Observations of vertical currents and convection in the central Greenland Sea during the winter of 1988–1989

Abstract

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 convection. Two ADCPs were looking up from about 300 m and combined with thermistor strings in the depth range 60--260 m, two were looking downward from 200 m, and one was looking upward from.1400 m. First maxima of vertical 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 wav!!S in the thermocline. Beginning in late November the marginal ice zone expanded eaktward over the central Greenland Sea, reaching its maximum extent in late December. In mid-January a bay of ice-free water opened over the central Greenland Sea, leaving a wedge of ice, the "is odden," curled around it along the axis of the Jan Mayen Current and then northeastward and existing well into April 1989. Below the ice a mixed layer at freezing temperatures developed 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 . Through brine rejection, mixed-layer salinity increased steadily, reducing stability to underlying weakly stratifi ed layers (Roach et al., 1993). During the ice cover period, vertical currents were at a minimum. After the opening of the ice-free bay, successive mixed-layer deepening to > 350 m occurred in conjunction with cooling events around February 1 and 15, accompanied by strong small-scale vertical velocity variations. Upward mixing of more saline waters of Atlantic origin during this phase reduced the stability further, generating a pool of homogeneous water of> 50 km horizontal extent in the central Greenland Sea, preconditioned for subsequent convection to greater depths. Individual convection events were observed during March 6--16, associated with downward velocities at the 1400-m level of about 3 cm s -1 • One event was identified as a plume of about 300-m horizontal scale, in agreement with recently advanced scaling arguments and model results, and with earlier 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 temperatures at 1400 m were generally higher than that of the homogeneous surface pool, suggesting entrainment of surrounding warmer waters on the way down. Mean vertical velocity over a period of convection events was indistinguishable from zero, suggesting that plumes served as a mixing 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 homogenized in a large patch by the sporadic plumes. Overall, and compared to results from the Gulf of Lions, convection activity in the central Greenland Sea was weak and limited to intermed...

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