Deep water changes at the western boundary of the subpolar North Atlantic during 1996 to 2001

Repeated hydrographic observations between 1996 and 2001 of the deep water mass distribution on four sections in the western Labrador Sea and northwestern North Atlantic at about 56degreesN, 53degreesN, 48degreesN and 43degreesN show significant changes in the water mass characteristics. These changes are spreading southward mainly with the Deep Western Boundary Current (DWBC). Shallower convection forms a convective water mass known as upper Labrador Sea Water (ULSW). During periods of deep convection in the Labrador Sea, ULSW was described to be formed in the western boundary current region. In the post deep convection period 1996 to 2001 ULSW was formed in the western and central Labrador Sea and spreads mainly westward towards and along the western boundary. At 53degreesN ULSW moves southward as a part of the deep Labrador Current, also constituting the upper part of the DWBC. In the early 1990s the deep convection produced a large volume of deep Labrador Sea Water (LSW) which filled intermediate layers of the central region of the Labrador Sea. After these years the convection became weaker, with no apparent LSW renewal in 1996, partial mixing down to 1500m in 1997 and no notable LSW formation between 1998 and 2001. At the southwestern exit of the Labrador Sea at 53degreesN the deep LSW in 2001 was least in thickness and highest in salinity and temperature compared to the years since 1996. This reflects restratification which resulted in an increase in the density stratification between 1000 and 2000 in in the central Labrador Sea as well as year-to-year transformation of the LSW core. LSW passes 43degreesN off the Grand Banks about 1 to 2 years after it was first seen at 56degreesN. At the 48degreesN and 43degreesN sections the northward flowing North Atlantic Current (NAC), farther offshore than the DWBC, complicates the property distributions. Saltier and warmer LSW recirculates northward with the NAC at 43degreesN. Between 1996 and 2001 the Gibbs Fracture Zone Water (GFZW) turned colder and fresher. The Denmark Strait Overflow Water (DSOW) showed two periods of cooling and freshening, separated by an abrupt (rapid) increase in temperature and salinity within a year. The arrival time of this increase at the different locations implies a DSOW spreading time that is no more than two years from 56degreesN to 43degreesN near the western boundary, or four years from the sill of the Denmark Strait to the Grand Banks. (C) 2004 Elsevier Ltd. All rights reserved.