Response of the lower Chesapeake Bay to forcing from Hurricane Floyd

Water density and velocity data from a ∼70-day period deployment across the entrance to the Chesapeake Bay were used in conjunction with wind velocity and sea level records to describe the response of the lower part of the estuary to forcing from Hurricane Floyd in mid-September 1999. Forcing associated with Floyd consisted of northeasterly winds during the 4 days prior to the passage of the storm eye over the lower Chesapeake Bay with hourly values that peaked at ∼21 m/s. After the passage of the eye of the storm on 16 September, winds shifted rapidly to northwesterly and peaked at 27 m/s. Buoyancy forcing from Floyd consisted of at least 2000 m3/s of direct freshwater input to the lower Chesapeake Bay. This was added to the runoff from the surrounding land that was already saturated with precipitation from tropical storm Dennis that affected the area of the lower bay in early September 1999. Northeasterly winds prior to the eye passage over the lower bay caused net inflow over the shallow northern half of the bay entrance and outflow in the deep channel to the south of the entrance. This outflow veered clockwise with depth. The change to northwesterly winds coincided with a pulse of freshwater that caused a salinity drop throughout the water column of up to 8 units in 1 day. Wind and river discharge set up a seaward barotropic pressure gradient force that drove net outflow everywhere across the entrance to the bay thus hindering any inflow and effectively flushing water out. The barotropic pressure gradient must have overcome the tendency of the baroclinic pressure gradient, related to the buoyancy input, to drive salty water into the estuary. It is estimated that approximately one-third of the net outflow was caused by wind forcing and two-thirds by freshwater discharge.