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Natural Climate Variability on Decade-to-Century Time Scales
Time series of winter anomalies east of Newfoundland of (a) SST (solid) and surface air temperature (dashed) and (b) zonal wind (solid) and geostrophic zonal wind calculated from sea level pressure differences (dashed). All curves are smoothed with a 5-point binomial filter. See text for definition of indices.
on the basis of globally averaged temperatures (cf. Folland et al., 1984) and North Atlantic basin-averaged temperatures (cf. Bottomley et al., 1990). The surface temperature, wind, and sea level pressure differences between the warm and the cold period are shown in Figure 7. A minimum of 20 winters was required for the period average in each grid square. The surface wind and sea level pressure fields (Figure 7a) are dynamically consistent, with negative pressure anomalies along 40°N and easterly (westerly) wind anomalies to the north (south). The maximum sea level pressure difference is approximately 3 mb, and the largest wind-
Difference between the periods 1939 to 1968 and 1900 to 1929 of winter (a) sea level pressure and surface wind, (b) SST, and (c) surface air temperature. In (a) the contour interval is 0.5 mb, with negative contours dashed. The lowest pressure anomaly is -3mb. Wind scale is indicated in lower right. In (b) light shading indicates values between 0.8°C and 1.0°C; heavy shading indicates values greater than 1.0°C. Contour interval is 0.2°C. In (c) light shading indicates values between 0.6ºC and 0.8°C; heavy shading indicates values greater than 0.8°C. Contour interval is 0.2°C.
speed anomaly is around 1.5 m s-1. This circulation pattern, which resembles the negative phase of the North Atlantic Oscillation (cf. Barnston and Livezey, 1987), represents an overall weakening of the basin-wide wind system.4 Similar
The North Atlantic Oscillation is the dominant mode of variability of the surface atmospheric circulation, accounting for 33 percent of the variance of the winter sea level pressure field during 1900 to 1989 (not shown).