142 Fluctuations 



high flow of the Florida Current by about a month. It is unlikely that the 

 atmospheric-pressure disturbance is the direct cause of the anomalous 

 flow, for two reasons. First, an adjustment to a pressure change would 

 occur with the speed of a long gravitational surface wave — that is, in 

 several hours, not several weeks. Secondly, the mass flux to compensate 

 the surface-pressure drop would be of the order of 3 x 10^^ m.^, whereas the 

 total excess flow of the Florida Current observed during each period is of 

 the order of 3 x 10^^ m.^. It seems more hkely that the anomalous currents 

 are caused by changes in wind-stress distribution over the ocean. But even 

 this influence is not direct, because the anomalous wind conditions asso- 

 ciate low winds wdth high discharge of the Florida Current. However, if it 

 is remembered that, because of the immense mass of water involved, the 

 dynamic topography of the North Atlantic subtropical gyre is constrained 

 to remain relatively unaffected by short-period (even thirty-day) wind 

 changes, it is possible to offer a rationahzation for this inverse relation in 

 the following manner : All along the Lesser Antilles, at the entrance to the 

 Caribbean Sea, the dynamic topography is such as to produce southwest- 

 flowing currents in the upper 400 m. In the very surface layers the normal 

 trades tend to produce a net northward drift (Montgomery, 1936 a). The 

 combined effect of these two tendencies is to produce an essentially west- 

 ward flow of the upper 100 m. at 25° N., and a southerly component below 

 this depth. The fact that Sargasso Sea water normally enters the Caribbean 

 only below 100 m. and not at the surface (Parr, 1939 a) lends support to 

 the explanation just outlined. 



When the trades cease to blow at 25° N., however, the northward Ekman 

 drift ceases abruptly (wdthin 18 hr.), and even the surface waters move 

 south west ward. Thus the flow into the Caribbean is increased. An order- 

 of-magnitude estimate of this increase may be obtained from the following 

 simple relation. In the case of a steady wind stress t^ acting in the 

 x-direction (toward the east), the northward Ekman transport rMyg (see 

 Ekman, 1905) is given by the relation 



TT 



rM,,= —f, (4) 



where/ is the Coriohs parameter, and r is the width of the ocean. Since Tj. 

 is normally very nearly — 1 dyne/cm.^ at 25° N. latitude, /= 0-6 x 10~*/sec., 

 and r = 5xl0*cm., the northward Ekman transport across the 25° N. 

 parallel is 8-3 x 10^ m.^/sec. When the trades cease, this flux is added to 

 the water entering the Caribbean. There is a distance of about 3000 km. 

 through the Caribbean to be traversed before this increase reaches the 

 Florida Straits . There is therefore the possibility of a lag of about fifteen days 

 before the first effect of the change in flow appears in the Florida Current. 



