Objectively mapped stream function fields on the Texas-Louisiana shelf based on 32 months of moored current meter data

On the basis of current measurements observed at 31 moorings during April 1992 through November 1994, as part of the Texas-Louisiana Shelf Circulation and Transport Processes Study, we describe the horizontally nondivergent part of the near-surface, shelf-wide-scale, low-frequency circulation and its variability on the Texas-Louisiana continental shelf by means of objectively fitted stream function fields. The 32-month mean velocity stream function patterns generally support the Cochrane and Kelly [1986] circulation schema for the shelf circulation, in which an elongated cyclonic gyre exists during the nonsummer period (September through May) but with flow in the summer (June to August) directed upcoast (toward the Mississippi delta) over the entire shelf. There exists significant interannual variability of the stream function fields, especially in the transition period from spring to summer to fall and along the shelf break. Strong upcoast (downcoast) currents dominate at the western shelf break associated with the presence of energetic anticyclones (cyclones). The dominant pattern of stream function variability deduced by empirical orthogonal function (EOF) analysis of 32 individual monthly mean patterns contains about 89% of the variance and shows a simple, shelf-wide, along-shelf pattern of circulation. The monthly evolution in amplitude of this dominant EOF pattern clearly mimics the monthly evolution of the along-shelf component of the wind with squared correlation of 0.83. This provides strong evidence that the seasonal variation of the shelf circulation is wind forced, in support of Cochrane and Kelly. The second and residual EOFs show more complex features near the shelf break that are associated with Loop Current eddies known to impact the continental slope and shelf break.