Cessation of Labrador Sea Convection by Freshening through (Sub)mesoscale Flows

Abstract

By ventilating the deep ocean, deep convection in the Labrador Sea plays a crucial role in the climate system. Unfortunately, the mechanisms leading to the cessation of convection and, hence, the mechanisms by which a changing climate might affect deep convection remain unclear. In winter 2020, three autonomous underwater gliders sampled the convective region and both its spatial and temporal boundaries. Both boundaries are characterised by higher sub-daily mixed-layer depth variability than the convective region. At the convection boundaries, buoyant intrusions--including eddies and filaments--primarily drive restratification by bringing freshwater, instead of warm warmer, and instead of atmospheric warming. At the edges of these intrusions, submesoscale instabilities, such as symmetric instabilities and mixed-layer baroclinic instabilities, seem to contribute to the decay of the intrusions. In winter, strong destabilising surface heat flux and along-front winds can enhance the lateral stratification, sustaining submesoscale instabilities. Consequently, winter atmospheric conditions and freshwater intrusions participate in halting convection by adding buoyant freshwater into the convective region through submesoscale flows. This study reveals freshwater anomalies in a narrow area offshore of the Labrador Current and near the convective region; this area has received less attention than the more eddy-rich West Greenland Current, but is a potential source of freshwater in closer proximity to the region of deep convection. Freshwater fluxes from the Arctic and Greenland are expected to increase under a changing climate, and our findings suggest that they may play an active role in the restratification of deep convection.