GEOPHYSICAL RESEARCH LETI'ERS, VOL. 28, NO. 20, PAGES 3935-3937, OCTOBER 15, 2001 Seawater Temperature Trends at USA Tide Gauge Sites GeorgeA. Maul and Andria M. Davis Departmentof Marine andEnvironmental Systems, Florida Institute of Technology JeffreyW. Simmons BahamasDepartment of Meteorology Abstract. Seawater temperatureshave been measuredat on the photocopiesare hand entries;monthly and annual UnitedStates tide gauges throughout most of the 20 th century. means were mechanicallycalculated. All availablerecords have been digitized, and the longest14 Temperaturesand densitiesfrom the tidal stationsheets have been analyzedby linear least-squareregression. The were placedinto spreadsheetsmanually. Plots of monthly largestpositive trend is from BostonMA (+3.6 + 0.4øC per data were made to determineerrors or outliers. Finally the century),and the largestnegative trend is at CharlestonSC digitizedtables were comparedto the photocopiesto ensure (-0.1 + 0.3øC per century). No consistentlatitudinal or east- accuratecopying. coast vs. west-coast patterns are discernable, but air As with meteorologicalstations, tide gauge stationsare temperature trends are typically greater than seawater moved from time to time. The usual emphasisin sea level changes. continuity is through benchmark checks by differential leveling. No suchpractice seems to have beenemphasized with the seawatertemperature or densityobservations, but Introduction consideringthe traditionof excellencewithin the U.S. Coast and GeodeticSurvey since its foundingby PresidentThomas Tidal observersat primarytide gaugesof the United States Jeffersonin 1807, it is reasonableto assume the historical Coastand GeodeticSurvey (now the NOAA National Ocean information is reliable. Service) routinely measured seawater temperature and Quantifying the standarderrors associatedwith these density.Many recordsextend back to the earlypart of the last monthly or annual means is problematic. The observer century, the longest record extant being San Francisco traditionallyvisited the tide gauge on a daily basis, and (Carvalhoand Maul, 1997). For the mostpart, these records recordedthe data by hand on summarysheets and on the havenot beendigitized nor analyzedfor climaticsignatures to madgramitself (USC&GS, 1929). Temperatureand density ourknowledge. typicallyare from bucketsamples and thusrepresent true sea Unfortunately,many of the 81 or so USA tidal station surface observations. Well-designed and calibrated temperaturerecords have missingdata or are too short for thermometersare usuallyread at leastto the nearest1 øF or to analysisof linear trend. Modem tide gaugestypically have the nearest0.5øC, and estimatingthe numberof observations seawatertemperature sensors reporting in their data- stream, per month to be 30, it seemsreasonable to assumethe but the traditionalanalog systems relied on the skill of the monthlymean standarderror is lessthan + 0.1øC. Observer observer to measure, record, summarize, and report error then is probably negligible for the purposesof temperatureand density. The missingrecords are probably determiningtrends. not the fault of field observers,and may well yet be Linear trend error estimatesreported below have not been discoveredin misfiledreports. adjusted for serial correlation (WMO, 1966), and thus Herein, recordsthat span most of the last 100 yearsare representan optimisticestimate. In seriallycorrelated data, enteredinto spreadsheetfiles, and are madeavailable. Most the effectivesample size (N') is oftenconsiderably less than of the digitizing is the productof work-studystudents at the numberof data points(N), increasingthe standarderror. Florida Tech. Quality controland quality assuranceutilize byxIN/•'. Thisadditional uncertainty isan aside to the main standardmethods including plots and visual outlier detection. All data were converted from Fahrenheit to Celsius. themereported herein. Results Data Sourceand Processing Table 1 summarizeslinear trends (AT) at fourteenof the Data were providedas photocopies(and more recently longestUSA stations.This subsetof the 81 uncoveredto date digitizedrecords) of the stationsheets filed with each tide all end in the 1990's and in so far as possiblerepresent a stationfolder. In most casesthe data includedmonthly and commonepoch but not necessarilya commonrecord length. annualmean temperatures, monthly and annual maximum and Also reportedare the standarderrors (SE) of the trend (+ øC minimumtemperatures, monthly and annualmean seawater per century).Figure I showsthe data. densityand maximum and minimum density. Almost all data A cursoryglance at the table showsthat the mostcomplete recordon the east coast USA during the 20 th century is from Charleston SC and on the west coast at La Jolla CA. Copyright2001 