Seasonal variation of the water mass along the Oregon-northern California coast

Temperature and salinity data collected at five Oregon-northern California coastal stations from 1960 to 1971 are used to assess the influence of local processes in modifying the water mass along the coast. A bivariate model is constructed for the summer and winter seasons at each coastal station permitting a quantitative measure of the degree of importance of each process. In summer the dominant modifying process is different at each shore station with local heating, upwelling, and mixing with shelf/slope water and Columbia River water being the major factors. In winter rainfall and runoff is the dominant process along the coast except near the northern border where mixing with shelf/slope water occurs slightly more frequently. In winter offshore surface waters within 81 km of the coast have nearly the same T-S as the waters at the coast. In summer offshore surface waters become cooler and more saline in the shorcward direction with the coolest and saltiest water occurring at the coast. Oceanographic features along a coastal boundary may vary markedly from one area to another due to local variations in those processes which influence the properties of coastal water masses. Within the coastal region, Dodimead et al. (1963) point out that local variations in runoff, heating and cooling, winds, etc. may cause variations in water properties, horizontal and vertical structure, and circulation patterns. To assess the magnitude of these variations, a program was instituted in 1960 to collect weekly temperature and salinity samples at selected locations along the Oregon coast. From these data Pattullo and Denner (1965) attempted to describe the variability of the coastal waters off Oregon, using a bivariate analysis of the temperature-salinity data to determine the dominant modifying processes prevalent in summer and winter and at various locations along the coast. Their analysis was based on only 2 years ( 1961-1962) of relatively sparse data. More frequent observations were deemed necessary, since some of the modifying 1 Present address: Naval Postgraduate School, Montcrcy, California 93940. ’ Deceased, 1972. processes were found to have periodicities of less than a week. In addition, fewer but more widely spaced short sampling stations were required since sufficient uniformity existed in the temperature and salinity distributions along the coast. Since 1964, therefore, sampling was conducted on a daily basis at five to seven coastal stations. The observations were taken by volunteer:;, so the sampling tended to ‘be intermittent. After 12 years ( 1960-1971) in operation, the coastal sampling program was formally terminated. This paper continues the study initiated by Pattullo and Denner (1965) and examines the physical processes important in modifying the coastal waters of Oregon and northern California. It is based on all the observations through mid-1971. We ale grateful to all the volunteers who collected the water samples, in fair weather and foul, over the past decade. Coastal station data To examine the uniformity of the coastal waters of Oregon and nort‘hern California, we analyzed temperature and salinity data from five nearly equally spaced shore stations ( Fig, 1). The data are listed in te,n LIMNOLOGY AND OCEANOGRAPHY 190 MARCH 1974, V. lO( 2) Seasonal variation of coastal waters technical reports of Oregon State University ( 1961-1972). The Seaside, Newport, and Crescent City stations are located on or near estuaries or bays. To preserve the oceanic character of the samples from these stations we used only data taken at high tide. The Cape Arago and Port Orford stations are located on the rocky outer coast and experience little freshwater dilution. Sample temperatures are accurate to about + 0.2”C. Most salinities were computed from hydrometer readings calibrated against an inductive salinometer and are considered accurate to f 0.2$/00. The data were arranged into summer and winter distributions. Each of these seasons may be considered as a time when dominant mean physical factors such as wind and current directions are in a quasi-steady state condition (Bourke et al, 1971). The data for July and August are combined to form the summer season for all stations except Crescent City, where June and July are used because these are the dominant months for coastal upwelling in this region. The winter season includes December, January, and February. The seasonal data at each shore station were then divided into class intervals of 1°C for temperature and O.Sp/,, for salinity. The frequency of occurrence of each temperature and salinity observation was recorded in its particular cell as defined by a pair of temperature and salinity intervals (Montgomery 1955). As examples, the summer and winter T-S fields for Seaside and Cape Arago are shown in Figs. 2 and 3. Because of the skewness of the temperature and salinity distributions, modal rather than mean values have been used as the more precise measure of central tendency. The single cell containing the greatest number of observations is termed the “central” or “modal” cell. The T-S characteristics of this bivariate mode make up a water type that is more prevalent than any other type and, following Pattullo and Denner ( 1965), is assumed to be representative of water typically found at the coast. Deviations from this water type are due to a change Fig. 1. Location of stations along the Oregonnorthern California coast. in one or more of the processes that alter the oceanic water from which the typical coastal water is formed. The modal cell water generally has characteristics different from oceanic water found farther offshore, especially in summer. The source of the modal cell water has not been rigorously investigated since this study is concerned with those processes which alter the water once it has reached the coast. However, we can reasonably speculate that coastal water is comprised of a mixture of oceanic surface water, upwelled water, and Columbia River water, with the contributions from each varying regionally and seasonally. A representative mixture might be 80% oceanic surface, 15% upwellcd, and 5% Columbia River water. Other cells containing large numbers of observations are found near the central cell and represent water types that have been modified in varying degrees by local processes. On each T-S fi,eld is an isopleth enclosing the