On the role of freshwater forcing on the convection intensity in the central Irminger Sea between 2002 and 2011

The Central Irminger Sea has long been identified as one of the open ocean deep convections regions in the North Atlantic Ocean. Here surface waters may reach densities that are high enough to contribute to the re-ventilation the Labrador Sea Water density range which is part of the North Atlantic Deep Water. The ‚efficiency‛ of the deep convection has been found to depend on local (Greenland Tip Jet) as well as large scale (North Atlantic Oscillation) atmospheric forcing. Most likely because of unavailability of data the role of the salinity in the near surface waters have been ignored in the investigations of deep convection in the Irminger Sea. Based on temperature and salinity time series data from late 2002 to now from an open ocean mooring in the central Irminger Sea (nominal 60°N/40°W) the convection depth at this location is derived. The time series are used to estimate the heat and freshwater fluxes and their respective contribution to the buoyancy budget. It is shown that the freshwater fluxes can have a significant influence on the evolution of the buoyancy budget and thus in setting the convection depth. In the period 2002 to 2007 the convection depth was less than 500m. In the last winters from 2007 onwards particular higher upper layer salinity contributed to a much deeper convection depth down to about 700m. However, the buoyancy flux was not strong enough to ventilate the NADW density range at this location. As the mooring is equipped with a real time data transmission buoy an estimate for the current (2010/2011) winter convection intensity is presented