The Labrador Sea is one of the deep convection sites in the world's oceans and the water masses formed here are an important component of the Atlantic Meridional Overturning Circulation (AMOC). To study this linkage, one study in particular used an idealized model of the Labrador Sea where the density variations consists only of temperature variations. In this study, it is questioned whether the assumption of neglecting salinity is appropriate, by analysing the pathways of water mass and water mass transformation in the Labrador Sea.
This is investigated by using that same idealized model (here called the reference run) and comparing this to a model where salinity variations are added whilst keeping density variations the same (Sconstant) to produce a similar circulation pattern. Furthermore, a model configuration is created which investigates if a seasonal cycle in salinity impacts the circulation pattern of the Labrador Sea (Sseasonal). The pathways of water masses in these model configurations are analyzed by Lagrangian particle tracking from A to B.
It was found that with the same initial density variations the maximum surface eddy kinetic energy (EKE) increases by 41 % when salinity is incorporated in the model. An increase in EKE is often associated with more water mass leaving the boundary current (BC) due to an increase in instabilities. Surprisingly, the opposite was found: 7.02 and 8.22 Sv are transported through the BC for the reference run and Sconstant, respectively. Furthermore it was found that most of the water mass leaves and re-enters the BC near the maximum EKE for each model configuration. An increase was found in maximum overturning in density space from an Eulerian perspective: from 3.9 to 4.8 Sv for the reference run and Sconstant, respectively, where about 10 % so called density compensation occurred for Sconstant. No significant annual changes are found when adding a seasonal cycle to the model. For all model configurations a large discrepancy exists between Eulerian and Lagrangian calculations in downwelling. This discrepancy is due to Lagrangian particles that reside in the models at the end of their simulation duration Thus the overturning in the Labrador Sea is significantly influenced by particles that have a long residence time (longer than 4 years in these model simulations). Between 22 and 25 % of the Lagrangian volume transport does not reach the outflow of each model simulation.
There are also properties that salinity did not influence: no significant changes were found between the model configurations for the overturning in depth space, the annual MLD and barotropic streamfunction. In conclusion adding salinity to the idealized model showed only minor changes in the pathways of water mass and water mass transformation: the order of magnitude of all analyzed properties stays the same. Density compensation however is neglected when no salinity variations are added in the model. This means that for a highly idealized model of the Labrador Sea, salinity variations can be neglected, when density variations due to salinity variations are represented by temperature variations. ... Read more

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. ... Read more