Evidence for Directional Wind Response in Controlling Inter-annual Variations in Duration and Areal Extent of Summertime Hypoxia in Western Long Island Sound

Abstract

Observations of summertime hypoxia duration and hypoxia areal extent in western Long Island Sound from 1991 to 2010 are correlated with local wind forcing. Results show a strong dependence on wind direction consistent with straining due to axial winds. The analysis of current data from moored ADCPs in the western Sound shows that the dominant mode of response is that of axial currents to axial winds. Estimates for the Wedderburn number (W) are relatively low ranging from 0.75 to 2.5 for prevailing winds, putting the western Sound in a regime more dominated by wind straining than by wind mixing. Estimates for the Kelvin number (Ke) range from approximately 1 to 6 in this diverging channel suggesting the importance of rotation; in the wide part of the basin, current observations show that significant vertically sheared lateral currents develop consistent with the tilting of planetary vortex lines by wind-driven axial current shear. Lateral straining, however, offsets longitudinal straining in the wide part of the basin supporting the hypothesis that simple longitudinal straining associated with axial winds exerts the most important control on the development summertime hypoxia in the constricted part of the western Sound.