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Natural Climate Variability on Decade-to-Century Time Scales
peratures respectively (see Figure 6 in M3), could result (via advection) in decadal-scale SST anomalies in the southwestern part of the subpolar gyre a few years later, an idea proposed by Mysak (1991). For example, our Figure 6, which depicts the SST anomalies averaged over 45°N to 55°N in the Atlantic Ocean, clearly indicates the existence of alternating cold and warm periods in the northwestern Atlantic, i.e., at around 30°W. To the left of the time axis is a sequence of solid and dotted lines that indicate heavy or light ice conditions, respectively, in the Greenland and Iceland seas. In several cases, we observe that heavy ice conditions precede negative SST anomalies in the northwestern Atlantic by approximately 5 years (the advection time from the Greenland and Iceland seas to this region); similarly, light ice conditions in the Greenland and Iceland seas precede positive SST anomalies, also by about 5 years. While the correspondence between heavy (light) ice conditions and warm (cold) SST anomalies in the northwestern Atlantic is not completely one-to-one, the relationship is sufficiently encouraging to warrant further investigation and increased monitoring of high-latitude climate parameters.
Low-pass filtered and detrended sea surface tempera- ture anomalies averaged over the latitude band 45°N to 55°N as a function of time and longitude in the North Atlantic and Baltic Sea. (Adapted from Bryan and Stouffer, 1991; reprinted with permission of Elsevier Scientific Publishers.) Solid and dotted lines on the time axis indicate, respectively, periods of relatively large and small (or near-normal) sea-ice extents in subregion D1 (see Figure 4) of the Greenland and Iceland seas, as estimated from the sea-ice data described in Figures 3, 10, and 15 of Mysak et al. (1990).
A second way in which Arctic sea-ice changes could induce climate variability at lower latitudes is via the surface albedo: Anomalously large sea-ice extents increase the albedo, and vice-versa. However, the change in the area covered by the sea ice that can be ascribed to natural decadal-scale variability is only of the order of a few percent; such variations are therefore unlikely to induce changes in the polar energy budget large enough to affect the lower latitudes via atmospheric circulation. The Southern Hemisphere sea-ice cover, however, may show larger changes on the interdecadal time scale, in which case Arctic albedo changes and climate need to be discussed together with the natural variability of the southern polar region.
The possibility that polar (in contrast to just Arctic) and high-latitude climate fluctuations on decadal time scales may be related, with the former driving the latter, is suggested by the spatial structure of the third empirical orthogonal function of SST that has been computed by Folland et al. ( 1986a). (For a clear picture of this EOF, which accounts for 6 percent of the SST variance, see Figure 2 in Bryan and Stouffer (1991).) The largest amplitudes of this EOF, whose temporal fluctuations have an interdecadal time scale, are in the northeast Pacific, the northwest Atlantic, and a broad range over the South Atlantic and South Indian oceans. (Folland et al. (1986b) have shown that this EOF is closely linked with Sahel rainfall.) At the same time, we note that because of the particularly large SST gradients in the northeast Pacific and northwest Atlantic, it is conceivable that this EOF's variations may be driven by interdecadal variability in the Arctic. A recent study by Walsh and Chapman (1990b) showed that monthly Arctic SLP fluctuations in winter are associated with or teleconnected to SLP changes in the northern North Atlantic. In a continuation of this study, using data from all seasons, Power and Mysak (1992) showed that at low frequencies (periods of 5 years and longer) this teleconnection pattern persists; in addition, they found that at low frequencies another teleconnection pattern exists between centers in the Arctic and the North Pacific. However, neither of these studies addresses the question of cause and effect, so this information should be taken mainly as a starting point for further work on determining how the polar regions (and the Arctic in particular) drive lower-latitude fluctuations.
Lower-Latitude Forcing of Arctic Fluctuations
The third viewpoint noted earlier, namely, that lower-latitude interdecadal fluctuations trigger Arctic climate fluctuations on this time scale, perhaps has the most widespread appeal. There have been many recent observations of interdecadal variability at middle and tropical latitudes in the climate system (for a list of references, see Weaver et al., 1991). Many investigators believe that these fluctuations may be due to internal oscillations in the thermohaline