VARIABILITY OF WATER PROPERTIES IN LATE SPRING IN THE NORTHERN GREAT SOUTH CHANNEL

Regional CDT/ADCP surveys made in the northern Great South Channel (GSC) in late spring of 1988 and 1989 show different patterns of surface salinity in the extent of the freshwater plume east of Cape Cod. In April 1988, the surface plume was just beginning to form along the outer coast of Cape Cod, while 6 weeks later in the season in 1989, the minimum salinity was about 1.5 less, and a large pool of water fresher than 31.6 had pushed eastward over much of the northern GSC region. The difference in the amount of freshening between these two years is due primarily to 6-week difference in the seasonal cycle and increased river discharge in 1989. The offshore spreading of the low-salinity plume was driven by the deeper circulation and upwelling-favorable winds. The distribution of Maine Intermediate Water (MIW) also significantly differed between April 1988 and June 1989. In April 1988, the seasonal thermocline was just beginning to form, and the spatial structure of MIW was relatively uniform. In June 1989, a narrow core of temperature minimum water (with T-min in a range of 3.2-4.4 degrees C) was found along the western flank of the northern GSC between 40 m and 130 m. This colder and fresher water spread to mix with the interior MIW as the core flowed southward into the central GSC. Hydrographic data plus satellite sea-surface temperature images showed a relatively permanent continuous thermal front (with a 10-km cross-isobath variation) along the eastern flank of Nantucket Shoals, across the northern shallow region of the GSC and along the northwestern flank of Georges Bank, which separated the well-mixed water over the shallow region of the GSC from stratified water in the center of the northern GSC. Comparison of the location of this front with theoretical predictions by LODER and GREENBERG [(1986) Continental Shelf Research, 6, 397-414] suggests that enhanced tidal mixing due to the spring-neap cycle is important in determining the relative balance between buoyancy import and tidal mixing in the GSC region.