change, curves were generated using data from 8,200 to 9,800 obser- 

 vations for each of the 12 mo of the year (Fig. 4). The frequency dis- 

 tribution of nontidaJ sea level fluctuations changes seasonally. In 

 April, for example, 73% of the observed sea levels were lower than 

 predicted, but in September, 81 % of the observed data were greater 

 than predicted. From March through May, observed sea levels tend 

 to be lower than predicted sea levels, probably due to offshore Ekman 

 transport, low water temperature, and atmospheric pressure effects as 

 discussed later. From July through January, observed sea levels are 

 higher than predicted due to atmospheric pressure and thermal 

 expansion effects during summer and fall, and to onshore transport, 

 pressure, and thermal effects during the Davidson Current period in 

 December and January. It is not clear why these seasonal differences 

 occur since one would expect seasonal effects to have been included 

 in the harmonic constituents. Perhaps the differences occur because 

 of variations in the frequency of occurrence of events in different 

 years. Thus harmonics generated from measurements in only a single 

 year may not be typical of other years. 



The distributions of differences for winter months are wider and 

 less peaked than those of summer months, indicating greater variabil- 

 ity and larger nontidal events such as winter storms. In contrast, the 

 distributions for July and August are narrow and more peaked. 



Monthly Mean Sea Level. — Averaging of hourly sea level values 

 over intervals of weeks to months removes the effects of the principal 

 diurnal, semi-diurnal, and other short-term tidal components from the 

 data to reduce the quantities of data to manageable size and to empha- 

 size the longer time scales. 



Monthly means of the hourly values were calculated for the period 

 July 1963 through August 1976 and were updated for the period Sep- 

 tember 1976 through December 1978 with monthly mean values pro- 

 vided by the NOS. Figure 7 shows the long-term monthly means, 

 standard deviations, and extremes of the monthly means of sea level at 

 Monterey and other stations along the coast. Mean sea level at Monte- 

 rey is lowest in April and highest in September, with a mean annual 

 range of 13.6 cm. Variability is highest during winter months, with 

 monthly standard deviations during winter being almost double those 

 for summer. The range between maximum and minimum monthly val- 

 ues reaches a high of 21.0 cm in January and a low of 8.5 cm in 

 August. 



Anomalies of monthly sea level were calculated as differences 

 between the monthly mean and the long-term mean for the same 

 month. Calculation of anomalies in this manner removes the annual 

 cycle from the data and allows examination of processes of nonannual 

 periods. Monthly mean sea levels and their anomalies are shown in tab- 

 ular and graphical form in Appendix B. In these figures extreme 

 monthly sea level anomalies are shown to range from -10.8 cm in 

 December 1975 to + 10.7 cm in January 1978. Periods of anomalously 

 high sea level occurred during 1969, 1972-73, 1976-77, and early 

 1978, and periods of anomalously low sea level occurred in 1964, 

 1970, 1971, 1973, 1975-76, and 1977. 



To statistically define the persistence of anomalous periods, the auto- 

 correlation function was used. This function describes the decay of the 

 correlation coefficient of the data series with itself as the date series is 

 time shifted relative to itself an increasing number of lag periods 

 (months). The autocorrelation function of monthly Monterey sea level 

 anomalies (Fig. 5) shows that sea level anomalies are correlated at the 

 5 % level of significance for lags of up to 5 mo, indicating that anoma- 

 lies persist over a period of several months. The autocorrelation func- 

 tion of the sea level series appears to decay exponentially for the first 8 

 mo or so, with significant negative autocorrelation coefficients occur- 

 ring from lags of 1 1 to 18 and 23 to 26 mo. 



-61 -30.5 30.5 61 cm 

 Observed Minus Predicted Tide Ht 



Figure 4. — Frequency of occurrence by month of differences 

 between observed and predicted hourly tide heights at Mon- 

 terey, Calif., 1963-76. 



