The influence of surface layer salinity on wintertime convection in Wilkinson Basin, Gulf of Maine

Abstract

Observations suggest that the interannual variability of wintertime convection in Wilkinson Basin (WB) during 1977–2005 is related to the variation in the surface layer salinity in the western Gulf of Maine (wGOM). When the winter convection is stronger (weaker), the resulting deep-layer temperatures in the wGOM are colder (warmer), and the likelihood of deep winter mixing is greater when the wGOM salinities are high. This hypothesis was tested using a one-dimensional mixed layer model to simulate the water column structure over the cooling period. Comparisons were made between the convection potential for the range of observed late-fall salinity values. Model results indicated that the mixed layer depth could deepen by as much as 50% when the surface layer salinities in the wGOM were high. Sensitivity tests revealed that the surface layer salinity variability, and its influence on the water column density structure, was a significant factor, along with local climate variability, in determining the observed time series trend in springtime bottom layer temperatures. Density distributions in the wGOM in high salinity years suggested that winter-cooled water from the adjacent shallow coastal areas could become sufficiently dense to cascade into the deeper layers of WB and enhanced the convective cooling. The interannual variation in wintertime convection and in the resulting density structure has important implications for biological processes. The associated variation in deeper layer temperatures has been shown to influence the spatial distribution, timing of spawning, and growth rates of many demersal and mid-water fishery populations.