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Natural Climate Variability on Decade-to-Century Time Scales
mates the change in temperature, salinity, or density for a unit change in the intensity of the THC.
In order to provide a large-scale measure of conditions in the sinking region, the regression coefficients between density (ρ') and the thermohaline circulation were averaged vertically and horizontally over the sinking region. As can be seen in Figure 6, the density time series leads the thermohaline circulation time series by a few years, suggesting that fluctuations in density in the sinking region induce the fluctuations in the THC. In addition, the density term is
At each grid point at each model level, linear regression coefficients were computed between the time series of annual mean density, temperature, and salinity at that grid point versus the time series of the thermohaline circulation. The regression coefficients were computed using various lags in order to obtain an estimate of the temporal evolution of the three-dimensional fields of density, temperature, and salinity with respect to variations in the thermohaline circulation. These regression coefficients were then averaged vertically and horizontally over the sinking region of the North Atlantic (defined in text) and plotted as a function of lag in years. For example, the regression coefficients plotted at lag −5 (+ 5) years indicate conditions 5 years prior (subsequent) to a maximum in the thermohaline circulation. The heavy, solid line (ψ') denotes the regression coefficients of the THC index with itself (thus representing a ''typical" fluctuation). The thin, solid line (ρ') represents the regression coefficients between density and the THC index. The dashed line (ρ'S) denotes the regression coefficients for the density changes attributable solely to changes in salinity versus the THC index, while the dotted line (ρ'T) represents the regression coefficients for the density changes attributable solely to changes in temperature versus the THC index. Note that ρ'T is inversely related to temperature, whereas ρ'Sis proportional to salinity. The units for ρ', ρ'T, and ρ'S are (g/cm3) 10−5/Sv. (From Delworth et al., 1993; reprinted with permission of the American Meteorological Society.)
Latitude by depth cross-section in the Atlantic basin of the zonal mean of the regression coefficients between the time series of density and the thermohaline circulation. The regression coefficients are computed at lag −3 years, indicating that the density values at year (n − 3) are correlated with values of the THC index at year n. This lag corresponds to the phase with the maximum vertically averaged density perturbation in the sinking region. Units are (g/cm3) x 10−5/Sv. Values less than zero are stippled. This cross-section can be interpreted as the anomaly density structure three years prior to a maximum in the thermohaline circulation. (From Delworth et al., 1993; reprinted with permission of the American Meteorological Society.)
decomposed into individual contributions from temperature and salinity anomalies, and the regression coefficients between these two terms and the THC are also plotted. This figure indicates that density fluctuations attributable to salinity anomalies (ρ'S) are almost in phase with, but slightly behind, the thermohaline circulation, whereas density fluctuations attributable to temperature anomalies (ρ'T) lead the thermohaline circulation by approximately 90° in phase. The sum of the effects of temperature and salinity anomalies results in a density time series that leads the thermohaline circulation by several years, as indicated in this figure. The phase differences between ρ'S and ρ'T shown in Figure 6 are important to the quasi-oscillatory nature of the fluctuations.
Temperature-induced density anomalies (ρ'T) serve as a negative feedback on the fluctuations of the THC. The largest negative ρ'T values (corresponding to the warmest vertically averaged temperatures) occur after the maximum in the THC, thereby reducing the mean density in the sinking region, and thus weakening the intensity of the THC. In contrast, the largest positive values of ρ'T occur before the maximum in the THC, thereby increasing the mean density in the sinking region and enhancing the intensity of the THC. Thus, temperature-induced density anomalies play and important role in these fluctuations.
To examine the latitude-depth distribution of the density