understood. This study uses a three-dimensional large-eddy simulation of a dense overflow to examine how surface cooling and the descending gravity current set the PV structure of the overflow and precondition the shelf for symmetric instability (SI). Surface cooling generates a geostrophically balanced, along-slope flow and topography-aligned isopycnals, while the descending gravity current injects positive PV at the shelf break that is then advected down the slope, capping the overflow with elevated PV in the stratified interior. This high PV contrasts with previous overflow simulations, which found thin bands of negative PV in the interior and attributed them to SI. Here the resolved bottom PV injection keeps the interior symmetrically stable. Instead, SI is only found on the shelf where surface cooling leaves a negative PV anomaly. The resulting SI cells propagate toward the shelf break, driving episodic subinertial off-shelf export of dense water. Together, these results show that boundary PV fluxes generated by dense overflows can control cross-shelf export and a lasting interior PV signature within deep boundary currents, with implications for representing water-mass transformation in the ocean overturning circulation. This is part I of II, with the companion paper focusing on how this background structure modifies the propagation of near-inertial waves.