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FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



briefly the general nature of the currents which 

 may be expected in such a region. 



The general nature of currents in the Gulf of Mexico 



Sverdrup (1942) ^ lists three different groups of 

 currents each of which is represented in the Gulf 

 of Mexico. These are: 



(1) currents that are related to the distribu- 

 tion of density in the sea, 



(2) currents that are caused directly by the 

 stress that the wind exerts on the sea surface, and 



i'.i) tidal currents and currents associated , 

 with internal waves. 



Tidal currents ' are caused by the tide-producing 

 forces. These forces result from differences 

 between the constant centrifugal force which acts 

 on any particle on the earth and the varying 

 gravitational attractions between the earth, the 

 moon, and the sun. These attractions are pro- 

 portional to the masses of the bodies and inversely 

 proportional to the squares of the distances 

 between them. Because of its very short dis- 

 tance from the earth the attraction of the moon is 

 large. The sun, on the other hand, although it is 

 at a much greater distance from the earth, is so 

 large that a tide-producing force results which is 

 as much as 46 percent of that of the moon. 



The direct result of tide-producing forces acting 

 upon the rotating earth is to raise and lower peri- 

 odically the level of the ocean's surface, i. e., to 

 create tides. Water which is required to raise sea 

 level at a particular location must be furnished by 

 horizontal movements within the ocean. These 

 are the tidal currents. Since the sun and moon 

 change their position with respect to a given part 

 of the earth's surface in a periodic fashion, the 

 tides and tidal currents are periodic. Because 

 the rotation of the earth affects movements of 

 water the tidal currents do not oscillate back and 

 forth on a straight Ime bu^ rotate. In the 

 Northern Hemisphere this rotation usually is in a 

 clockwise direction except where modified by 

 other factors. At times, interference i)et\veen 

 tidal waves or the influence of other forces is such 

 that the rotation may be counterclockwise. 



Along the Gulf coast there are many bays and 

 lagoons which have relatively restricted outlets to 



' References are listed ;it the end of the chapter. 



3 Tides in the (Julf of Mexico are discussed separately in the article by 

 H. A. Marmer, pp. 101-118. 



the sea. If the water level in these bays is to be 

 raised by tidal action, all of the water required for 

 the change in level must flow into the bay through 

 these restricted channels. Therefore, the tidal 

 currents in such channels may be quite large, 

 particularly at certain stages of the tide. 



The great width of the shallow continental shelf 

 along the Gulf coast results in tidal current veloc- 

 ities which are relatively high considering the 

 small range of tide. This is because the change 

 of water level of this large area over the shelf 

 must be brought about by flow across the shallow 

 shelf. Since the depth of the moving water is 

 small, its velocity must be relatively great to 

 provide the volume needed for change in sea 

 level. The high velocities and the changing di- 

 rection and speed of these tidal currents may 

 lead to considerable turbulence and stirring in 

 certain localities. 



Oscillating currents related to internal waves 

 may be important in this region, but little infor- 

 mation now is available on this subject. 



Currents caused by tiie stress of the wind upon 

 the sea surface are particularly important on the 

 Gulf coast. The most widely known phenom- 

 enon which results from the action of such cur- 

 rents is the storm tide or general rise in water 

 level which precedes winds of hurricane velocities. 

 Storm tides are discussed by Cline (1920) and 

 Tannehill (1927). Some of their results are sum- 

 marized in the chapter on meteorological phe- 

 nomena. 



Wlien a wind starts to blow over the ocean it 

 exerts a frictional force or drag upon the sea 

 surface. If the wind persists the surface layers of 

 the water start to move and they in turn act 

 upon the deeper layers and set these in motion 

 also. The two forces which are involved in setting 

 up such cm-rents are the frictional force, and the 

 Coriolis force which is the apparent force due to 

 the rotation of the earth. If the wind blows long 

 enough for a state of equilibrium to be reached, 

 the surface waters away from tlie influence of the 

 coast will be moving in a direction approximately 

 45° to the riglit of the wind direction in the North- 

 ern Hemisphere. A north wind sets up a surface 

 current toward the southwest. The surface veloc- 

 ities may reach 1 to 2 percent of the wind velocity. 

 Currents at greater deptlis will flow at greater 

 angles to the wind and at speeds which decrease 

 with depth. 



