For an estimate of the accuracy of these results, 

 consider the topography near a western boundary current. 

 Sturges (1968) has calculated the sea-surface topography 

 between Bermuda and the east coast of the United States, 

 using ship-drift observations to describe the velocity 

 field, as shown in Figure 2. The results are not for an 

 instantaneous current, but represent a climatological 

 mean. This topography is important because it represents 

 a result similar to the previous figure, and the two are 

 independent. If we examine the annual mean of the absolute 

 difference in sea level between the east coast of the 

 United States and the offshore edge of the Gulf Stream by 

 the two methods, the change in level is 100 cm; the two 

 independent estimates agree to within 10 cm. The calculated 

 annual mean effect of wind on the sea- surface topography 

 between Bermuda and the U.S. coast is 4 cm, which can 

 only be considered an order- of-magnitude estimate. 



The numerical values of the contours in Figure 2 

 are based on an arbitrary mean value of 200 cm at Bermuda. 

 These contours can be reconciled with those of Fig. 1, 

 which is based on geopotential anomaly relative to 1000 db . 

 The annual mean geopotential anomaly of the sea surface 

 at Bermuda, relative to 1000 db, is 157 cm. Therefore, 

 by subtracting 43 cm from the contours of Figure 2, they 

 become equivalent with those in Fig. 1. 



The primary uncertainty in interpreting the topography 

 shown in Figure 1 as the physical sea surface, to high 

 accuracy, arises from the question of whether the 1000 decibar 

 surface is a level surface. Certainly at 1000 meters depth 

 the currents are much weaker than .-it the sea surface, so 

 the 1000 db surface should be much more nearly level than 

 the sea surface. The 2000 db surface will be a better 

 reference level, as has been discussed by Montgomery (1969). 

 We know something about the velocity distribution at 1000 

 to 2000 meters. Away from strong boundary currents, there 

 are amplitudes of 10 to 15 cm/sec as transients, but the 



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