SECT. 5] SEASONAL CHANGES IN SEA-LEVEL 491 



3. Factors Influencing Variations in Sea-Level 



Many explanations of variations in sea-level have been proposed ; Rossiter 

 has summarized them for all long-period changes in Volume I, Chapter 16. 



There has been, unfortunately, some confusion in discussions of cause and 

 effect in this field, due principally to the fact that meteorological and oceano- 

 graphic phenomena are so inextricably interdependent. It appears more 

 suitable at this point, instead of attempting to search out all "initial causes", 

 to consider one by one the other physical phenomena to which changes in sea- 

 level must somehow be related. This approach offers the advantages that these 

 accompanying phenomena can in most cases be measured, although few have 

 yet been studied in any detail, and one can hope that eventually the causes of 

 each will be thoroughly explored. The following table lists the various pheno- 

 mena and the maximum seasonal rise in sea-level that is known or estimated to 

 accompany each. 



Table I 



Seasonal Deviation in Sea-Level (cm) and Associated 

 Phenomena 



10 20 



(a) Fall in local atmospheric pressure (total air-mass 



over oceans constant) x 



(b) Increase in heat content of water (mass of sea- 

 water constant) x 



(c) Decrease in salinity of water (mass of sea-water 



constant) x 



(d) Increase in speed of onshore component of wind '! 



(e) Increase in speed of longshore component of 



current (sign of effect depends on direction of flow) ? 



(/) Approach of annual or semiannual high of astro- 

 nomic tide x 

 (g) Increase in total mass of water in oceans x 

 (/?) Decrease in mixing of water ? 



The amplitudes given are estimated for the open ocean or exposed coastline; 

 in restricted bodies of water, local conditions must be more completely con- 

 sidered. Where "x" appears in the column the estimates have been derived 

 from the following sources: (a) Lisitzin and Pattullo (1961): (6), (c) and (/), 

 Pattullo et al. (1955) ; and (g) van Hylckama (1956). Question marks represent 

 the author's estimates as to the probable maximum amplitudes associated with 

 the phenomena indicated. 



If these estimates have any reality at all, it appears that the larger of the 

 variations in sea-level with season are related to the first five phenomena listed. 

 Terms (a), (b) and (c) considered together constitute what has been called the 

 isostatic part of sea-level variations, because they are not accompanied by 



