Influence of wind stress and discharge on the mean and seasonal currents on the Alabama shelf of the northeastern Gulf of Mexico

Analysis of a relatively long (3.33 years) time series of velocity in 20 m of water on the Alabama shelf of the northeastern Gulf of Mexico in conjunction with long-term records of wind stress and discharge reveal a better understanding of the seasonal currents. Analysis of the mean and seasonal signals of the depth averaged velocity shows virtually no mean flow, but a relatively small yet significant seasonal signal. The 3 cm s(-1) seasonal signal is primarily rectilinear in the along-shelf direction with peak eastward (westward) flow during the late spring (late fall), consistent with the patterns reported in previous basin scale studies of the region. The two prominent regional forcing functions, wind stress and freshwater discharge, show clear seasonal signals. The seasonal vertical profiles also show two distinctively different patterns, most clearly observed in the fall with a westward maximum flow at the surface that decreases with depth and spring with a subsurface eastward flow and a reduced westward or even eastward surface flow. Separation of the current velocity into a wind-driven component and a non-wind-driven component demonstrates their counteracting influence on the mean current and maximizes seasonal effects where the minimum (maximum) seasonal wind-driven signal roughly corresponds to the maximum (minimum) non-wind-driven signal in late spring/early summer (late fall/early winter). On the basis of several sources of indirect evidence, it is hypothesized that a freshwater discharge generated barotropic pressure gradient is the primary forcing of the seasonal signal in the non-wind-driven current.