Variability and structure of currents over the Texas-Louisiana shelf: A view from a shipboard acoustic Doppler current profiler

The structure and variability of currents over the Texas-Louisiana shelf are described using ADCP measurements, hydrographic data, and moored current meter records taken during February, July and November of 1993. For these three surveys, complex EOF analysis of ADCP velocity profiles demonstrates that the first three empirical modes account for more than 90 percent of the observed energy along the 50-m and 200-m isobaths over the shelf. The dominant EOF modes are nearly barotropic. The second empirical modes infer that the baroclinic motion during February is weakest among these three cruises.; The vertical shears of geostrophic and ADCP current were compared using bivariate EOF analysis to assess the validity of geostrophic approximation over this shelf. The first and third EOF modes indicate prominent consistency of these two shears, but the second modes suggest that the current measurements contain significant ageostrophic components. These ageostrophic components in the ADCP data presumably result from high-frequency current fluctuations and frictional effects.; Calculations of transverse and longitudinal velocity correlations versus separation distance were made from the near surface ADCP measurements. From these, the correlation of an associated geostrophic streamfunction can be inferred. For the along-shelf transects, the spatial correlation scales of streamfunction are 32 km during July and 35 km during November. For the February survey, the correlation scale for the cross-shelf transects is 19 km. This coincides with the scale of geopotential anomaly deduced from the hydrographic data, but the scale for the along-shelf transects is shorter.; For the majority of the shelf, data from current meter moorings indicate that low-frequency currents are stronger than high-frequency currents during February and July. The high-frequency currents are stronger in the eastern shelf than the western shelf. Optimal estimation of low-frequency currents was implemented by the statistical objective analysis of ADCP velocity measurements. Results agree well with the circulation pattern inferred from the geopotential anomaly for July and November. However, due to the strong temporal variability of the currents, a synoptic flow field for the February cruise was not achieved.