Temperature, Wind and Wave Climatologies, and Trends from Marine Meteorological Buoys in the Northeast Paci®C

15 DECEMBER 2002 GOWER 3709 Temperature, Wind and Wave Climatologies, and Trends from Marine Meteorological Buoys in the Northeast Paci®c J. F. R. GOWER Institute of Ocean Sciences, Sidney, British Columbia, Canada (Manuscript received 16 May 2000, in ®nal form 28 February 2002) ABSTRACT Time series of sea surface temperature, wind speed, and signi®cant wave height from meteorological buoys off the west coast of Canada and the adjacent United States are long enough and of suf®cient quality to be useful for studying interannual variability and trends. Long-term averages of data provide a precise climatology of surface temperature, wind speed, and wave height for locations along- and offshore. Data from many of the buoys suggest a warming trend, but only three buoys show statistical signi®cance, and sheltered buoys show no increases. Both the wind speed and the wave height data show an increasing trend with more statistical signi®cance. Future data from these buoys should bene®t from better calibration and a wider variety of sensors, as well as from longer time series. 1. Introduction indicator of long-term trends, and to show the signature of short-term ``climatic'' effects such as El NinÄo. Any An array of 26 meteorological buoys off the west apparent trends in temperature, wind speed, and wave coast of the United States and Canada provides weather height need to be evaluated as a check on the quality and surface ocean data for coastal and offshore waters of the buoy data. Wind and wave data from the Canadian in the Gulf of Alaska north of 458N and east of 1608W buoys have been used in validation of satellite sensors (Fig. 1). The ®rst buoys were 12- and 10-m discus buoys such as the TOPEX/Poseidon altimeter (Gower 1996) installed in the 1970s by the U.S. National Data Buoy and of the Comprehensive Ocean±Atmosphere Data Set Center. These are now replaced by an expanded network (COADS; Cherniawsky and Crawford 1996). of 6-m Navy Oceanographic Meteorological Automatic This paper is based on monthly average values of sea Device (NOMAD) buoys offshore, and 3-m discus buoys surface temperature, wind speed, and signi®cant wave near shore and in sheltered waters, deployed by the Unit- height (SWH, the height exceeded by one-third of the ed States and by the environment and ®sheries depart- waves, H1/3, computed as 4 times the standard deviation ments of the Canadian federal government. The ®rst U.S. buoys were deployed in 1972±77 at six offshore locations of the measured height values) in the time period 1972± numbered 46001 to 46006. Four U.S. nearshore buoys 99. Recent Canadian data for the period May 1990 to were added in 1984±91. The ®rst Canadian buoys were May 1999 were derived from a new compilation pro- installed in 1987, at which time location 46004 was trans- vided by Environment Canada for this study. Data from ferred to Canadian responsibility. The Canadian network the U.S. buoys are available on the Web at http:// was brought up to its full strength of 16 operational buoys www.ndbc.noaa.gov. in 1993. An additional experimental buoy was added in 1998 for testing sensors for biological and optical vari- 2. Buoy data ables. The buoys measure wind speed and direction, wave height and spectrum, surface water and air temperature, The positions of the 26 buoys in the west coast U.S. and atmospheric pressure. All data are transmitted in real and Canadian networks are shown in Fig. 1. Offshore time at hourly intervals. buoys were originally 10- and 12-m discus buoys, but Although the operational purpose of the buoys is these were replaced in 1982±92 by NOMAD shiplike short-term weather forecasting, the time series of data hulls measuring 6 m 3 4 m. All other buoys have 3-m from the buoys are long enough (6±27 yr) to be a useful discus hulls. The offshore buoys are located about 400 km away from the coast, with the exception of 46006, Corresponding author address: Dr. J. F. R. Gower, Institute of which is positioned about 1000 km offshore to give Ocean Sciences, P.O. Box 6000, Sidney, BC V8L 4B2, Canada. greater warning of weather systems coming from the E-mail: [email protected] west. The weather ships at Ocean Weather Station Papa Unauthenticated | Downloaded 09/25/21 07:04 PM UTC 3710 JOURNAL OF CLIMATE VOLUME 15 FIG. 1. Map showing the locations of the buoys whose data are discussed here, and of weather station P, indicated by P at 508N, 1458W. (position indicated by the letter P in Fig. 