CalCOFI Reports, Vol. 11, 1967 - California Cooperative Oceanic ...

160CALIFORNIA COOPERATIVE OCEAKIC FISHERIES INVESTIGATIOKSThe papers cited have described the major featuresof the variations in temperature and salinity in theCalifornia Current region. The present study employsa totally different method of analysis which substantiatesthe previous findings. Detail is provided in someareas and for some months that has not previouslybeen available. Almost all of the statistical treatmentis new.California Cooperative Oceanic Fisheries InvestigationsAtlas No. 1 (1963) contains distributions chartsof 10-meter temperature and salinity for each Cal-COFI cruise, 1949-59. The atlas also presents chartsdrawn from 10-year monthly means of temperatureand salinity. Near the end of this paper a brief comparisonis made between the mean distribution chartsof the atlas and those derived from the present analysis.THE DATAThe CalCOFI data through 1959 are published inthe series, Oceanic Observations of the Pacific. Datafor more recent cruises are available in the data reportseries (unpublished) of the University of California Scripps Institution of Oceanography.Records for 222 stations were selected for analysisfrom the CalCOFI 10-meter data for 1950-62 accordingto criteria of occupancy and location (Figure 2).Thirty percent of the stations were occupied 14 to50 times, and the remainder 51 to 119 times. The stationswith the lesser frequency of sampling are foundat the seaward extreme of the station pattern, offCabo San Lazaro and south, and scattered among thestations north of Point Conception. These stationrecords were used to extrapolate charting of thedistributions beyond the better sampled areas. Theobservations are not evenly distributed throughout theyear but occur more frequently in April-July, andless frequently in September, November, and December.Figure 13 shows time plots of data records withall years folded into one 12-month period. (An explanationof Figure 13 is given under STATIONREGRESSION CURVES.) Only one observation permonth was used ; when duplicate observations weremade, usually the one nearest mid-month was used.In addition to the CalCOFI data, portions ofrecords from five shore stations (surface only) wereanalyzed :Temperature SalinityBlunts Reef _~ ___-_--_ __-_____ 1955-62 1957-62S.E. Farallon Island _--_ _ ____-- 1955-62 1957-62La Jolla ..................... 1950-62 1956-62 *Guadalupe Island --_-_ _ _----__ 1956-60 -Cedros Island ~ ~ ~ _ ~ ~ _ 1957-62 _ ~ _ _ ~ ~ ~ _ ~ _Observations were made daily. The records were collectedby the U.S. Coast and Geodetic Survey andScripps Institution of Oceanography (unpublished).For this analysis, monthly averages (daily observationsaveraged for each month of each year) wereentered as initial data. Thus, there is one value permonth comparable in number to the CalCOFI samplingValidity of salinity observations for 1944-55 were questioned byRoden (1961) : hence, these observations were not includedin this analysis.program, but with some high frequency variationfiltered out.CHOICE OF THE 10-METER LEVELHydrographic casts were made during about 80percent of the station occupations ; the remaining 20percent were “net-haul” stations, where work consistedof biological sampling and a 10-meter temperatureand salinity measurement. The 10-meter levelwas chosen to represent the upper mixed layer in lieuof a surface sample to avoid such transient conditionsas might be caused by rain or river runoff and bydiurnal heating and cooling. In some places a shallowsummer thermocline may develop. When this thermoclineis shoaler than 10 meters it is readily subjectto wind stirring; hence, its existence is usually brief.These arguments do not obtain in the vicinity ofCab0 San Lucas3 where oceanic fronts and othercomplex features persist (Griffiths, 1965).ANALYSISAn expression of the mean seasonal variation of acharacteristic may be obtained from a digital recordby Fourier polynomials, a method of harmonicanalysis. Because the time intervals between datameasurements are irregular, standard textbook formulasderived for processes sampled at equallyspacedintervals are unsuitable and hence an approachfrom basic concepts is necessary. A detailed descriptionof each data record by harmonic analysis is notnecessary ; the only harmonics needed are those whichcontribute significantly to the description of the seasonalvariation. This consideration leads to a differentbut totally equivalent approach which has an addedadvantage. The mean seasonal variation may be obtainedby least squares regression of the data (consideringeach station record as a time series) toannually periodic sinusoids. Van Vliet and Andersonanalysed sea surface temperature records for seasonalvariation by fitting annual and semiannual harmonicsto the observed data by regression analysis. Theiranalyses were performed on long records of dailytemperature observations at four shore stations andtwo weathership stations. Least squares regressionanalyses for curve fitting is identical to the morecommon application of estimating linear relationships.The added advantage of this method is seen inthe statistical approach; it is a natural adjunct ofregression analysis to compute measures of dispersion,correlation coefficients, and significance parameters.Though less natural, such computations can bemade with truncated Fourier polynomials.Natural events driven by insolation tend to varywith an annual cycle that may be roughly describedby a sinusoid. However, because the effect of insolationis often indirect, the rough approximation providedby the annual sinusoid can usually be refined byGeographical locations are identified in Figure 2.4 Statistical analyses of sea surface temperature time series (unpublishedmanuscript). U.S. Navy Electronics Laboratory,San Diego, Calif.