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

Regional CDT/ADCP surveys made in the northern Great South Channel (GS C) 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 Ap ril 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 min imum salinity was about 1.5 less, and a large pool of water fresher th an 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 riv er discharge in 1989. The offshore spreading of the low-salinity plume was driven by the deeper circulation and upwelling-favorable winds. T he distribution of Maine Intermediate Water (MIW) also significantly d iffered between April 1988 and June 1989. In April 1988, the seasonal thermocline was just beginning to form, and the spatial structure of M IW 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 a long the western flank of the northern GSC between 40 m and 130 m. Thi s colder and fresher water spread to mix with the interior MIW as the core flowed southward into the central GSC. Hydrographic data plus sat ellite sea-surface temperature images showed a relatively permanent co ntinuous thermal front (with a 10-km cross-isobath variation) along th e eastern flank of Nantucket Shoals, across the northern shallow regio n of the GSC and along the northwestern flank of Georges Bank, which s eparated 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 GREEN BERG [(1986) Continental Shelf Research, 6, 397-414] suggests that enh anced tidal mixing due to the spring-neap cycle is important in determ ining the relative balance between buoyancy import and tidal mixing in the GSC region.