Variability in Labrador Sea Water formation

Abstract

The Atlantic Meridional Overturning Circulation (AMOC) transports of a large amount of heat towards the North Atlantic region. Since this circulation is considered to have shown pronounced variability in the past, and a weakening is projected for the 21st century, it is very important to understand and monitor the mechanisms that determine its variability. Deep water formation is one of the most important of these mechanisms as it plays an important role in setting the shape and strength of the AMOC. It only takes place in a few locations in the ocean, one of which is the Labrador Sea. In this thesis two processes that play an important role in determining the variability of Labrador Sea Water formation are studied as well as the possibility to monitor this variability using satellite altimetry measurements.

The first process study focused on the restratification period after a deep convection event. The dense water in the area affected by deep convection is then (partly) replaced by more buoyant water originating from the boundary currents that encircle the interior. Using a numerical model in an idealized configuration, the roles of three eddy types that are known to play a role in the restratification process were studied. It was found that the presence of Irminger Rings is essential for a realistic amount of restratification in the Labrador Sea.

The second process study focused on the effects of a very fresh surface layer, which makes the surface layer lighter and can, if light enough, inhibit convective mixing. The well-known case of the Great Salinity Anomaly (1969-1971), which was fortuitously well documented by the measurements taken at ocean weather station “Bravo” in the central Labrador Sea, has been analyzed. In contrast to what is commonly assumed, only a combination of the fresh surface layer and the very mild winter conditions in 1969 could have started the convective shutdown, and only a combination of the extremely harsh winter and a salinification of the upper water column could have caused its resumption in 1972. Moreover, two so far undiscovered positive surface feedbacks were found (both acting through the low wintertime sea surface temperature) that limit the buoyancy flux to the atmosphere and thereby actively reinforce the shutdown state.

Apart from understanding the variability in Labrador Sea Water formation, it is also important to monitor this variability. The network of satellite altimeters does not suffer from limitations as harsh winter conditions and poor coverage and can therefore give valuable additional information to in situ measurements. Altimeters can detect the sea surface lowering that accompanies the densification of the water column during deep water formation. Although this signal is small compared to variability in sea surface height induced by other processes, still the approximate depth of deep convection (less than 1000 m, between 1000 and 1500 m or more than 1500 m depth) and the location of the convection area at a larger scale can be determined