Hydrographic variability in the Irminger Sea

This thesis deals with the hydrography of the northwestern North Atlantic Ocean, particularly the Irminger Sea. The data sets used for this study include historical observations (since 1950), near-annual observations (since 1990) of the AR7E section from Greenland to Ireland and daily observations (between 2003 and 2008) from two moorings in the centre of the Irminger Sea. A multi-decadal variability is seen in the upper 2 km of the Irminger Sea and the nearby Labrador Sea. This variability includes a maximum in temperature and salinity around 1970 followed by a minimum in the late 1980s and early 1990s followed by an increase until present. This multi-decadal variability seems to be caused by correlating changes in the atmospheric heat flux and the wind stress curl. Deep convective mixing in the Labrador Sea, occurring every 10 year, distributes the sea-to-air heat loss over the water column. Through advection, the convective mixing in the Labrador Sea largely deter mines the hydrography of the North Atlantic. The convectively formed Labrador Sea Water (LSW) spreads to the Irminger Sea (2 year) and the eastward located Iceland Basin (5 year). The LSW decays by advection and lateral mixing with the more saline Icelandic Slope Water. The low temperature and salinity signal of the LSW returns in the North East Atlantic Deep Water in the Irminger Sea, with a delay of 2 year. For the Denmark Strait Overflow Water, the fast hydrographic variability described by Dickson et al. (2003) reappears in the observations included here. Although, the freshening trend observed between 1965 and 2000 changed into a weaker salinifying trend observed between 2001 and 2008. Notably, the range of the sub-annual variability exceed the inter-annual variability at all depths. Denmark Strait Overflow Water (DSOW) shows occasional rapid drops in salinity and temperature as well as a non-linear annual cycle in temperature. Both types of variability are assumed to be related t o changes in the atmospheric forcing over Denmark Strait. In the upper layers, convective mixing down to at least 400 m is seen in each winter in the Irminger Sea. The deepest mixed layers (1000 m) were observed during the cold winter of 2007-2008. The local mixing modifies the Subpolar Mode Water of the central Irminger Sea, and through advection add to the preconditioning of the Labrador Sea. A 1D model study showed that preconditioning is most important for deep convective mixing in the Irminger Sea, more so than in the Labrador Sea. A large part of the heat loss in winter is utilized to remove the stratification over the upper 500 dbar. Very cold and long-lasting winters are needed to reach deep below this level. A model comparison study showed that most coupled climate models have difficulty in simulating the hydrography of the study area. This seems to be due to drift of the oceanographic state during the spin up period of the model and large differences between the simulated and observed mixing regimes.