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Natural Climate Variability on Decade-to-Century Time Scales
Heavy, solid line: spectrum of sea surface temperature from observed data (ocean weather ship India. 59°N, 19°W). Light, solid line: spectrum of model sea surface temperature from grid point closest to 59°N, 19°W. The arrows in the lower left corner denote 95 percent confidence intervals about the spectra. (From Delworth et al., 1993: reprinted with permission of the American Meteorological Society.)
Stream function of the meridional overturning in the Atlantic Basin. Flow follows the direction of the arrows. Units are sverdrups (106 m3/s). The dashed lines denote the sinking region as defined in the text. (From Delworth et al., 1993: reprinted with permission of the American Meteorological Society.)
region defined by this latitudinal belt and extending from the western to the eastern boundaries of the Atlantic basin.
In order to concisely describe variations in this meridional overturning, one needs a measure of the intensity of the thermohaline circulation in the North Atlantic. This intensity is defined each year as the maximum stream function value representing the annual mean meridional circulation. A time series of this index, which represents the fluctuations in the intensity of the annual mean THC, is shown in Figure 3a. Visual inspection of this figure reveals substantial variability on interdecadal time scales. The spectrum of this time series, shown in Figure 3b, demonstrates enhanced variance in the 40-to-50-year time scale. Analyses of an extension of this integration (to 1000 years in length) suggest that these irregular oscillations are present in the period beyond 200 years, but are characterized by a somewhat longer time scale. In addition, the spectrum from the longer THC time series contains a more clearly defined broad spectral peak.
It should be noted that the index, by definition, primarily describes variations in the North Atlantic section of the THC. Subsequent analyses show that the climatic variations in the model associated with variations in this index are not hemispheric in nature, but are better characterized as regional to the North Atlantic and Arctic.
Sea Surface Temperature. Salinity, and Surface Air Temperature Variations
The spatial pattern of the changes of model sea surface temperature associated with fluctuations in the intensity of the thermohaline circulation is shown in Figure 4a. These differences are computed by subtracting the mean of four decades having anomalously small values of the THC index from the mean of four decades with anomalously large values of the THC. The pattern of SST change bears an encouraging resemblance to a pattern of observed interdecadal SST variation computed by Kushnir (1994) and shown in Figure 4b. The field in Figure 4b as computed by