Cold-front-induced flushing of the Louisiana Bays

Abstract

Water level variations were used to calculate cold-front-induced water exchange fluxes for three Louisiana bays. Twenty-nine cold fronts were identified from weather maps between September 2006 and April 2007. Cold front passages cause water to flush out of the bays, with large variability in volume fluxes. Due to the differences in water body area and basin geometry, the Atchafalaya/Vermilion Bays have exchange rates an order of magnitude higher than the Barataria Bay. We identified five largest flushing events which were all associated with migrating extra-tropical cyclones with frontal orientations perpendicular to the coastline, suggesting that wind direction is an important component in determining the flushing rate. Both alongshore and cross-shore winds play important roles in bay-shelf water exchange, and northwest winds appeared to be most effective in flushing the bays. Strong cold fronts may flush more than 40% of the bay waters out onto the continental shelf within a less than 40-hour period. An analytical model is applied for the cold front wind induced flushing process which reveals that the amplitudes of water level variations induced by alongshore and cross-shore winds have the same order of magnitude, indicating that they play almost equally important roles in driving the subtidal water level changes inside the bays.