85 



2.2.13 



PHYSICAL OCEANOGRAPHY OF THE TEXAS-LOUISIANA SHELF 



Dr. William J. Wiseman, Jr. 



Coastal Studies Institute 



Louisiana State University 



Baton Rouge, LA 70803 



Circulation on the shelf west of the Mississippi River Delta is strongly influenced by the relative strengths and 

 phasing of river runoff and seasonal meteorological changes. Immediately following the spring flood of the 

 Mississippi River, discharge from both the bird-foot delta and the Atchafalaya Delta combine to form a well- 

 developed coastal boundary layer of low salinity water which hugs the coast, is isolated from the mid-shelf waters 

 by a strong salinity front, and flows at least as far south as the Texas-Mexican border. There is good evidence 

 that it flows much further southward along the Mexican coast. As the winds over the western Gulf of Mexico 

 become more southerly in the late spring and early summer, the coastal winds along the south Texas coast 

 become upwelling favorable. Flow within the coast boundary layer reverses direction and flows northward and 

 eastward. A convergence develops somewhere along the TEXLA coast and low salinity water flows offshore. 

 A mean eastward flow over the outer shelf and upper slope appears to be present throughout the year. Cochrane 

 and Kelly propose the existence of a series of ephemeral counter rotating gyres over the shelf. 



On shorter time scales, atmospheric frontal passages drive strong currents over the shelf. On time scales of two 

 to ten days, much of the current variance over the shelf can be explained as a simple wind-driven sloshing of 

 water along the shelf. 



A potentially important mode of shelf break exchange is driven by air-sea interactions during winter cold front 

 passages. Mid-shelf waters over the western Louisiana shelf are rapidly cooled by the atmosphere. The shallow 

 inshore waters are cooled even more. Being fresher, though, these inshore waters remain lighter than the mid- 

 shelf waters. The outer shelf waters lose nearly as much heat as the mid-shelf waters, but they are deeper and 

 the resultant temperature drop is less. Furthermore, the salinity gradient between the mid-shelf waters and the 

 outer shelf waters is weak. Consequently, the mid-shelf temperature change is often sufficient to result in mid- 

 shelf waters being denser than the outer or inner shelf waters following a cold air outbreak. These mid-shelf 

 waters sink and flow offshore carrying suspended and dissolved material with them. 



