Winter Mixed Layer Restratification Induced by Vertical Eddy Buoyancy Flux in the Labrador Sea

Numerical model studies have shown that the lateral buoyancy transports from eddies restratify the convection region in the Labrador Sea. However, restratification by vertical motion during and after convection has been underestimated. Here we use a model with 1 & DEG;/60 & DEG; resolution which can resolve mesoscale and submesoscale motions, and find that negative feedback that includes the generation of vertical eddy buoyancy flux (VEBF) committed to restratify the mixed layer. In winter, VEBF compensates for nearly half of the surface buoyancy loss and is as important as the lateral buoyancy fluxes in the eddy-rich region, which results in restraining the development of deep convection. During this period, the surge of VEBF was due to seasonally enhanced frontogenesis, mixed layer instability and the interaction between strong surface winds and eddies on a 10-day timescale. Therefore, well parameterizing VEBF is important in improving the representation of the deep convection in coarse-grid climate models. Wintertime deep convection in the Labrador Sea deepens the surface mixed layer to exceed 1 km. Thus, a deep-water mass will be formed when the sea surface is restratified in spring. This deep-water mass contributes to the North Atlantic Deep Water and the Meridional Overturning Circulation. Simulating deep convection accurately is significant in climate prediction. Hence, restratification processes are a vital factor in modulating deep convection and need to be understood and accurately simulated. However, climate models that fail to resolve the eddy induced stratification significantly overestimate the convection strength. Though the previous generation of eddy-rich models (& SIM;1 & DEG;/12 spatial resolution) could resolve the mesoscale eddies that bring buoyancy laterally to enhance restratification and thus decrease mixed layer depth (MLD) in winter and spring, a gap between model and observational MLD still existed. Now we use an extra-high resolution ocean model (& SIM;1 & DEG;/60) which can partly resolve submesoscale fronts and eddies in the mixed layer and demonstrate the vertical motion associated an upward buoyancy flux can enhance the mixed layer restratification during and after the deep convection. It is as important as the lateral buoyancy fluxes in the upper ocean buoyancy budget and further restricts the MLD to close to the observation. Finally, we find that upward buoyancy flux can be attributed to the intensified fronts in the strain field and wind forced mixing induced balance at fronts in this fine resolution model, which mechanically become a negative feedback effect in restraining MLD deepening. Vertical eddy buoyancy flux is as important as the lateral buoyancy fluxes in compensating surface buoyancy loss in winterVertical eddy buoyancy flux is produced by the combination of the frontogenesis process and wind-induced mixing in the mixed layerResolving the vertical eddy buoyancy flux generation mechanisms in models will enhance simulation of deep convection