Wind, river discharge, and shelf-break eddies control,in some measure, slowly varying (sub-tidal)currents and associated water-level relief (surface topography or geopotential anomaly) on the Texas-Louisiana continental shelf. Alongshore wind and its variability play a major role in governing currents on the inner shelf, where water is 50 meters deep or less. One study emerging from the LATEX program focuses on computer simulation of daily to monthly variations in inner-shelf currents.
How does wind drive circulation?
To produce ocean circulation, particularly shelf circulation, momentum and energy must continually be added to overcome dissipative processes. Wind velocity, or stress, provides momentum to ocean and shelf waters, and simultaneously provides mechanisms for altering surface topography. The mechanisms, Ekman transport and Ekman pumping, are fundamental to creation and maintenance of ocean circulation. Due to Earth's rotation, sustained wind stress produces Ekman transport of water to the right of the wind-stress vector (in the northern hemisphere). Ekman pumping occurs when water transported from different locations converges or water is transported into a fixed barrier.
Downcoast winds on the Texas-Louisiana shelf transport water toward the coastal barrier and induce convergent pumping. This causes higher water levels near shore, which should be associated with a downcoast current. If alongshore wind lasts a week or more, the alongshore current's maximum speed occurs when resisting stress on the seabed balances wind stress. If wind stress lasts only a few days or less before it reverses, the magnitude of current response is smaller.
This scenario neglects variations in current and water level, as well as the important effects of the shelf-bottom slope. When there is any cross-shelf component of flow, as can occur with alongshore variations in water level, the flow pattern and associated water-level relief tend to propagate downcoast as a shelf wave.
Model application
Our computer simulations of evolving, wind-driven patterns of flow and water level on the inner shelf employ a representation of the patterns based on shelf-wave theory. Possible cross-shelf patterns, called modes, are evaluated from known bathymetry along each transect over the modeled shelf. The model predicts the amplitude characterizing each mode at each transect for a given wind-stress value, from which we evaluate simulated current and water-level patterns over time for selected positions on the shelf.
We used wind-stress fields derived from meteorological observations over the 32-month LATEX measurement program to simulate currents at inner-shelf mooring positions. We used current observations from moorings at 90.5°W to describe currents at the edges of the modeled region. After required tuning, the model was run to simulate the 32-month period.
Conclusion
A possible conclusion of this study is that at least 60% of the kinetic energy variability on the inner shelf can be explained by wind-stress forcing. Improvements to the model may increase this percentage, but substantial increases in the predictability of currents on the inner shelf will probably have to account for the effects of river discharge and possibly offshore eddies.
Modeled results compared to actual measurements.
Comparison of observed and modeled annual cycles of alongshore currents.
