236 KNAUSS [chap. 10 



which the water particles may move. Although the method is indirect, it is still 

 responsible for most of the little knowledge available on the deep circulation. 

 More particularly, Defant (1936) and Montgomery (1938) used this method in 

 describing the circulation in the equatorial Atlantic. 



C. Geostrophic Currents 



The third method is the calculation of velocity from the pressure field using 

 the geostrophic approximation. Apparently the first use of the geostrophic 

 calculation in the description of the equatorial currents was made by Sverdrup 

 (1932) using the Carnegie data. It has since been used by many investigators to 

 describe the equatorial circulation and has recently been used to find a new 

 equatorial current, a South Equatorial Countercurrent in the Pacific, which 

 flows eastward between about 5°-10°S and is most pronounced at a depth of 

 about 400 m (Reid, 1959). 



Because an important horizontal component of the Coriolis force becomes 

 zero at the equator, there has been some question in the past as to how close 

 to the equator the geostrophic approximation is valid. It has recently been 

 shown that geostrophic balance exists in the Pacific Equatorial Undercurrent 

 (the Cromwell Current) to within half a degree of the equator where the value 

 of the Coriolis acceleration was 1 to 2 x 10 -4 cm/sec 2 (Knauss, 1960). 



D. Velocity Measurements 



The fourth method is the use of direct current measurements. It seems the 

 obvious way to study the circulation, but reliable measurements in the open 

 ocean have until recently been difficult and time-consuming. Accuracy and 

 reliability are still problems. The discovery and further measurements of the 

 large sub-surface current at the equator in the Pacific, the Cromwell Current, 

 were accomplished by direct current measurements (Cromwell, Montgomery 

 and Stroup, 1954; Knauss, 1960). 



2. Gross Circulation Pattern 



The great equatorial currents are zonal. Except where they infringe upon 

 continents they move almost due east and west. Unlike the western boundary 

 currents, there is no change of water-mass, water color or marked change in 

 surface temperature as one moves out of one current and into the next. At least 

 in the Pacific there is no surface phenomenon of any kind that gives indirect 

 but conclusive evidence of entering or leaving one of the equatorial currents. 



The classic picture of the equatorial circulation has been that of three 

 currents : the westward-flowing North and South Equatorial Currents, and be- 

 tween them, the eastward-flowing Equatorial Countercurrent. These currents 

 correspond closely to the analogous wind system — the northeast and southeast 

 trades separated by a zone of weak and variable winds, the doldrums. The real 

 situation is more complicated and care must be taken in extrapolating the 

 conditions in one area to that of another. What Clarence Palmer has called 



