SECT. 5] SEASONAL CHANGES IN SEA-LEVEL 49f> 



E. Regional Effects 



Regional effects are superimposed on all of these general variations, and, 

 indeed, at times are the largest and most interesting features observed. Monsoon 

 effects are large and complicated around all of the coastlines of southeast Asia 

 from Arabia to Japan. In these waters, precipitation, wind and advection of 

 different water-masses throughout the year are all important. Local effects of 

 similar types must be considerable around the tropical coasts of Mexico and 

 Australia, and have been shown to be of great influence on the shores of northern 

 Europe. 



F. Isostasy 



Finally, for most of the deep-water areas of the open ocean, the seasonal 

 variation in sea-level is part of an isostatic variation in the distribution of 

 mass in the hydrosphere and troposphere. That is, the total mass of air plus 

 water above a point on the sea floor is very nearly constant throughout the 

 year. However, the fraction of the mass that is water (as opposed to air) and 

 also the specific volume of the water change with season, and both effects are 

 reflected in changes in elevation of the air-sea interface. Along most coastal 

 areas, non-isostatic changes of relatively local origin must be appreciable and 

 obviously cannot be summarized here. The climatology of each such region must 

 be examined in detail before a complete discussion can be given. 



5. Conclusions 



Although many able workers have devoted themselves to the study of 

 seasonal variations in sea-level, only a few facts can be considered definitely 

 established. The principal features of the large-scale variations are now evident, 

 and about a half dozen local areas have been thoroughly studied. Much work 

 has been done in correlating changes in sea-level to changes in meteorological 

 phenomena, in particular to variations in atmospheric pressure and in local 

 winds. A little is known about oceanographic phenomena such as the variation 

 in specific volume of the water throughout the year. On the other hand, rela- 

 tively little has been accomplished as yet in the study of seasonal variations in 

 currents, or in distinguishing between the barotropic and baroclinic portions of 

 these. Only a few workers have been able to make any quantitative estimate as 

 to the contribution of advection to the observed changes in local sea-water 

 characteristics which result in what we have called steric levels. Observations 

 of set-up in response to winds have been, perhaps necessarily, confined to en- 

 closed basins. It is to be hoped that the exchange of data and ideas initiated in 

 1933 by the International Association of Physical Oceanography, and the 

 added impetus provided by the opportunities of the International Geophysical 

 Year, will lead to fruitful work on some of these problems. 



