The Annual Cycle of Upper-Ocean Potential Vorticity and Its Relationship to Submesoscale Instabilities

Xiaolong Yu,Alberto C. Naveira Garabato,Adrian P. Martin,David P. Marshall

doi:10.1175/jpo-d-20-0099.1

Abstract

AbstractThe evolution of upper-ocean potential vorticity (PV) over a full year in a typical midocean area of the northeast Atlantic is examined using submesoscale- and mesoscale-resolving hydrographic and velocity measurements from a mooring array. A PV budget framework is applied to quantitatively document the competing physical processes responsible for deepening and shoaling the mixed layer. The observations reveal a distinct seasonal cycle in upper-ocean PV, characterized by frequent occurrences of negative PV within deep (up to about 350 m) mixed layers from winter to mid-spring, and positive PV beneath shallow (mostly less than 50 m) mixed layers during the remainder of the year. The cumulative positive and negative subinertial changes in the mixed layer depth, which are largely unaccounted for by advective contributions, exceed the deepest mixed layer by one order of magnitude, suggesting that mixed layer depth is shaped by the competing effects of destratifying and restratifying processes. Deep mixed layers are attributed to persistent atmospheric cooling from winter to mid-spring, which triggers gravitational instability leading to mixed layer deepening. However, on shorter time scales of days, conditions favorable to symmetric instability often occur as winds intermittently align with transient frontal flows. The ensuing submesoscale frontal instabilities are found to fundamentally alter upper-ocean turbulent convection, and limit the deepening of the mixed layer in the winter-to-mid-spring period. These results emphasize the key role of submesoscale frontal instabilities in determining the seasonal evolution of the mixed layer in the open ocean.

Highlights

The ocean’s surface mixed layer is a vital component of the global climate system, as it mediates the exchanges of physical and biogeochemical tracers between the atmosphere and the ocean interior
Our results portray a picture of the mixed layer as a highly variable and dynamically active boundary layer experiencing large changes in depth and stratification on time scales as short as days, and whose seasonal cycle arises as a small residual between much larger contributions from destratifying and restratifying processes
The diabatic potential vorticity (PV) flux dominates on time scales of months, and underpins the local seasonal cycle in mixed layer depth and stratification

Summary

Introduction

The ocean’s surface mixed layer is a vital component of the global climate system, as it mediates the exchanges of physical and biogeochemical tracers between the atmosphere and the ocean interior. Cumulative integrals of the positive and negative subinertial changes in H (Fig. 4) indicate that the mixed layer base undergoes frequent and large vertical excursions in winter to midspring, but is relatively invariable during the rest of the year This suggests that the winter-to-mid-spring period hosts a persistent and vigorous competition between destabilizing atmospheric forcing and restratifying upper-ocean processes, such as submesoscale frontal instabilities. Our results suggest that this type of event was a relatively frequent occurrence in our study area during the winter-to-mid-spring period

Upper-ocean potential vorticity budget

Findings

Conclusions