appearance of forerunners over a long stretch of the United States 

 Gulf coast. 



Forerunners in a basin like the Gulf of Mexico may be explained 

 in terms of the large scale barotropic normal gravity modes of the 

 Gulf. Even though the Gulf is relatively small, the size of 

 hurricanes still gives a mismatch of forcing and response scales. 

 However, normal mode excitation is not limited to open ocean 

 atmospheric forcing in the Gulf of Mexico. Platzman (1972) obtained 

 a long-period Helmholtz mode of 21.2 h in a numerical study of the 

 Gulf. Reid and Whitaker (1981) also obtained a Helmholtz mode for 

 the Gulf, but with a period of 28.5 h. This mode is characterized by 

 nearly uniform phase and amplitude, except near the ports. This is 

 indicative of a co-oscillating port-driven Gulf -Caribbean system. 

 Conceivably, some hurricanes could provide port forcing to elicit 

 this long period motion. 



Another possible large scale barotropic response of the Gulf to 

 hurricane forcing typically has relatively small surface expressions 

 but large vorticity (circulation). These are the vorticity modes 

 generated by planetary or topographic vortex stretching and they have 

 been observed in enclosed basins. Say lor et. al (1980) found that 

 the observed oscillation in Southern Lake Michigan with a period of 4 

 days was a lake-wide barotropic topographic vortex mode. 

 Calculations show that if the response of the Gulf of Mexico to a 

 hurricane was a Gulf -wide barotropic vortex mode it would have a 

 period of about 5 days. Muller and Frankignoul (1981) however, 

 showed that realistic topography causes the vorticity modes to have 



The term "ports" denotes the model representation of the Yucatan and 

 Florida Straits. 



