Influence of Gulf Circulations 



Large-scale circulations in the Gulf of Mexico, such as the Loop 

 Currents, may affect the coastal currents in the study area. To examine this 

 effect, the limited-area grid used here needs to be coupled with a global grid 

 for the entire Gulf. The dynamic coupling procedure as described in detail by 

 Sheng (1975) could be used to ensure proper transfer of information from one 

 grid to another. The basic hydrodynamic model described in this report could 

 be readily extended to study the entire Gulf circulation. 



5.3 Density-Driven Currents 



Density gradients resulting from spatial variation in temperature and/or 

 salinity could induce currents in coastal waters. Relatively strong currents 

 could be induced in an estuary where the light fresh river water meets with 

 the heavy salt water from the ocean. However, transient adjustment of the 

 density distribution is a very slow process compared to the tidal response or 

 the wind-driven response in coastal waters. To illustrate this, we have 

 performed a simulation in a two-dimensional estuary with a length of 15 km and 

 a uniform depth of 10 m. River flow with a uniform velocity of 10 cm/sec at 

 all depths enters through the left boundary. Along the open-ocean boundary, 

 the salinity distribution varies from 20 ppt near the surface to 28 ppt near 

 the bottom. Initially, the salinity is assumed to vary linearly in the 

 horizontal direction from the river to the open boundary. The simulation was 

 performed until the salinity distribution within the estuary changes little 

 with time. 



Current distribution at this steady state is shown in Figure 5.33. 

 Strong surface currents up to 50 cm/sec are in the seaward direction while 

 appreciable bottom currents on the order of 20 cm/sec are directed towards the 

 river. The strong vertical shear is a direct consequence of the relatively 

 small vertical eddy coefficient used (1 cm^/sec). The river inflow in this 

 case is relatively strong. If the river inflow is reduced to 1 cm/sec, the 

 steady-state currents are reduced to approximately 15 cm/sec. Bottom 

 topography could also affect the current magnitude. Within the relatively 

 deeper navigation channel, the density-driven currents are usually stronger 

 than those in the shallower surrounding water. The time to reach the above 



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