Effects of Salinity Variations in the Labrador Sea

Abstract

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.