by theAmerican Geophysical Union. Carvalhoand Maul (1997) reporteda lineartrend of +0.3øC Papernumber 2001GL013458. per century for their extended analysis of San Francisco 0094-8276/01/2001GL013458505.00 coveting1855-1993, includinga data gap 1878-1920. The 3935 3936 MAUL ET AL.' SEAWATERTEMPERATURE TRENDS AT USA TIDE GAUGESITES Table1' SeawaterTemperature Trends (AT øC. 100yr -•) + moved 14 times and few detailsof stationlocation were kept. StandardError (SE + øC.100yr -•)fiom SA. Tide s They cautiouslychose not to attempta trend analysisof air Station Years N AT SE temperatures,but a visualinspection suggests no discernable Boston 1921-1994 69 3.6 0.3 change. New York 1926-1994 61 1.8 0.4 At San Francisco,Carvalho and Maul (1997) reportedan AtlanticCity 1911-1991 54 0.9 0.4 air temperaturetrend of + 1.6øCper century,a ratesome three Baltimore 1914-1993 65 0.9 0.5 timeslarger than the seasurface temperature at the tide gauge. Charleston 1921-1992 70 -0.1 0.3 Other air temperaturetrend sitesinclude Baltimore (+2.0øC Mayport 1944-1993 46 0.2 0.6 percentury), Boston, Charleston, and Mayport (all +0.6øCper Key West 1926-1994 36 0.0 0.3 century),and Neah Bay (+I.0øC per century). In eachcase Galveston 1921-1992 36 -0.1 0.5 La Jolla 1916-1996 79 0.7 0.3 exceptBoston (very curiously),air temperaturetrends are Los Angeles 1923-1991 40 0.8 0.4 equalor higherthan seawater temperature trends. San Francisco 1921-1994 57 0.5 0.4 An exhaustiveinquiry into air temperaturetrends is beyond Seattle 1922-1994 41 0.0 0.4 the scopeof this paper. Sufficeto saythat for the mostpart, Neah Bay 1935-1994 53 1.1 0.4 the urban air seemsto be warming fasterthan the nearby Seward 1925-1993 29 0.1 0.4 water. Urbanization effects in climatic records are well known, but adjusting for them in time series analysis ultimatelyis an issueofjudgrnent for the investigator. linear trend during 1921-1994 at San Franciscois +0.5 + Discussion 0.4øCper century. Lineartrend at Boston'sprimary tide station(N=69) is the The tide gaugesites at eachof the citiesreported in Table 1 highestin Table 1 at +3.6 + 0.3øCper century.At Charleston were originallychosen for the purposeof observingtides. SC, a stationwith 70 recordedannual means, the trend is -0.1 Due to the effortsof USC&GS hydrographicsurveyors, the + 0.3øC per century,not unlike othernegative trends at Key tide gaugeshave proven useful for long-termsea level studies West FL, and GalvestonTX. Exceptfor Key West, perhaps as well. It is not as clear that they are equally useful for the most"oceanic" site on the USA eastcoast is AtlanticCity discerningclimatic temperature trends. NJ (1911-1991)which showsa linearwarming trend of +0.9 Bostonharbor for exampleis a semi-enclosedbasin with + 0.4øC per century. only a smallriver to flushthe estuary. SanFrancisco is near West coast sites show a similar mixture of linear the mouth of a large bay, and Seattleis well insidePuget temperaturetrends as on the east coast. Highly urbanized Sound. Each site has experiencedsignificant population SeattleWA shows0.0 + 0.4øCper century,and nearbyNeah growth in the last 100 years, and yet their seawater Bay WA, a much more remote site, is +1.1 + 0.4øC per temperaturetrends are all very different. Certainly,with the century.The longestreportable record fiom Alaskais Seward increasein maritimetraffic and dischargeof wastewaterone (+0.1 + 0.4øC per century);no recordslonger than N=19 wouldexpect water temperatures to rise. (Honolulu,1975-1993) are recovered in Hawaii. If projectionsof globalwarming are correct,one would also expectto find larger seawatertemperature trends in Comparisonwith Air Temperatures northernsites, decreasing to the south. While that couldbe arguedto be the caseon the eastcoast of the USA basedon Trends fiom air temperaturerecords are often more theseresults, just the oppositeoccurs for the mostpart on the problematicthan sea surfacetemperature. For example, westcoast. Clearly other factors are at work in theseplaces. Hansonand Maul (1993) analyzedthe recordat Key West Roemmich(1992) reportsthat the watertemperature at the fiom 1851 to 1986. They found that the stationhad been La Jollatide gaugeduring the 1980swas the warmestdecade 30 © Atl Cty, NJ [] Bal, MD 25 & Bos, MA X Chas, SC • 20 IK NY, NY o SF, CA • 15 + La J, CA - Key W, FL - Sea, WA O Sew, AK 5 ß Gal, TX & N Bay, WA 0 1900 1920 1940 1960 1980 2000 Year Figure 1. Timeseries of seawatertemperatures •om USA tide gaugesites. MAUL ET AL.: SEAWATER TEMPERATURE TRENDS AT USA TIDE GAUGE SITES 3937 between1950 and 1991. Suchwarming toward the end of a trends are much larger than seawater trends too is recordcertainly could increasetrends. Unfortunatelymany unanticipated. On the time scalesinvestigated herein, one otherrecords reported herein are not as