1.0-r 



0.5-- 1 



0.0 



.5-- 



-1.0 1 - 



■^c 



S\ 



'^A S 7 5° 



Lags (in months } 

 5* Sig. Level 



Figure 5.— Autocorrelation function for anomaly of monthly mean sea level at 

 Monterey, Calif. The number of data points is 180 and the significance level is 

 computed assuming a normal distribution of correlation coefficients. 



Relation to Other Pacific Coast Tide Stations 



We have seen that mean monthly sea level anomalies at Monterey 

 tend to persist for up to 5 mo. The question naturally arises as to 

 whether these anomalies are of local or regional geographic extent. 

 To determine the spacial and temporal coherence between the 

 monthly anomalies at Monterey and those observed at neighboring 

 tide recording stations, monthly mean data were assembled for 15 

 tide stations along the Pacific coast ranging from Sitka, Alaska, to 

 Callao, Peru (Fig. 6). These data were obtained from Klaus Wyrtki 

 of the University of Hawaii and from the NOS. Stations selected for 

 analysis were those having the best combination of the following 

 characteristics: 1) representativeness of open ocean conditions, 2) 

 long and continuous data record, 3) a constant tidal reference datum, 

 and 4) suitable spacing between station locations along the coast. For 

 each station, long-term monthly means were calculated from the 

 available data for the period 1963 to 1978 and monthly sea level 

 anomalies were derived (Fig. 7). 



For stations north of Crescent City, frequent energetic winter 

 storms cause the time series of anomalies to have only moderate per- 



^^" 



N 



1 



Maiat Ian , Me 



Figure 6. 



-Location of 15 tide stations along the « est coasts of North and South 

 America «hose data «ere used in this study (see text). 



sistence whereas stations south of San Francisco have much greater 

 persistence of anomalies. Perhaps the most striking feature of the 

 time series is the high visual correlation of anomalies along the coast 

 (Bretschneiderand McLain 1979; Enfield and Allen 1980). The peri- 

 ods of anomalously high sea level at Monterey during 1969, 1972- 

 73, 1976-77, and 1978 were common to most stations where data are 

 available. Similarly, the periods of anomalously low sea level seen at 

 Monterey in 1964, 1970, 1971, 1973, 1975-76, and 1977 occurred at 

 most of the other stations. 



Correlations of the monthly sea level anomalies between stations 

 were calculated using the BMDP8D statistical program (Dixon 1975) 

 and are tabulated in Table 1 . The correlation of the selected tide sta- 

 tions relative to Monterey is shown graphically in Figure 8. Correla- 

 tion of the Monterey anomalies is seen to be highest with San 

 Francisco (r = 0.85) and lowest with Sitka (r = 0.15). Note also that 

 the correlation coefficient drops off more rapidly with distance to the 

 north of Monterey than to the south, due to the different space scales 

 of the processes affecting sea level to the north and south. 



Osmer and Huyer (1978) suggested the existence of two domains 

 of coastal sea level fluctuations, with a boundary located south of San 

 Francisco in winter and north of Crescent City in the spring and sum- 

 mer. The general location of their break-point is in agreement with 

 the findings of Zee (1975). who suggested that sea level anomalies at 

 stations from San Francisco southward to the Equator were related to 

 nonseasonal vertical movements of the thermocline. That an oceano- 

 graphic gradient or boundary may exist between northern and south- 

 ern stations is further suggested by Nelson (1977) who showed that 

 the area off northern California near Cape Mendocino is one of 

 marked change in the seasonal surface wind stress field. The mean 

 seasonal wind stress field over the coastal ocean south of Cape Men- 

 docino is alongshore (southward) all year while the stress field north 

 of Cape Mendocino is strongly onshore in winter and alongshore 

 (southward) in summer. 



The geographic coherence of sea level anomalies observed at 

 Monterey with the neighboring tide stations along the coast was fur- 

 ther examined in a time-distance domain. The monthly anomalies 

 from the series of 15 coastal stations from Sitka, Alaska, to Callao, 

 Peru, were plotted and contoured at 5 cm intervals for the period 

 1963 to 1974 (Fig. 9). Data for the years 1975-78 were not available 

 for several of the stations so plots for these years are not included. 

 The monthly anomalies have recognizable patterns which are coher- 

 ent in both time and space. For example, large negative anomalies 

 can be seen in January 1963 extending from Crescent City to Sitka 

 and large positive anomalies in the same region occur in the subse- 

 quent fall and winter. 



Anomalies of greater magnitude and stronger gradients in time and 

 space occur northward of a boundary zone lying between Crescent 

 City and Monterey, than to the south. Anomalous events north of this 

 zone tend to occur simultaneously along the coast and persist for 1 or 

 2 mo. Anomaly magnitudes and gradients are also generally larger 

 southward of a second, less well-defined boundary zone lying 

 approximately between Manzanillo and Quepos. Between these 

 boundary zones, gradients of the anomaly field are relatively weak. 

 Southward of the zone between Crescent City and Monterey, sea 

 level anomalies are of relatively long duration, as was noted earlier. 



A particularly interesting event is the anomalously high sea level 

 during the period October 1972 through February 1973 between Callao 

 and San Francisco. This was a period of strong El Nino activity in the 

 eastern tropical Pacific. During El Nino occurrences warm advection 

 occurs into the eastern tropical Pacific Ocean and high SST's are 

 observed. Sea level rapidly rises in the eastern tropical Pacific and falls 

 slowly in the western Pacific (Wyrtki 1977). A peak sea level anomaly 



