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Natural Climate Variability on Decade-to-Century Time Scales
(a) Time series of the annual mean intensity of the index of the meridional overturning in the North Atlantic. Units are sverdrups. (The long-term mean of 18.28 sverdrups has been removed from the time series prior to plotting.) Heavy, solid line is a smoothed time series computed by applying a 13-point binomial filter to the annual mean data (approximately a 10-year low-pass filter). (b) Heavy, solid line denotes spectrum of the thermohaline circulation index time series shown in (a). Thin, solid line denotes the least-squares best fit of a theoretical red-noise spectrum to the spectrum of the THC. Dashed lines denote 95 percent confidence limits about the red noise spectrum. Note: The spectrum was computed by taking the Fourier transform of the autocovariance function, using a maximum of 50 lags and a Tukey window (Chatfield, 1989, Chapter 7). (From Delworth et al., 1993; reprinted with permission of the American Meteorological Society.)
(a) Differences in annual mean model sea surface temperature between four decades with anomalously large thermohaline circulation index values and four decades with anomalously small thermohaline circulation index values. Units are °C. Values less than zero are stippled. (b) Difference in observed sea surface temperature between the periods 1950-1964 (warm period) and 1970-1984 (cold period). Units are °C. Values less than zero are stippled. (Adapted from Kushnir, 1994; reprinted with permission of the American Meteorological Society.) (c) Differences in annual mean model sea surface salinity, computed in the same manner as for Figure 4a. (From Delworth et al., 1993; reprinted with permission of the American Meteorological Society.)