An estimation of Ekman and geostrophic current over the Texas-Louisiana shelf

Abstract

An estimation of wind driven Ekman current and geostrophic current over the Texas-Louisiana (TX-LA) continental shelf is demonstrated in this paper. One should expect the current field is a combination of two parts (G and E). Part G is the near surface part of the geostrophic current generated by the broad-scale wind over the shelf, plus possible influence from the offshore eddies. Part E is the near surface current produced by local Ekman dynamics. One should also expect that the direction and magnitude of part E is controlled by the local wind vector, while part G will tend to be constrained heavily by the local bathymetry of the shelf, which contributes the largest variance of the total near surface current. In order to indirectly obtain geostrophic current, the Ekman current can be determined using a direct calculation from the local wind stress vector. Since the local wind can be in any direction relative to the local shelf topography, a complex linear regression analysis statistical method to get the Ekman flow correctly over the shelf is used. The data set analyzed to obtain regression coefficient and correlation is simulated surface current time series at selected locations over the TX-LA shelf from September 10 to November 3, 2001, total 65 days in this long time series. The complex regression coefficient can also be expressed in polar form as a modulus and phase, where the phase gives the angle between the wind vector and the Ekman current vector. Negative angle representing clockwise rotation of the current vector from wind stress vector should be expected in the northern hemisphere. The observations from Texas Automated Buoy System (TABS) moorings R, B, F, D and J for this 55-day window are also used for comparison.