Convection and restratification in the Labrador Sea, 1990–2000

Abstract

Temperature, salinity and other property distributions observed across the central Labrador Sea in the early summers between 1990 and 2000 reveal a 4-year period of exceptionally intense convection followed by 5 years of restratification. The intense convection led, in the centre of the Sea, to mixed layers increasing in density and depth to a maximum of 2300 m thereby creating a fresh deep pool of Labrador Sea Water (LSW). In the second half of the decade, warmer winter weather limited the depth of convection to ∼1000 m. The shallower convection isolated the deep reservoir of homogeneous LSW between 1000 and 2000 m from renewal: this reservoir slowly diminished in volume as the layer became more stratified. In addition, the mean temperature and salinity of the 1000–2000 m layer increased by 0.4°C and 0.025 as warmer more saline water was mixed into the central region from the boundaries. In the upper layer between 150 and 1000 m the restratification processes led to an increase in temperature of 0.6°C but no significant change in salinity. The upper 150 m also showed no discernible trends in salinity but did participate in the warming trend. Interannual variability in local atmospheric forcing accounts for much of the observed change in heat content in the convectively overturned part of the water column during both the convection and restratification phases. It is proposed that constant horizontal fluxes transport heat and salt from the boundaries into the centre of the Sea. When the heat loss from the sea surface is greater than the horizontal flux the mixed layer becomes colder and denser and the depth of convection increases. When the heat loss is less than the horizontal flux and the convection remains shallow the temperature rises in both the 0–1000 m and the 1000–2000 m layers and salinity increases in the deeper layer. In both situations salinity in the upper 1000 m remains roughly constant as the horizontal salinity flux approximately offsets the annual input of fresh water of 60±10 cm into the surface layer.