F. Schott et al., OBSERVATIONS OF VERTICAL CURRENTS AND CONVECTION IN THE CENTRAL GREENLAND SEA DURING THE WINTER OF 1988-1989, J GEO RES-O, 98(C8), 1993, pp. 14401-14421
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.