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Natural Climate Variability on Decade-to-Century Time Scales
(a) SST (bold solid and dashed contours) and wind anomalies (arrows) regressed on the time series of EOF 2 SST. Also shown is the climatological SST distribution (thin contours). Note that EOF 2 of SST here has polarity opposite to that shown in Figure 1c; it has been smoothed in space with a 3-point binomial filter. (b) Combined EOF 2 of sea level pressure and zonal wind, based on winter means of 1900 to 1989. The meridional wind field was obtained by regressing meridional wind anomalies on the time series of the combined EOF. This mode explains 15 percent of the variance in the combined pressure and zonal wind fields.
bined EOF. The wind and pressure distributions in Figure 4b are consistent with the geostrophic relation, lending credence to the patterns. The wind patterns in Figures 4a and 4b are similar, indicating that E2 SST is coupled to a dominant mode of variability in the atmosphere. The power spectrum of the second combined EOF of pressure and zonal wind, shown in Figure 5, exhibits peaks at biennial and decadal time scales, similar to those found for E1 air temperature and E2 SST. The circulation mode represented by the combined EOF corresponds to the West Atlantic pattern as defined by Wallace and Gutzler (1981), although the northern center of action in the sea level pressure field is centered about 5° of longitude east of the corresponding feature in the West Atlantic pattern.
Time series of the various surface parameters to the east of Newfoundland (the center of actions of E1 air temperature and E2 SST) are presented in Figure 6. These records have been low-pass filtered with a 3-point binomial filter. Air temperature and SST were averaged over the region bounded by [52°-40°N, 50°-30°W]; zonal wind was aver-
Power spectra as in Figure 3a, but for EOF 2 of the combined fields of sea level pressure and zonal wind.
aged over the region within [48°-38°N, 52°-22°W] and the geostrophic zonal wind was calculated from sea level pressure differences between [40°-32°N, 52°-22°W] and [52°-44°N, 52°-22°W]. Note that the air and sea temperatures and the winds and pressures were measured independently, so the comparisons provide a means of assessing data quality. All four parameters exhibit prominent decadal variability in the area east of Newfoundland; the amplitude of the decadal surface temperature (wind) fluctuations is a b o u t 1 . 5 ° C (1.5 m s-1). The correlation between the air temperature and SST time series is 0.88, and that between the measured and geostrophic wind series, 0.80. The high correlations attest to the reality of the decadal signal. The squared coherence between the SST and zonal wind time series (not shown) exceeds the 99 percent significance level at the decadal period, with a phase lag of 180° between the two parameters.
It is apparent from the time series in Figure 6 that the time scale of the quasi-decadal fluctuations was longer after about 1940 than it was before then. This is borne out by calculations of the power spectra for the pre- and post-1944 periods. During the period 1900 to 1944 the decadal spectral peak is centered at 9 years, whereas during 1945 to 1988 it is centered on 12 years (not shown). Thus, the decadal fluctuations are by no means regular.
Gulf Stream Pattern
The dominant features of the time series of El of SST and E2 of air temperature are the cold period of 1900 to 1929 and the warm period of 1939 to 1968 (Figures 1b and 2d). These cold and warm epochs may also be identified