Convective mixing in the central Irminger Sea: 2002–2010

Abstract

A near-continuous time series of 8 years of daily hydrographic profiles, recorded between fall 2002 and summer 2010 by moorings located in the central Irminger Sea, is presented. This record shows that convective mixing down to 400m depth occurs in most winters. Under favorable conditions, convective mixing is seen to reach much deeper. During the cold winter of 2007–2008 mixed layers reached depths of 1km. In the subsequent, more moderate winter of 2008–2009, a stronger preconditioning of the Irminger Gyre led to mixed layers down to 800m depth. The convectively formed waters in the Irminger Sea are more saline and warmer than those formed in the Labrador Sea, but potential vorticity is reduced to 0.7 10−12m−1s−1 in March 2009. Following the local wintertime convection of 2007–2008, columns of relatively fresh water were seen to arrive in the Irminger Sea in spring 2008. A comparison with float data suggests that this water mass was likely formed south–southwest of Cape Farewell. The relatively fresh water replaced the locally formed convective water mass and continued to dominate the upper 1000m of the water column until the following winter. In the winter of 2008–2009 the Irminger convective mixing strongly increased the salinity in the upper 800m, but left the intermediate salinity minimum intact. Although the water mass formed by convective mixing in the Irminger Sea differs from that formed in the Labrador Sea, the heat lost per unit area by convection is of the same order of magnitude in both basins.