Water Mass Circulation and Variability in the Subpolar North Atlantic

The present study deals with the formation of Upper and classical Labrador Sea Water (ULSW and LSW) which occurs in the western part of the subpolar North Atlantic (Labrador Sea). Both water masses represent the upper part of North Atlantic Deep Water. As part of the cold limb of the thermohaline circulation they spread into the world ocean. Various publications revealed that water mass properties of classical LSW are subject to variability on intraseasonal to interdecadal time scales. A significant weakening of the Labrador Sea convection intensity throughout the 1990s resulted in the formation of a lighter water mass, termed ULSW. The focus of the present study is on estimating the variability in the formation of ULSW. This is compared to formation of the denser classical LSW. The analysis is based on a comprehensive hydrography/tracer data set from the years 1997, 1999, and 2001. Horizontal distributions of water mass layer thickness and mean concentrations of anthropogenic chlorofluorcarbon (CFC, component CFC-11) have been constructed from single profiles. The function describing the input of these gases into the oceans is well known. Different methods have been compared to handle data gaps which occurred in the fields of 1999 and 2001. Within the error margins these gave consistent results. For the ULSW layer a significant increase in the CFC-11 inventory was found in the period 1997-2001. At the same time the CFC-11 inventory of classical LSW reduced. The CFC inventories have been used to infer water mass formation rates. A significant formation of ULSW was found during 1997-1999 (6.9-9.2 Sv). In the subsequent two years the ULSW formation rate reduced to 3.7-4.0 Sv. LSW formation was absent during these four years. Water mass properties of ULSW and LSW have been compared in the central Labrador Sea based on data from 1990-2001. Coinciding with weakening convection the isopycnal σΘ =27.74 kg/m3 shifted to greater depths. Per definition this density surface separates ULSW from classical LSW. The 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 surface fluxes (momentum, heat, buoyancy) indicate a change in the atmopsheric conditions after 1995/96. The convection activity was, however, sufficient to ventilate the lighter ULSW layer. So far ULSW formation was only observed in the boundary current of the Labrador Sea. The period of weakened convection, however, favoured considerable ULSW formation in the central Labrador Sea. A reconstruction of decadal time series indicated strong variability and a significant anti-correlation of ULSW and LSW formation (r = -0.85). Furthermore, deep water spreading pathways have been analysed in the Newfoundland Basin. The question is addressed whether deep water export pathways additional to the boundary current exist. The analysis of deep and bottom water properties indeed provided evidence for the existence. The estimation of LSW spreading time scales point to a fast spreading in the boundary current (1-2 years from the Labrador Sea to 43 ◦N) and a slow spreading in the interior Newfoundland Basin (3-6 years, depending on location). The analysis of horizontal gradients of sea surface temperatures revealed similar spatial patterns as was found in the deep water circulation of model studies. A relation between deep water circulation and variability in the pathway of the North Atlantic Current is considered as a likely cause for offshore deep water spreading. The reconstruction of ULSW/LSW time series in the boundary current system of the Newfoundland Basin revealed a strong correlation with changes in the formation region.