Seasonal and interannual variability in the properties of the surface waters of the Gulf of Maine

Abstract

The MARMAP hydrographic data set (1977–1987) is used to determine the mean annual cycle of temperature, salinity, and density structure of surface waters throughout the Gulf of Maine. The temperatures follow the expected seasonal warming and cooling pattern. In the eastern Gulf the salinity cycle is dominated by influx of low salinity Scotian Shelf water which enters near Cape Sable in the winter, and in the western Gulf by the local spring runoff. Phasing of temperature and salinity cycles in different parts of the Gulf results in the western Gulf of Maine being more strongly stratified in the summer and more vertically uniform in the winter than is the eastern Gulf. The interannual variability, derived by subtracting the annual cycles from the original data, reveals relatively little temperature variability (1–2°C) during the period 1977–1987, compared to observed changes of 4–6°C in previous decades. Large interannual changes in salinity (0.5 psu), however, are evident in the data. The salinity variability is shown to be due primarily to changes in local fresh water sources—precipitation and runoff. Comparison of salinity changes in the Gulf of Maine with data from Georges Bank and the Middle Atlantic Bight shows that the salinity variability is coherent over the northeast continental shelf region from the western Gulf (Wilkinson Basin) to Cape Hatteras.