6 TJie Ocean 



the sea-level as a geopotential surface will no longer have the same simple ellipsoidal 

 form but will show little variations to either side. This irregularly shaped surface is 

 called in the theory of the Earth figure the "geoid". The geoid can be regarded as dis- 

 placed from the surface of the rotational ellipsoid by the distortions of the continental 

 masses. The geoid rises on passing from the sea towards the continents and falls on 

 passing towards the sea again. Figure 4 illustrates the undulations of the geoid around 



Ocean Continent 



Rototionol ellipsoid ^^llXlIIJilL-J 'n^^^^^ 



Fig. 4. Undulations of the geoid about the rotational ellipsoid. 



the rotational ellipsoid. The ideal sea-level (geoid) lies below the rotational elHpsoid in 

 sea areas and above it in land areas. The magnitude of these deviations depends on the 

 magnitude of the gravitational anomahes in the upper crust of the Earth. It was at first 

 thought from theoretical considerations that the undulations of the geoid must be 

 rather large. However, it was found that due to the almost perfect isostatic adjustment 

 of the masses of the outer crust (hydrostatic equilibrium), these remain rather small 

 and amount to not more than rhlOOm. The forces that cause periodic variations 

 of the actual sea-level from the geoid were mentioned above. Amongst these are the 

 forces due to the attraction of the sun and the moon which produce the tides in the 

 ocean and the tangential force of the wind on the surface of the sea which causes 

 ordinary sea waves. Both of these effects on the sea-level initiate waves that can be 

 considered as oscillations to either side of a mean sea-level. It can be fixed at any 

 coastal station by continuous observation of the water level, because the influence of 

 the tides can be excluded if full-yearly observations are available while the effect of 

 the ordinary wave motions disappears in a daily mean of observations. 



Other forces affecting the ideal sea-level may cause long lasting displacements of the 

 actual sea-level from the geoid. If these forces are steady the corresponding displace- 

 ments will also be steady and give a static equilibrium state. Also in the case of slowly 

 changing forces the time will be sufficient for the sea-level to follow the changes. If, 

 however, there are rapid changes in the intensity of the force the situation will be 

 more complicated and an oscillation may develop depending on the size of the water 

 masses involved. 



An important source of steady displacements of this kind from the ideal sea-level 

 is the effect of barometric pressure. The ocean reacts to steady changes in the atmos- 

 pheric pressure on the surface like an enormous water barometer: as the atmospheric 

 pressure rises the sea-level will fall below the geoid, as the atmospheric pressure falls 

 it will rise above it. When conditions are stationary there can be no pressure difference 

 between two points at the same level within the ocean. The pressure at a depth h^ 

 below ideal sea-level in a homogeneous sea of density pq will be 



