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Natural Climate Variability on Decade-to-Century Time Scales
the time series of annual mean temperature at 52.5°N, 35.5°W taken from objective analyses of SST observations from the Comprehensive Ocean Atmosphere Data Set (COADS) of surface marine observations (Woodruff et al., 1987). The COADS data have been gridded and analyzed using the same procedures described by Levitus (1982) as part of a joint project with A. Da Silva of the University of Wisconsin. (We emphasize that the OWS "C" time series are averages of the data taken by ships at the OWS "C" location.) Figure 5a shows the annual mean temperature at this grid point as computed from the 12 monthly fields of objectively analyzed SST data for each year. Figure 5b shows the annual mean time series for the OWS "C" surface data and the COADS data time series plotted together to facilitate comparison. Some differences exist between these two data sets, but the comparison indicates that both data sets display the same negative trend as well as the same decadal-scale oscillation. The year of maximum temperature of the decadal-scale oscillation in each series may differ by 1 to 3 years in each figure. We attribute this to differences in data density and differences in the way these annual means have been computed. The decreasing SST of the North Atlantic Ocean over this time period has been described in previous studies (e.g., Folland et al., 1984).
To further describe the temporal variability at this location, Figure 6 presents the time-depth temperature field at OWS "C" over the 0-125 m depth range. This figure makes it very clear that both the negative trend and decadal-scale oscillations noted in Figures 4 and 5 extend coherently over the upper water column. There is no phase propagation with depth of the interannual variations.
Beginning in the mid-1960s, very-low-salinity water was observed at various locations along the periphery of the subarctic gyre of the North Atlantic. Dickson et al. (1988) have suggested that this low-salinity (and relatively cold) water advected out of the Arctic Ocean into the subarctic gyre of the North Atlantic and circulated around this gyre.
Annual mean temperature (°C) as a function of depth and time, based on the OWS "C" time series data.
They identified changes in salinity that occurred at OWS "C" in the 1970s as being associated with the "Great Salinity Anomaly" (GSA). Levitus (1989b) also presented evidence for this. Dooley et al. (1984), however, suggested that the changes at OWS "C" were associated with horizontal movement of the OPF. The fact that several oscillations have occurred in the temperature data at OWS ''C" suggests that the OPF might be shifting its axis. It does not seem likely to us that the cold, fresh-waters of the GSA have traveled around the subarctic more than once. Further examination of the historical data will be required to test these different hypotheses. Perhaps both phenomena are responsible to some degree for the decadal-scale variability at OWS "C". Quite possibly the GSA was a phenomenon independent of the process responsible for the oscillations we describe in Figures 4 to 6.
To place the variability described by Figure 6 in perspective, Figure 7 presents the climatological annual cycle of OWS "C" upper-ocean temperature data. At a 100 m depth the annual range of temperature is approximately 1.8°C. A clear propagation of phase with depth is observed.
Variability of Deep-Ocean Temperature and Salinity at OWS "C"
To document the variability of deep ocean conditions at OWS "C," Figure 8 presents time series of temperature and salinity for the 1964-1990 period. Again, all data for the 1964-1973 period are U.S. Coast Guard observations (Hannon, 1979), and the 1975-1990 data are Russian observations. While only a few observations were available for the
Climatological annual cycle of temperature (°C) based on the OWS "C" data series.