586 



Basic Principles of the General Oceanic Circulation 



A remarkable feature is a counter current east of the main current, with a magnitude 

 of 17% of that of the main one. There can be little doubt that such counter currents 

 exist, although this fact has been obscured in some instances by the smoothing of 

 data. This theoretical result has in fact been shown to be in agreement with observa- 

 tions (see p. 536 et seq.). 



The total transport of the western current and counter current is found by putting 

 numerical values of A' into X.14) giving 



"Av 



M7/-;8-icurl, r. 



(XVIII. 16) 



The resulting expressions are independent of A and the transport can be computed with 

 a relatively high degree of accuracy; the uncertainty is of the same order as that in the 

 calculation of wind stress. Table 1 50 gives a comparison between the transport values 

 of some western currents determined from oceanographic observations, and those 

 computed from the zonal wind stress using equation (XVIII. 14). The two sets of values 

 are of the same order of magnitude, but the calculated transport values differ from the 

 observed values by a factor of as much as two ; the discrepancy is not surprising when 

 it is considered that amongst other uncertainties the wind and current data are not for 

 the same year, nor necessarily for the same time of the year. Another source of error 

 may be due to possible underestimation of the wind stresses at low wind speeds. It 

 can be assumed, in accordance with views held at the present time, that the dependence 

 of wind stress on the wind velocity is given by /c = 0-0026 at high wind speeds and 

 K r-^ 0-008 at low speeds, with the discontinuity at Beaufort 4 (see p. 421 and especially 

 MuNK, 1947). This assumption, however, does not appear to be absolutely certain 

 and further investigations are required. 



Table 150. The mass transport of some western currents determined from the 

 wind stress and from oceanographic observations 



* Sverdrup et al. (1942), pp. 605, 761. 



t Adjusted for a supposed southward motion of 19 x 10^^ g of slopewater. 



+ For August (Uda, 1938). 



Away from both boundaries the stream-line function X reduces to 



-^central = 1 " 



which gives the central oceanic drift; this is a broad constant drift that compensates 

 for the swift shallow western currents. Equation (XVIII. 17) also gives 



(XVIII. 17) 





(XVIII. 18) 

 which agrees with the relationship derived by Sverdrup (see p. 580, equation XVIII. 5). 



