OBSERVATIONS OF VERTICAL CURRENTS AND CONVECTION IN THE CENTRAL GREENLAND SEA DURING THE WINTER OF 1988-1989

During the winter of 1988-1989 five acoustic Doppler current profilers (ADCPs) were moored in the central Greenland Sea to measure vertical currents that might occur in conjunction with deep mixing and convecti on. Two ADCPs were looking up from about 300 m and combined with therm istor strings in the depth range 60-260 m, two were looking downward f rom 200 m, and one was looking upward from 1400 m. First maxima of ver tical velocity variance occurred at two events of strong cold winds in October and November when cooling and turbulence in the shallow mixed layer generated internal waves in the thermocline. Beginning in late November the marginal ice zone expanded eastward over the central Gree nland Sea, reaching its maximum extent in late December. In mid-Januar y a bay of ice-free water opened over the central Greenland Sea, leavi ng a wedge of ice, the ''is odden,'' curled around it along the axis o f the Jan Mayen Current and then northeastward and existing well into April 1989. Below the ice a mixed layer at freezing temperatures devel oped that increased in thickness from 60 to 120 m during the period of ice cover, corresponding to an average heat loss of about 40 W m-2. T hrough brine rejection, mixed-layer salinity increased steadily, reduc ing stability to underlying weakly stratified layers (Roach et al., 19 93). During the ice cover period, vertical currents were at a minimum. After the opening of the ice-free bay, successive mixed-layer deepeni ng to >350 m occurred in conjunction with cooling events around Februa ry 1 and 15, accompanied by strong small-scale vertical velocity varia tions. Upward mixing of more saline waters of Atlantic origin during t his phase reduced the stability further, generating a pool of homogene ous water of >50 km horizontal extent in the central Greenland Sea, pr econditioned for subsequent convection to greater depths. Individual c onvection events were observed during March 6-16, associated with down ward velocities at the 1400-m level of about 3 cm s-1. One event was i dentified as a plume of about 300-m horizontal scale, in agreement wit h recently advanced scaling arguments and model results, and with earl ier similar observations in the Gulf of Lions, western Mediterranean. The deep convection occurred in the center of the ice-free bay; hence brine rejection did not seem necessary for its generation. Plume tempe ratures at 1400 m were generally higher than that of the homogeneous s urface pool, suggesting entrainment of surrounding warmer waters on th e way down. Mean vertical velocity over a period of convection events was indistinguishable from zero, suggesting that plumes served as a mi xing agent rather than causing mean downward transport of water masses . However, different from the surface pool that was governed by mixed- layer physics, the water between 400 and 1400 m was not horizontally h omogenized in a large patch by the sporadic plumes. Overall, and compa red to results from the Gulf of Lions, convection activity in the cent ral Greenland Sea was weak and limited to intermediate depths in winte r 1988-1989.