1) provided of 10 m, and water temperature at a depth of 1.5 m. this same warning for Canada from 1949 to 1981. Signi®cant wave height accuracy is that maintained by The offshore buoys are listed in tables in this paper calibration on shore prior to deployment. Sensors are in an anticlockwise order starting at 46003. Data from rejected if errors measured in predeployment tests are the line of nearshore buoys, moored in exposed positions outside the 10% range. within 100 km of the coast, are listed in a north-to-south Buoys are rotated among locations on the annual ser- order from 46205 off the north end of the Queen Char- vice cruises, so that a reasonable error model is to as- lotte Islands south to 46027 off northern California. A sume independent random errors of the above-speci®ed further eight buoys are located in more sheltered Ca- magnitudes that remain ®xed for each year. On this ba- nadian coastal waters, behind the Queen Charlotte Is- sis, calibration errors will limit the accuracy with which lands and Vancouver Island. Two of these buoys (46131 average values and trends in the data can be determined and 46146) are located in the Strait of Georgia. A further to the values listed in Table 1 for time series covering two (46181 and 46134) are located in narrow coastal 5, 10, and 20 years. inlets. Buoy 46134 was installed only recently and its Both winds and temperatures are averages over 10- data are not considered here. min periods computed once each hour. Winds are vector Nominal measurement accuracy for the Canadian averages. The wave heights are deduced from vertical buoy data is 60.58C for sea surface temperature, 60.3 ``heave'' accelerometers mounted inside the buoys. In ms21 for wind speed, and 10% for signi®cant wave the NOMAD hulls, the sensors are gimballed to remain height (Axys Environmental Consulting Ltd., 2001, per- vertical when the buoy tilts. In the 3-m hulls, the ac- sonal communication). Water temperatures are mea- celerometers are strapped down, and buoy tilt is a pos- sured by thermistor at a depth of 80 cm. The wind speed sible source of error. Buoy wave measurements are av- accuracy is that speci®ed for the Young Wind Monitor erages of 37 min of data collection. Model 05103, and is assumed to apply to all deployed This study is based on monthly mean values of sea instruments. An additional source of error at low wind surface temperatures (SST), wind speeds, and signi®- speeds is the threshold wind speed required to turn the cant wave heights computed from the hourly buoy mea- rotor, which is speci®ed as 1 m s21. Wind speed ac- surements up to and including May 1999. Means are curacy for U.S. buoys is quoted as 61ms21 or 10%, computed for all months for which at least 300 hourly whichever is greater (Hamilton 1980). Wind speeds are values (approximately 40% of the maximum possible) measured by two propeller anemometers mounted at 3.7 are collected. For computing anomalies from the month- and 4.7 m above the water. The higher anemometer is ly values, a mean seasonal cycle is computed from all used unless its data are suspect. On the older 10- and available data for each buoy. The cycle is assumed to 12-m discus buoys, wind speed was measured at a height consist of an annual sinusoid and a second harmonic (2 Unauthenticated | Downloaded 09/25/21 07:04 PM UTC 15 DECEMBER 2002 GOWER 3711 TABLE 1. Errors in long-term trends introduced by calibration uncertainties discussed in the text, for an avg of 1 yr of data, and for time series of 5, 10, and 20 yr. SST Wind speed SWH Length Mean Trend Mean Trend Mean Trend (yr) (8C) (8Cyr21) (m s21) (m s21 yr21) (m at 3 m) (m yr21 at 3 m) 1 0.50 0.30 0.30 5 0.22 0.12 0.13 0.08 0.13 0.08 10 0.16 0.06 0.10 0.03 0.10 0.03 20 0.11 0.03 0.07 0.015 0.07 0.015 cycles per year) whose amplitudes and phases minimize 48C offshore, but is less near the coast where upwelling the standard deviation of the residual monthly values, tends to reduce summer temperatures. Coastal upwelling obtained by subtracting these cycles from the data. reduces the amplitude of the annual cycles at lower Sections 3 and 4 of the paper discuss ®rst the cli- latitudes from 3.88C off the northern Queen Charlotte matologies and then the trends in sea surface temper- Islands to 28C at buoy 46050 off central Oregon and ature, wind speed, and wave height. Response of the 0.68C (less than the residual variation in monthly mean buoys to the major El NinÄo events in 1982/83 and 1997/ values) at buoy 46027 off northern California.

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