122 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



94° 92* 90* 88° 86° 84° 8 2° 



Figure 35. — Surface ocean currents in the Gulf of Mexico in December. 



80° 



78° 



are involved, one of these being the CorioHs 

 force and the other the "pressure force" which is 

 a force that depends upon the water density 

 distribution in the earth's gravitational field. 

 The pressure force tends to make water flow from 

 a region of high pressure toward a region of low 

 pressure just as water poured into less dense oil 

 will flow outward from the point at which it is 

 poured. When the movement related to the 

 pressure gradient has begun, the Coriolis force 

 acts toward the right of the movement in the 

 Northern Hemisphere and the resulting equili- 

 brium between the two forces is associated with a 

 steady current flowing almost perpendicular to a 

 line connecting the regions of high pressure and 

 low pressure. This flow is such that in the 

 Northern Hemisphere the more dense water is on 

 the left of a person standing with his back to the 

 current and the less dense water is on his right. 



Since temperature is one of the major factors 

 influencing density, it may be inferred that the 

 cold water is on the observer's left and the warm 

 is on his right when he is standing as described 

 above with relation to the current. Thus, he 

 can tell something about the currents if he knows 

 the distribution of temperature, or he can tell 

 something about the temperature if he knows the 

 distribution of currents. 



There are a number of difficulties which arise in 

 applying the current computation method. These 

 occur partly because the basic assumptions under- 

 lying the theory are not always fulfilled. How- 

 ever, despite these difficulties the method has been 

 found to be the one which provides the most 

 infoimation for a reasonable amount of work. 

 It is not known how accurately the Gulf currents 

 in deep water may be determined by this method, 

 but there is reason to believe it to be the most 



