170 



would be formed around the low pressure area. Stevenson (in press) reported 

 on shallow water ocean conditions associated with hurricane CARLA in 196I. 



A MODEL 



The observations collected on the GUS III cruise may be fitted into a 

 simple model based primarily upon the concept of wind-driven current. V. W. 

 Ekman, 1905^ concluded that the effect of the wind upon the sea surface is 

 to set up a surface current ^5 to the right of the wind. As depth in- 

 creases, the direction of the flow turns to the right and the speed decreases. 

 When the ends of vectors representing velocity at different depths are 

 projected on a horizontal plane they form a spiral which has been called the 

 Ekman Spiral. The net transport of water in this layer of wind-driven current 

 can be shown to be 90 to the right of the wind direction in the Northern 

 Hemisphere . 



Figure ^4- is a schematic diagram of a hurricane similar to HILDA. As 

 may be observed, the winds at any given time create an Ekman net transport 

 outward from the center. 



Since the hurricane is moving and since the actual transport direction 

 and speed vary with depth the full pattern is more complicated. In all events, 

 the passage of a hurricane causes the surface layers of the ocean to diverge 

 from the center. The surface waters which are moved aside must be replaced, 

 and the only source of replenishment is from below. Thus, cold water from 

 considerable depths appears at the surface in the hurricane eye . 



Figure 5 shows schematically the change in isotherms in a section across 

 the hurricane path. It allows a comparison of the isotherms as they would 

 appear in the normal summer situation in the Gulf with those which would 

 appear after a hurricane passage . 



Since the upwelled water arrives at the surface only after the winds 

 have blown for a considerable time, these waters would lose little heat to 

 the atmosphere through evaporation and conduction. The major loss of this 

 type would be from the warm mixed layer of surface water upon which the winds 

 acted directly and which was pushed aside in the Ekman drift. 



THE DATA 



With this model in mind, the data may now be considered. The surface 

 temperature pattern prior to the hurricane was shown in Figure 2. The 

 comparable pattern after the passage of the hurricane is given in Figure 6. 

 The darkest shaded areas indicate warm water, above 28 . These areas appear 

 to represent the original warm surface layer which has undergone the normal 

 seasonal cooling through the 12 to ik days involved. The cold upwelled 



