Water Mass Circulation and Variability in the Subpolar North Atlantic

Abstract

This study focuses on estimating the variability in the formation of Upper and classical Labrador Sea Water (ULSW and LSW). Both are formed by winterly convection and spread into the world ocean as part of the cold limb of the thermohaline circulation. Analyses are based on a large-scale hydrography/tracer data set from the years 1997, 1999, and 2001. Horizontal fields of water mass layer thickness and mean concentrations of chlorofluorcarbon (CFC) have been constructed to determine the CFC inventory of each water mass and to infer water mass formation rates. The years 1997-2001 showed a significant increase in the CFC inventory of ULSW, while the inventory of classical LSW reduced. During 1997-1999 formation of ULSW was strong (6.9-9.2 Sv). From 1999 to 2001 the ULSW formation rate reduced to 3.7-4.0 Sv. LSW formation was absent during these four years. Historical hydrographic data from the Labrador Sea have been used to compare water mass properties of ULSW and LSW on longer time scales. Time series indicate strong variability and a significant anti-correlation of ULSW and LSW formation. Coinciding with weakening convection the density surface that separates ULSW from classical LSW shifted to greater depths. Water layer lying on top of LSW revealed an increasing stratification which is presumably strengthened by warm and saline water intruding from the West Greenland Current into the interior Labrador Sea. Time series of sea surface fluxes indicated a change in the atmospheric conditions after 1995/96. The convection activity at that time was, however, sufficient to ventilate the ULSW layer. Analyses of deep and bottom water properties provided evidence for the existence of export pathways in the Newfoundland Basin that are additional to the Deep Western Boundary Current (DWBC). LSW spreading time scales point to a fast spreading in the DWBC (1-2 years from the Labrador Sea to 43°N) and a slow spreading in the interior Newfoundland Basin (3-6 years).