Enhanced stratification in the lower Chesapeake Bay following northeasterly winds

Abstract

Density data from a lower Chesapeake Bay transect obtained after two northeasterly wind events were used to describe the effects of these events on the density field in the lower estuary. The first northeaster occurred in early August 1995 and the second northeaster was related to the passage of hurricane Felix off the lower bay in mid-August 1995. The latter northeaster prolonged a period of persistent winds from the N and NE that began in early August and caused storm surges of similar magnitude to the former northeaster. The salinity fields observed after the early August event suggested encroachment of coastal waters into the lower bay as reflected by high salinities throughout the transect. Two days after weakening of the winds related to Felix, the density distribution across the lower bay showed strongly stratified conditions. This behavior suggested that the inflow of coastal water into the lower bay and the wind mixing related to Felix combined to produce a vertically uniform density gradient perpendicular to the bay entrance that relaxed after the winds weakened. This weakening of the winds coincided with neap tidal currents, which were not energetic enough to maintain vertical homogeneity and must have allowed the self-adjustment of the density gradient and the seaward advection of relatively buoyant waters near the surface. These mechanisms were illustrated with simplified numerical experiments. The findings of this study are used to propose the hypothesis that, in general, enhanced stratification and flushing in the lower bay will ensue the relaxation of a northeasterly wind event, provided that this relaxation coincides with a weak friction regime, i.e., neap tides.