Subtidal across‐shelf velocity structure and surface transport effectiveness on the Alabama shelf of the northeastern Gulf of Mexico

A 3.33 year time series of velocity and hydrographic data from a mooring site on the 20 m isobath of the Alabama shelf in the northeastern Gulf of Mexico are used to examine across‐shelf circulation. The flow structure and surface transport are determined on this wide shallow shelf system, in which wind stress is a primary forcing mechanism, over a wide range of environmental conditions. The relatively long data set allows the along‐ and across‐shelf wind stress responses to be separated so that their individual contributions to the flow structure can be analyzed. This study finds that both along‐ and across‐shelf wind stress play a role in the across‐shelf circulation. While the along‐shelf wind is correlated with the currents during all seasons, the across‐shelf shelf wind is most clearly correlated with the currents during fall and winter when the water column is least stratified and the across‐shelf wind stress is strongest. In addition, wind stress magnitude, mid‐depth vertical shear of the horizontal velocity, and stratification all show significant relationships with across‐shelf transport effectiveness to varying degrees. The wide range of stratification conditions provides new insight on the influence of stratification on transport effectiveness and across‐shelf wind stress forcing. Under very low stratification conditions, there is no apparent relationship between stratification and transport effectiveness, and across‐shelf wind stress can generate a significant forcing contribution. As stratification increases, across‐shelf wind stress becomes less important and the transport effectiveness increases to a point, above which, there is again no clear relationship with stratification.