pressure surface, as this is appropriate for velocity 

 calculations. For these results to be compared with 

 direct measurements of the physical sea surface, as from 

 a tide gauge, altimeter, etc. it is necessary to subtract 

 the local variation of atmospheric pressure. It should 

 be recognized that atmospheric pressure over the ocean 

 must be known at least as well as the resolution demanded 

 of the sea-surface topography. The average effect of 

 atmospheric pressure has a range of about 30 cm; daily 

 values are considerably larger. 



Some Oceanographic Problems 



Before the use of the Swallow float (or neutrally- 

 buoyant float), or the wide-spread use of moored, recording 

 current meters, one of the classical problems in oceanography 

 was the "level of no motion", i.e., the question of the 

 slope of a reference pressure surface for performing 

 geostrophic calculations. From the density field, velocities 

 can be calculated only relative to the velocity at some 

 reference pressure surface, whose slope is unknown. This 

 difficulty leads to relatively small errors in velocity 

 at the sea surface in the Gulf Stream, for example, but 

 to large errors in the total mass (or heat, etc.) transport. 



It has been suggested that satellite altimetry could 

 determine the slope of the sea surface well enough to be 

 of some value in connection with the level-of-no-motion 

 problem in the major boundary currents. The slope of the 

 2000 db surface beneath the Gulf Stream, however, as discussed 

 above, is only about 10 cm across the entire stream. To 

 be useful, therefore, the satellite data would have to provide 

 information referred to a level surface at an accuracy 

 considerably better than 10 cm. Furthermore, the use of 

 moored current meters in this application is preferable 

 to a knowledge of sea- surf ace slope, for two reasons. We 

 need to know a time history of the currents, detailed enough 

 to resolve tidal-period motions, and long enough to average 

 over the time scale of the density measurements. It would 

 also be preferable to have information about the reference 



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