Characterization of the mean and seasonal flow regime on Georges Bank from shipboard acoustic Doppler current profiler data

Abstract

From 1995 through 1999 the Northwest Atlantic GLOBEC program sponsored over 100 oceanographic cruises to Georges Bank and the surrounding waters. About 12,000 hours of shipboard acoustic Doppler current profiler (ADCP) data were collected during the program on more than 80 of these cruises, forming one of the most extensive three‐dimensional current measurement data sets ever collected. These data have been used here to examine the seasonal variability of the bank’s barotropic flow field. The variability has been described using the 5 years of ADCP observations to derive both small area statistics and seasonal least squares fits, and a more flexible description based upon interpolating functions describing the spatial and temporal dependence of the bank‐wide stream functions. Both methods show the seasonal acceleration of the bank gyre in the summer with larger velocities along the north flank and smaller seasonal variations over the southern flank. The stream functions show the seasonal change from a winter condition with on‐bank flow from the northwest through to the acceleration and ecologically critical closure of the anticyclonic Georges Bank gyre during the summer. On the basis of annual and semiannual decompositions of the seasonal progression, the closure of the gyre in the area of the Great South Channel appears to occur by June, first in the shallow waters and then progressively southward, but it does not extend as far as the middle of the southern flank. A series of monthly Lagrangian drift predictions based upon the stream function analysis, suggest that the observed seasonal movement of the larval fish from their spawning grounds over the Northeast Peak in March, through to their late summer concentration over the central bank, can generally be ascribed to the seasonal barotropic flow over the inner portion of the bank. The analysis also showed that those larvae found over the outer part of the bank are, almost certainly, doomed to be swept out of the banks’ ecosystem, to either the west or south.