b) The 6.5 day tilt mode 



Time histories of volume transport obtained from the 21 day 

 simulation of HURB with rotation are presented in Fig. 96. A 

 prominent feature is the 6.5 day period oscillation for the 

 transports through both Florida and Yucatan Straits. 



The water level from this long simulation was sampled at 12 

 stations, six of which are the same as those given in Fig. 12. The 

 remaining six stations are Panama City, Tampico, Coatzacoalcas , Dimas 

 (northern coast of Cuba), Central Gulf, Eastern Gulf and Western 

 Gulf. The hydrograph at Key West (Fig. 97) shows a strong 6.5 day 

 mode with an average amplitude of 0.50 m. Superimposed on this long 

 period mode is the 28 h volume mode. Careful inspection of the 

 remaining hydrographs indicates the presence of the 6.5 day 

 oscillation but with a much smaller amplitude as compared to that in 

 the Key West hydrograph. Hydrographs at Galveston (Fig. 98) and 

 Dimas (Fig. 99) show that the water levels at the two stations are 



o 



180 out-of -phase. For example, at 1600 h on day 8 the water level 

 at Galveston is 0.05 m while at Dimas it is -0.05 m. The three deep- 

 water hydrographs are near zero at this time (note that the Central 

 Gulf station is approximately midway between Dimas and Galveston) . 

 This 0.1 m differential in water surface elevation is possibly a 

 geostrophic tilt since all the forcings were set to zero well before 

 this time. Large space and time scales and a small surface 

 expression are indicative of a vorticity (or quasi-geostrophic tilt) 

 mode. 



161 



