1989. Individual monthly means are computed as the average of all daily mean profiles in each month. Each annual mean is computed as the average of the available monthly means for each year. All years plotted in Figure 4a had at least one monthly mean for each of the four seasons. The vertical bars centered at each annual mean represent ± 1 standard error of the monthly means in each year about the annual mean.
There are two prominent features of this series. The first is the approximately decadal-scale oscillation of about 2°C magnitude, with peaks occurring in 1956, 1965, and 1979. The years for which we have both station data and MBT data indicate good agreement between these two different sets of measurements, and give us confidence in the accuracy and representativeness of annual mean MBT data that have been averaged as we have done. Although the accuracy of any individual MBT measurement is an order of magnitude less than that of a reversing thermometer measurement (0.2°C versus 0.02°C), the accuracy of monthly and annual mean values is increased by averaging many observations. Figure 4a clearly demonstrates that the signal-to-noise ratio
is small enough that averaged MBT data are adequate to define the observed signal (2°C peak-to-trough difference in annual mean temperature).
The monthly mean data used in Figure 4a was derived from both MBT and station daily means. The MBT data was available from 1946 through 1981. During the 1947-1954 period and again in 1960-1967, measurements were taken quite frequently, sometimes daily. During the 1970s relatively few observations were made, although a couple of times the annual frequency again reached 150 or higher (as it had been during 1955-1959). The station data, which covers 1964 through 1990, reflects reasonably frequent observations (again, about 150 per year) for the period 1967-1973, and then near-daily readings for 1976-1985. Both the MBT data and the station observations were scattered fairly evenly throughout the year.
Figure 4b shows the time series of annual mean sea surface temperature (SST) for OWS "C". The same features observed at 100 m occur at the sea surface. To provide further evidence of the existence of both the downward trend and the decadal-scale oscillation, Figure 5 presents