Warming of Greenland Sea Deep Water Induced by Abyssal Mixing

Abstract

In the absence of deep convection, the Greenland Sea Deep Water has experienced a slow but significant warming during the 1980s and ´90s. Enhanced vertical mixing can explain the observed trends of several propertiesincluding anthropogenic tracers, but the necessary mixing ratesare at the higher end of those observed elsewhere in the ocean.In this work, the mixing is studied by means of the spatial distribution and strength of eddy diffusivities. These are calculated from energy dissipationrates, which in turn are estimated with two complementary methods.Thorpe scales provide a direct estimate of the dissipation from densityoverturns. The second method uses a spectral estimate of the finescale variances of velocity shear density strain as proxies for the energy content of the internal wave field, and the equlibrium energy dissipation rate. Both methods are applied on a data set obtained duringsummer 1998, comprising loweredADCP measurements of the velocity field in the central Greenland Sea and near the surrounding ridge systems, as well assupplementary temperature and salinity measurements.The diapycnal diffusivities observed in the Greenland Sea are highenough to account for the changes in deep water. The mean diffusivity across the 2000~m isobath is 1.2 x 10:sup:-3:/sup: m:sup:2:/sup:/s,two orders of magnitude larger than the typical deep ocean background.The highest values occur in the deep basin, with a moderate amplification in the vicinity of rough topography at mid depth.In the upper layer, the locations of strongest mixing are close tothe fronts of the boundary currents.Enhanced mixing in the deeper layers is not confined to rough topography, but occurs throughout the whole basin. The critical latitudes of most semidiurnal tides are located in the Greenland Sea, therefore thisdistribution is interpreted as a result of the resonant breakdown ofthe tidal waves.