Sea Level Variations at Monterey, California 



DALE EMIL BRETSCHNEIDER and DOUGLAS R. McLAIN 1 



ABSTRACT 



Sea level data from Monterey, Calif., during the period 1963 through 1976 were compared with data from 

 coastal stations from Peru to Alaska. Sea level fluctuations at Monterey were correlated with data from these 

 stations, particularly those to the south. The causes of sea level fluctuations at Monterey were investigated by 

 correlation, regression, and spectral analysis of sea level with atmospheric pressure, zonal and meridional 

 wind stress, Ekman and Sverdrup transport, surface temperature and salinity, and dynamic height data from 

 nearby locations. Of these variables, dynamic height was the best predictor of sea level fluctuations. Atmo- 

 spheric pressure, surface temperature, and meridional wind stress were of secondary importance. The predic- 

 tion was better during the Davidson Current period than during the upwelling period. 



INTRODUCTION 



Sea level and its fluctuations have interested man for centuries. His- 

 torical sea level time-series data are unique among marine data sources 

 in that they have been obtained continuously and inexpensively over 

 periods of decades or longer at many coastal and island locations 

 worldwide. Sea level records include not only periodic fluctuations due 

 to astronomic tides but also nontidal. low frequency fluctuations result- 

 ing from various oceanic and atmospheric processes. The nontidal 

 components can be isolated by filtering out the astronomic tides, thus 

 making measurements of sea level useful as a spacially integrated index 

 of nearshore and offshore ocean changes. 



This paper examines the character of sea level anomalies at Mon- 

 terey, Calif. , and the relative importance of the large-scale atmo- 

 spheric and ocean processes which may cause nontidal, low 

 frequency fluctuations. An understanding of these processes will 

 allow the use of the abundant historical records of sea level data to 

 reconstruct changes in the past oceanographic environment of the 

 California Current system, which, in turn, may aid in understand- 

 ing past changes in distribution, abundance, and availability of 

 marine fish populations. In particular, the study was designed to 

 examine the utility of sea level data for identification of anomalous 

 environmental periods and for monitoring of changes in coastal 

 oceanographic conditions. 



EARLIER STUDIES ON 

 SEA LEVEL VARIATIONS 



Sea level variations along the Pacific coast and their relationship to 

 various environmental phenomena have been examined from a number 

 of different points of view. In addition to the well-understood astronom- 

 ically induced periodicities, it is widely recognized that coastal sea 

 level measurements are influenced by: 1) wind waves and swell, 2) 

 wind set-up or set-down against the coast due to storms, 3) changes in 

 atmospheric pressure over the ocean surface, 4) redistribution of water 

 mass due to wind stress, 5) changes in average density of the sea water 

 column, 6) long period astronomic tides, 7) subsidence or uplift of the 

 land upon which the tide gage is located, and 8) changes in total mass 

 of water in the oceans associated with the glacial ice budget. These 

 physical processes are discussed by Montgomery (1938). LaFond 

 (1939) found close agreement between weekly mean sea level mea- 



sured at La Jolla, Calif., and offshore geopotential topography, thus 

 directly relating ocean currents to sea level. Jacobs (1939) suggested 

 that the relationships observed by LaFond were not entirely due to 

 changes in the density of surface water but rather to actual slopes 

 induced by wind-driven water transport along the coast. Pattullo et al. 

 (1955) found that south of lat. 40°N in the North Pacific Ocean, the 

 seasonal variation of steric elevation and sea level are in phase, both 

 having a maximum elevation in late summer or early fall and a mini- 

 mum elevation in winter. This they took as a consequence of seasonal 

 heating and cooling. These investigators further found that seasonal 

 variations in sea level north of lat. 40°N along the northwest coast of 

 the United States could not be explained by steric considerations alone, 

 suggesting that nonisostatic processes such as wind and currents can 

 lead to appreciable regional deviations. Roden ( 1960) used autocorrela- 

 tion and spectral techniques to examine the relationship between 

 monthly mean sea level pressure, wind, and sea surface temperature 

 (SST) at several stations along the Pacific coast. He found good coher- 

 ence between anomalies of sea level and atmospheric pressure, moder- 

 ate to poor coherence between SST and sea level depending on the 

 location of the station, and moderate coherence between anomalies of 

 sea level and north-south component of the geostrophic wind. Sturges 

 ( 1974) found high correlations between occasional steric observations 

 and 3-d mean sea levels at Neah Bay, Wash., and San Diego, Calif. 

 Reid and Mantyla (1976) demonstrated that the winter increase in sea- 

 sonal sea level elevation along the northern North Pacific coast results 

 from increased overall flow in the North Pacific subarctic cyclonic 

 gyre. : 



OCEAN AND ATMOSPHERIC PROCESSES 

 NEAR MONTEREY 



Monterey Bay is located along the central California coast, about 

 120 km south of San Francisco. The bay, which is bisected by a deep 

 submarine canyon, is a large, semi-elliptical coastal feature measur- 

 ing about 37 km wide at the mouth and about 19 km from the mouth 

 to the innermost point. 



The bay lies inshore of the broad, diffuse, southward flowing Cali- 

 fornia Current. The strength of the Current is affected by the winds 

 over the Current which, in turn, are controlled by the strength and 

 location of the Aleutian low-pressure cell located over the Aleutian 



'Southwest Fisheries Center Pacific Environmental Group, National Marine 

 Fisheries Service. NOAA, P 0. Box 83 1. Monterey. CA 93940. 



2 After this paper was completed, the thesis of Chelton (1980) became available. 

 The reader is referred to it for additional information on processes affecting sea 

 level along the coast. 



