FISHERY BULLETIN: VOL. 70, NO. 3 



The data have been less utilized in the study of 

 specific oceanoofraphic changes. Stewart, Zetler, 

 and Taylor (1958) pointed out the rise in sea 

 level along the west coast of North America dur- 

 ing the warming period of 1957-58. Bjerknes 

 (1966) also noted changes in sea levels along the 

 equator during El Nifio, i.e., warming which oc- 

 curs off the Pacific coast of South America, dur- 

 ing the same 1957-58 period. 



In this paper the use of sea level information 

 in the northeast Pacific Ocean to augment infer- 

 ences about oceanographic changes is explored. 

 The geostrophic equation requires that the speed 

 of the surface current be proportional to the 

 transverse slope of the sea surface. Thus time 

 series of differences in sea level between the Ha- 

 waiian Islands and the California coast could 

 be an index to broad-scale changes in surface 

 current in the eastern North Pacific Ocean. Some 

 earlier exploratory work on sea level differences 

 between Honolulu and San Francisco through 

 1962 was reported orally at meetings of the 

 American Geophysical Union (Saur, 1966). 

 These preliminary results indicated that a pro- 

 nounced weakening of the strength of the south 

 flowing current around the eastern limb of the 

 North Pacific current gyre was associated with 



140' 150' 



WEST LONGITUDE 



Figure 1. — Station locations and general pattern of 

 surface currents (solid contours: geopotential anomaly 

 at the sea surface relative to the 1,000 db surface, in 

 dynamic meters) after the NORPAC Atlas (Oceanic 

 Observations of the Pacific, 1960) but modified near the 

 Hawaiian Islands per Reid (1961) and Seckel (1962). 

 Dashed lines are great circles joining pairs of tide sta- 

 tions for which sea level differences are calculated. 



the warm winters of 1940-41 and 1957-58, which 

 have been reported for the California Current 

 region (Reid, 1960; Robinson, 1961). The oc- 

 currence of El Nino in these same periods 

 (Bjerknes, 1961, 1966) further indicated the 

 possibility of a relaxation in strength of ocean 

 current systems in the eastern Pacific on a broad 

 scale. 



No time series of direct observations of ocean 

 conditions exist to study such broad-scale 

 changes in current, and certainly not of the con- 

 tinuous nature of sea level data. In this paper 

 therefore, the normal seasonal cycles of sea level 

 differences between the Hawaiian Islands and 

 the California coast are presented and some in- 

 ferences are drawn from the 65-year record of 

 Honolulu-minus-San Francisco differences about 

 the character of changes of surface current in 



the region. 



EARLIER STUDIES 



Montgomery (1938) was the first to use the 

 cross-current difference in sea level in the geo- 

 strophic equation to estimate the range of fluc- 

 tuation of ocean current. Using changes in sea 

 level difference between Charleston, S.C, and 

 Bermuda, he found the range of seasonal var- 

 iation was about 32 '^r of the average difference. 

 Stommel (1953) used fluctuations in cross-cur- 

 rent sea level differences between Havana and 

 Key West and between Cat Key and Miami as 

 a measure of current in developing a model of 

 the structure of the Florida Current. More re- 

 cently, Wunsch, Hansen, and Zetler (1969) mea- 

 sured statistical variability of the Florida Cur- 

 rent by spectral and coherence analyses of longer 

 sea level records at the same four stations. They 

 found that the apparent seasonal variation of the 

 Florida Current accounts for only about 10 9r 

 of the root-mean-square modulations for periods 

 from 2 days to 1 year. They further concluded 

 that monthly mean sea levels could be used to 

 indicate long-period fluctuations. 



Changes in mean sea levels at shore stations 

 have been favorably compared by other investi- 

 gators with changes in geopotential height of the 

 sea surface in deep water offshore as traditional- 

 ly determined from observed subsurface distri- 

 bution of density of the water column. Mont- 



620 



