Analysis on Governing Processes of Mixed Layer Depth Variability in the Labrador Sea

Abstract

As a key component to the bottom limb of the Atlantic Meridional Overturning Circulation (AMOC), the Labrador Sea is one of the regions where deep ocean convection takes place. This convection is driven by atmospheric cooling during winter, which brings the surface water into the intermediate and deep layers by uniformizing water mass properties. This homogeneous layer is called Mixed Layer (ML). As a result of this convection, stratification is no longer maintained, and the Mixed Layer Depth (MLD) deepens. During this deepening, an enormous amount of potential energy is converted to kinetic energy, and meso- and sub-mesoscale instabilities develop. After wintertime, the MLD starts to shallow again. Atmospheric-induced convection ceases or decreases significantly and physical components return to stratified conditions. Baroclinic instabilities grown to mesoscale or geostrophic scale play a role in restratifying the ML through the formation of coherent ocean eddies. This chain of processes follows a seasonal cycle that strongly depends on the imbalance between horizontal and vertical buoyancy gradients. A practical way to quantify this imbalance is the use of the Ertel potential vorticity or a derived magnitude as the Richardson angle, which allow to infer the existence of instabilities and to classify them respectively.
This study analyzes the physical processes behind the MLD seasonal variability in the Labrador Sea. To this end, high-resolution model data (1/12° × 1/12°) from a global simulation has been used. An evaluation of spatial and temporal patterns of the MLD and energy conversion is provided, and the dominant types of instabilities are determined. It is hypothesized that these instabilities drive the energy conversion and the growth of coherent mesoscale eddies, which can modify the MLD and restratify the ocean. Finally, the sequential interactions among the processes are investigated to provide better understanding about seasonal MLD variability. This study shows that the density-based MLDs with a threshold of 0.03 kg m^-3 are the most credible values, and the spatial and temporal patterns of energy conversion and gravitational/symmetric instabilities are in phase with the MLD variability. The energy conversion is investigated by means of the available potential energy (APE), kinetic energy (KE) and Energy Ratio (ER) which is introduced in this study, and a large amount of gravitational and/or symmetric instabilities is found within ML, especially in the upper ocean layers. The role of baroclinic instabilities is investigated with the Eady growth rate, while the presence of coherent mesoscale eddies is inferred from the Okubo-Weiss parameter and the Eddy Kinetic Energy, whose size is limited by the internal Rossby radius. This study shows that the MLD variability is the result of changes in the conversion between the available potential energy (APE) and kinetic energy (KE) as well as of the competition between ravitational/symmetric and baroclinic instabilities. The former favoring MLD deepening, and the latter favoring MLD shallowing.