average speeds are at least an order of magnitude smaller. 

 Within the Gulf Stream, however, the speeds are consistently 

 large enough to be measured. Observations with neutrally 

 buoyant floats (Warren and Volkmann, 1968) or by moored 

 current meters or transport floats (Richardson and Knauss, 

 1971) show clearly that an average speed of 10 cm/sec at 

 2000 meters in the Gulf Stream is a reasonable upper limit. 

 Such speeds lead to a slope of the 2000 db surface of only 

 10 cm across the stream, with the inshore edge lower than 

 offshore. The change in level of the 1000 db surface across 

 the Gulf Stream is about 20 cm. That is, if the results 

 shown in Figure 1 were to be interpreted as the physical 

 sea surface, they could be improved slightly by compensating 

 for the slope of the reference surface in the major boundary 

 currents . 



Anati (1971) has mapped the 1000 db surface relative 

 to the 2000 db surface, and shows that over most of the 

 Atlantic Ocean the total relief is less than 10 cm. We 

 may conclude that errors of the order of 10-20 cm are to 

 be expected in Figure 1, from a slope of the reference surface, 

 Seasonal effects contribute about 10 cm (see, for example 

 Donn, Pattullo, and Shaw 1964) so that an uncertainty of 

 30 cm would be a reasonable estimate of accuracy. If known 

 slopes of the reference surface (e.g. beneath the Gulf Stream) 

 are compensated, and seasonal effects are removed, good 

 coverage with modern data could reduce the absolute error 

 of a map, based on 2000 db, to approximately 10 cm. 



Montgomery (1969) has shown that the 1.3-m surface in 

 Fig. 1 lies near the Geoid, and is the surface which coincides 

 with mean sea level along the Pacific coast of the United 

 States. More work and more recent data could probably 

 improve the details of this result, but it seems unlikely 

 that this average value could be in error by more than 10 

 to 20 cm. 



The arguments above concerning the shape of the sea 

 surface refer to a surface which is assumed to be a constant 



24-7 



