Hydrodynamic response to cold fronts along the Louisiana coast

Abstract

Cold fronts play important roles in flushing water out of the Louisiana estuaries. This study is aimed at examining the impact of cold front passages on the hydrodynamics in autumn-winter-spring of 2006-2007, and tries to determine the geographic difference, correlation and relative importance of winds, tides, and river discharge on water level variability and flow field. The amplitude spectra of water level reveal that diurnal tides dominate most stations. Areas west of 91°W have relatively high semidiurnal tides. The subtidal fluctuations are mainly wind-driven. Only the station in the Atchafalaya River shows obvious response to the spring flood of the Mississippi/Atchafalaya Rivers. Coastal bays have different water exchange rates depending on their water body area and geomorphology. Five largest flushing events correspond to migrating extratropical cyclones with frontal orientation perpendicular to the coastline, suggesting that wind direction is one of the controlling factors in the flushing rate and total transport. Both alongshore and cross-shore winds may effectively induce bay-shelf exchange. Northwest/north winds appear to be the most effective wind forcing in driving water movement from bay to shelf. Strong cold fronts may flush more than 40% of the bay waters onto the shelf within a period less than 40 hours. The near-surface current on the Louisiana inner shelf is mainly wind-driven, but tidal forcing becomes more important in the sub-surface layers or in the vicinity of the coastline of shallow waters. A prevailing down-coast flow occurs 81% and 70% of the time at CSI-6 and CSI-3, respectively. Strong cold front events may disturb this down-coast flow system by inducing a 1- to 3-day up-coast flow. At CSI-6, the Mississippi river discharge has little influence in non-flood seasons. During the period of spring flood, however, the large amount of freshwater exerts significant barotropic and baroclinic forcings on the current field and reinforces the down-coast flow. The analytical model reveals that the amplitudes of water level variations induced by alongshore and cross-shore wind forcings have the same order of magnitude (i.e., 10-1 m), indicating that they play almost equally important roles in driving the subtidal water level variability inside the bays.