1.0-1 



1.CH 



100 75 50 25 

 Period ( days ) 



Figure 12.— Amplitude response function for the 30-d running mean filter used 

 to low pass filter hourly sea level data from Monterey, Calif. 



occurred in the 0.04-0.08 cpd frequency band (24-12 d). This peak 

 was present in all series and was significant at the 95 % confidence 

 level for pressure but not for sea level. Fluctuations of longer period 

 than this peak appear to be more important for sea level than for pres- 

 sure. The filters used in the analysis had been designed to isolate vari- 

 ations with periods 2-10 d (0.5-0.2 cpd) but did not reveal any 

 significant spectral peaks in that region. 



The coherence (squared) between sea level and atmospheric pres- 

 sure was found to be significant and independent of frequency in the 

 upwelling period (Fig. 14). but in the winter period (Fig. 13). 

 decreased in magnitude at frequencies greater than 0.5 cpd (<2d). The 

 nearly constant 180° phase angle between the two series reflects the 

 inverse response between atmospheric pressure and sea level as 

 expected from the hydrostatic equation. 



In order to better examine the relationship of wind stress and sea 

 level, the low-passed 6-h sea level series was adjusted for atmospheric 

 pressure effects and detrended using the 30-d running mean filter 

 described previously. Auto- and cross-spectra were then calculated for 

 the 6-h adjusted sea level and meridional wind stress series (Figs. 15, 

 16). Like the atmospheric pressure and unadjusted sea level series, 

 meridional wind stress had a concentration of energy at low frequencies 

 with large variations occurring in the 0.04-0.08 cpd frequency band, 

 and the winter season power spectra contained more energy than that of 

 the upwelling season. Coherence between adjusted sea level and 

 meridional wind stress is generally low. The phase angles provide little 

 information because of the low coherence. 



cpd 



.O-i 



o 



c 



0) 



t_ 



<u 



.c 

 o 

 O 



.6- 



.4 — 



.2- 



0.0 



o o o o 



OO 



O o 



o o 



95* 



0.1 



.6 cpd 



0—1 



ro 



180 



•0—1 



o o 



0.1 



-inr 



6'0 6 6 o o'o 



-rWi 



cpd 



95% 



I 



SUMMARY 



Analysis of 13 yr of hourly sea levels indicates that nontidal sea 

 level variations are small compared with the normal tide range in the 

 area. The largest nontidal deviation observed was 39.6 cm. A sea- 

 sonal change revealed by monthly frequency distributions of hourly 

 nontidal sea level variations was found, with observed sea levels 

 being generally less than the predicted during March through May 

 and greater than the predicted from July through January. 



Monthly sea level anomalies at Monterey are correlated with 

 anomalies at tide stations from Prince Rupert. Canada, to Callao. 

 Peru, but are most closely related to events affecting sea levels in the 

 group of stations from Crescent City. Calif., to Quepos. Costa Rica. 

 Processes producing the El Nino phenomenon in the eastern tropical 

 Pacific affect sea level at Monterey with a lag of about 6 mo. 



Figure 13.— Spectral plots of 6-h atmospheric pressure (Press) and unadjusted 

 sea level (SL) for the winter period (df = 90) at Monterey, Calif. The horizontal 

 axes are frequency in cycles per day (cpd). The upper plot shows spectral den- 

 sity of pressure (in mb 2 /cpd) and sea level (in cm 2 /cpd); the middle plot shows 

 the squared coherence of the two series; and the lower plot shows the phase. 



Multiple regression analysis indicates that monthly anomalies of 

 dynamic height and meridional wind stress account for most of the 

 monthly sea level variability at Monterey during both the Davidson 

 Current and upwelling seasons. Atmospheric pressure and SST 

 account for an additional portion of sea level variability during the 

 upwelling season. 



There is good agreement between the behavior of sea level and 

 dynamic height in both a seasonal sense and in interyear variability. 

 The close agreement between sea level and dynamic height, and the 

 high correlation of sea level at Monterey with that at adjacent tide sta- 



18 



