ALBERT

Description: The depth of winter convection in the central Labrador Sea is strongly influenced by the prevailing stratification in late summer. For this late summer stratification salinity is as important as temperature, and in the upper water layers salinity even dominates. To analyze the source of the spring and summer freshening in the central region, seasonal freshwater cycles have been constructed for the interior Labrador Sea, the West Greenland Current, and the Labrador Current. It is shown that none of the local freshwater sources is responsible for the spring–summer freshening in the interior, which appears to occur in two separate events in April to May and July to September. Comparing the timing and volume estimates of the seasonal freshwater cycles of the boundary currents with the central Labrador Sea helps in understanding the origin of the interior freshwater signals. The first smaller pulse cannot be attributed clearly to either of the boundary currents. The second one is about three times stronger and supplies 60% of the seasonal summer freshwater. Transport estimates and calculated mixing properties provide evidence that its source is the West Greenland Current. The finding implies a connection also on interannual time scales between Labrador Sea surface salinity and freshwater sources in the West Greenland Current and farther upstream in the East Greenland Current. The freshwater input from the West Greenland Current thus also is the likely pathway for the known modulation of Labrador Sea Water mass formation by freshwater export from the Arctic (via the East Greenland Current), which implies some predictability on longer time scales.

Description: This study analyzes the predictivity of convection depth in the Labrador Sea based on data from the period 1992-2003 - floats, moorings, hydrographic sections, satellites and ice cover data. We show that taking into account atmospheric forcing observations, lateral fluxes and knowledge about prevailing stratification in this convection area in late summer we can infer convection depth the following winter with good accuracy. The relevance of the parameters for prediction of stratification in late summer is analyzed. Previous convection events, atmospheric forcing and eddy activity influence stratification locally. Remotely, the major variable appears to be the West Greenland Current with its upstream sea ice melt. It is possible to predict the central Labrador Sea stratification with the mentioned parameters reasonably well. This makes it possible to analyze the relevance of these remote parameters for prediction of convection depth, showing the importance of the West Greenland Current.

Description: The main focus of the thesis is the analysis of freshwater variability in the upper Labrador Sea on seasonal to decadal time scales. The seasonal freshening of the Labrador Sea and its variations play a key role in Labrador Sea deep water formation, since the freshwater has large impact on the stratification of the water column. This stratification as well as atmospheric forcing define in first order the intensity and density - thus depth - of the convection. Therefore it was the aim of this study to understand the origin, variability and path of the freshwater better. A large amount of data sources got combined to get the best possible results, including two online databases of CTD data, float data from the "Labrador Sea Deep Convection Experiment" (1996-1999), ARGO-floats and thermosalinograph data from the North Atlantic. The analyzes concentrate on 9 region within the Labrador Sea that have low horizontal salinity gradients and represent all important surface water masses. The best possible climatological seasonal salinity cycle was constructed for every region. This is for instance important for judging on anomalies in decades with only isolated measurements in a few months. The climatological salinity cycles confirm qualitatively my preliminary work in Schmidt and Send (2007). The further use of satellite SSH-anomaly measurements derived geostrophic surface currents and eddy kinetic energy (EKE) allowed a selection of high and low EKE years from the last 13 years of satellite data. These years show significant hydrographic differences in the central Labrador Sea. Years with low EKE in the Labrador Sea show an early freshening between April and May. The existence, variability and origin of this freshening was so far unsure. The freshwater pulse is not existing in years with high EKE. On the basis of changes in geostrophic surface current and variabilities within the seasonal cycle of some regions I develop a hypothesis. This hypothesis describes the origin, pathway and occurrence of the early freshening pulse in the central Labrador Sea. High EKE in the Labrador Sea seems to reduce the mean velocities of the southern West Greenland Current branch or even stops it. Since this branch is a pathway of freshwater into the northern Labrador Sea and the convection area, high EKE suppresses the freshwater flux into the Labrador Sea. Finally I analyze the pathway of decadal variations in salinity like the Great Salinity Anomaly (GSA) of the 70's. Measurements in the West Greenland Current region during times of large anomalies in the central Labrador Sea ('57, '70, '85), show the origin of these anomalies in the salty Irminger Sea waters with salinities above 34.7. With an average lag of two years these anomalies are found in the fresh shelf water of polar origin, thus significantly past the occurrence in the central Labrador Sea. This order suggests that an origin in the source of the Irminger Current, the North Atlantic Current is more likely than in the Nordic Seas. This would contradict the general believe of Great Salinity Anomalies originating from the Arctic.