Meeting Documents

Temperature and salinity stratification and the subpolar North Atlantic overturning

Abstract

The diapycnal overturning of the subpolar North Atlantic (SPNA) depends on cooling by air-sea fluxes overcoming the concurrent freshening to densify inflowing Atlantic Water. The Irminger Sea, where stratification is largely dominated by temperature, thus contributes more to the observed overturning than does the Labrador Sea, where the salinity gradient is more important. Ocean models with different background stratification in the Labrador Sea also show unrealistic overturning, hinting at the sensitivity of the SPNA diapycnal overturning to potential shifts in background stratification due to changes in Atlantic Water properties driven by meridional freshwater fluxes from the subtropics or the Arctic.

We use two observation-based products, the ECCOv4 state estimate and the EN4 analysis, to investigate the impacts of salinity and temperature changes in the SPNA on the distributions of temperature-stratified (alpha), salinity-stratified (beta), and mixed or variable (transition) oceans and on the components of the diapycnal overturning. Years when the SPNA is colder and fresher on average are associated with a larger role for salinity, making the Labrador Sea solidly beta to at least 500 m, while the transition zone in the Irminger Sea expands from the edge of the East Greenland Current to cover a broad area of the basin. In years with a warm and salty SPNA, there is a lightening of the density of maximum overturning, the Irminger Sea becomes more strongly alpha, and the transition zone moves into the southern Labrador Sea; the East Greenland current and the area off the southern tip of Greenland, however, remains a beta ocean. The time scales and depth ranges where changes in the contributions to stratification are associated with the largest differences in overturning are investigated.