Currents Driven by a Westerly Wind 



Wind-driven currents in the Mississippi Sound depend strongly on the wind 



2 

 direction. For example, assuming a uniform wind stress of 1 dyne/cm from the 



west, our model results showed relatively stronger currents in the along-shore 



direction (Figure 5.29). Notice the near-surface and near-bottom velocities 



differ not only in magnitude but also in direction at some locations. This is 



partially associated with the pressure gradient caused by the wind set-up. As 



shown in Figure 5.30, wind set-up on the order of 20 cm occurs across the 



Mississippi Sound. The bottom stress distribution in the study area is shown 



in Figure 5.31. According to our laboratory flume study on the erodibility of 



the Mississippi Sound sediments, it is expected that the bottom shear stress 



generated by the strong westerly wind in winter will cause significant 



resuspension of sediments. The exchange of water mass between the Sound and 



adjacent offshore waters may result in transport of sediments into or out of 



the Sound. 



Effect of Lateral Boundary Condition 



The results shown in Figures 5.29 through 5.31 were computed with an open 

 boundary condition assuming the surface displacement remained to be zero along 

 the two open boundaries. While this condition is a reasonable assumption for 

 the southern open boundary where the water is quite deep, it may be somewhat 

 restrictive for the transverse open boundary along the east. To test the 

 sensitivity of model results to the open boundary condition, we have performed 

 a simulation with a zero surface slope condition along the eastern boundary. 

 As shown in Figure 5.32, the near-surface currents and the near-bottom 

 currents have increased from those in Figure 5.29 by more than 50%. Along the 

 eastern boundary, currents are directed out of the computational domain. The 

 zero surface slope condition may be valid in such a situation, but may not be 

 valid where currents along the open boundary are directed into the 

 computational domain. 



Ill 



