SAUR: SEA LEVEL DIFFERENCES 



gomery (1938) cautioned that he had only a 

 meager number of hydrographic stations for 

 this, but other studies since that time tended to 

 confirm his results. 



For the southern California coast, LaFond 

 (1939) found good agreement between weekly 

 sea level and contours of dynamic height 

 (0/500 db) extrapolated to the tide station. Spe- 

 cial observations of sea level and temperature 

 structure were carried out at many island sta- 

 tions during the International Geophysical Year 

 from which Lisitzin and Pattullo (1961) con- 

 cluded that in the open ocean most of the devi- 

 ations from mean sea level can be explained bj^ 

 combined atmospheric pressure effects and steric 

 effects, the latter being defined as those due to 

 changes in the specific volume of the water col- 

 umn, i.e., those measured by dynamic height 

 anomalies. Shaw and Donn (1964) had 173 hy- 

 drographic stations, taken approximately bi- 

 weekly by the Panulmis oflf Bermuda over a 

 period of nearly 7V2 years, to compute steric 

 levels for comparison with sea levels. They 

 found that about 80 7r of the variance of raw 

 sea levels, which included the seasonal cycle, 

 resulted from a combination of the atmospheric 

 pressure effect, which was weak, and the steric 

 effect, which was dominant. 



Sturges (1966) has shown high correlations 

 between steric levels and mean sea level at two 

 Pacific coast locations. The least squares regres- 

 sion of steric levels computed for the coast 

 against 3-day sea level (adjusted for atmospher- 

 ic pressure) for Neah Bay, Wash., was 0.97 with 

 a standard error of estimate of 4.1 cm and a cor- 

 relation of 0.904. The regression coefficient at 

 San Diego, Calif., was only 0.61 with a standard 

 error of estimate of 2.2 cm and a correlation of 

 0.914. Theoretically, for variations of sea level 

 adjusted for pressure to agree with steric level, 

 the slope of the regression should be unity. 

 Sturges also estimated the wind set-up effect at 

 Neah Bay, which has a narrow continental shelf, 

 to be negligible. 



Such studies indicate, as summarized by Donn, 

 Pattullo, and Shaw (1964) , that the combined at- 

 mospheric pressure and steric effects account 

 for most of the sea level variations of periods 

 longer than a few months. Thus, the interpre- 



tations made here assume that sea level differ- 

 ences, suitably adjusted for trends and atmos- 

 pheric pressure, are a reasonable measure of 

 changes in broad-scale geostrophic currents. 



SURFACE CURRENTS OF THE REGION 



Our area of interest is shown in Figure 1. Sea 

 level differences between Honolulu and San 

 Francisco and between Hilo and Avila are to be 

 studied. The great circles joining each pair of 

 stations span the same region of the eastern 

 North Pacific Ocean. 



The currents in the region are part of the 

 eastern limb of the major anticyclonic current 

 gyre of the North Pacific Ocean. The general 

 pattern is shown in Figure 1 by the 0/1,000 db 

 contours of dynamic height anomaly. The sur- 

 face current is generally to the southeast, nearly 

 normal to the great circles over most of the dis- 

 tance. At these latitudes the California Current 

 extends from the California coast to about long 

 130°W, or somewhat farther, and about one-half 

 of the change in geopotential anomaly takes place 

 across this current, i.e., in less than one-third 

 the distance between the stations. As a typical 

 eastern boundary current (Wooster and Reid, 

 1963), it is broad, sluggish, and reinforced by 

 coastal upwelling during the spring and summer 

 months. During the winter months, December 

 through February, a narrow north flowing 

 countercurrent, often referred to as the David- 

 son Current, frequently occurs at the surface 

 along the central California coast. 



The California Current feeds into the North 

 Equatorial Current, the axis of which lies south 

 of lat 20°N (Seckel, 1962) so that there is gen- 

 erally a southward component across the great 

 circle near the Hawaiian Islands. Occasionally, 

 however, there is a west-northwestward flow 

 along the east side of the islands, as evidently 

 occurred at the time of the 1955 Norpac survey 

 (Oceanic Observations of the Pacific, 1960). 

 Corresponding to this return flow across the 

 great circle, the sea levels at the Hawaiian sta- 

 tions would be lower than sea level northeast 

 of the islands. Such localized conditions can- 

 not be revealed by the sea level data, and thus 

 are one source of "noise" in the data. 



621 



