338 VON ARX [OHAP. 16 



Kuroshio, would be expected to develop transverse slopes in the order of one part 

 in lO -5 (2 seconds of arc). Technically impossible as the measurement of such 

 small slopes are at the moment from ships, their importance to oceanography 

 (and marine geodesy) is so great and the rate of technical advancement in 

 gyroscopic systems is so rapid that a few experiments have been made in 

 anticipation of the day when sensing of vertical to the required accuracy may 

 become feasible. 



Since the real fluid covering the earth consists of two components, air and 

 water, it is to be expected that motion in either component will be accompanied 

 by a surface-pressure anomaly and a deformation of the interface. Because of 

 the relatively great depth of water in the major ocean basins, it is possible for 

 the gravity wave developed by an anomaly of atmospheric pressure to keep 

 step with the relatively slower motions of atmospheric high- and low-pressure 

 centers that cross the ocean surface. It can be shown that the velocities of deep 

 water needed to satisfy continuity, as the ocean surfaces elevated or depressed 

 by atmospheric pressure change, are so small that the readjustments of the 

 pressure field are quasi-hydrostatic within the volume affected. Only when a 

 very low pressure system, such as a hurricane, or possibly a sharp front passes 

 over water of coastal depths at speeds approaching or exceeding that of a 

 shallow-water gravity wave will non-hydrostatic effects appear. In deeper 



Table II 

 Magnitudes of Regional Sea-Surface Slopes Produced by Synoptic Influences 



Equilibrium tidal slopes in mid-ocean 10~ 10 



Wind set-up in open ocean (trades) 10 -7 



Sea-surface slopes accompany- 

 ing baroclinic or barotropic 

 geostrophic flows (middle 

 latitudes) 



Shelf tides ; Tsunamis in deep 1 -5 



water 



Quasi -hydrostatic barometric loading 



Margins of anticyclones 10~ 5 



Cores of extra-tropical cyclones 10~ 5 



Hurricanes (near eye) 10 -4 to 10 -3 



Wind set-up by coastal gales 10~ 4 



Tidal slopes in embayments and canals 10 -4 



Bores, capillary and ordinary gravity waves (not regional) 



water the changing loads of atmospheric pressure on the sea surface are very 

 soon (within 12 pendulum hours) compensated by changes in the surface 

 elevation (about 1 cm change of water elevation per mb of atmospheric pressure 

 change) which cancel the horizontal gradients of pressure in the remainder of 

 the water column. Since the slopes of the sea surface associated with barometric 



1 cm/sec 10 7 



10 cm/sec 10 -6 



100 cm/sec 10" 5 



