Decade-to-Century-Scale Climate Variability and Change A Science Strategy (1998) / Chapter Skim
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3 Modes of Climate Variability
3 Modes of Climate Variability
Pages 25-38
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From page 25... ...
These patterns vary over a broad range of space and time scales, and their relative phasing can dominate global and regional temperature variations. They often show regional and global teleconnections, involve a number of distinct climatological variables, and apparently focus different forcings and processes into single coherent responses.
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From page 26... ...
Correlation of the NAO index with surface air temperature and sea surface temperature (SST) further reveals the extent of the atmospheric connection between the Norm Atlantic and the northern portion of Europe, and part of northern Asia (Hurrell and van Loon, 1996; Hurrell, 1995~.
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From page 27... ...
It appears as four distinct cells in the 500 hPa (hPa are equivalent to mb) geopotential height field near Hawaii, over the North Pacific, over Alberta in Canada, and over the Gulf Coast of the United States.
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From page 28... ...
The term therefore includes coupled modes, but also refers to patterns in each component that are simply coherent. Tropical Atlantic Variability Numerous studies have found a robust relationship between SST anomalies in the tropical Atlantic and changes in soil moisture, albedo, and surface-roughness over North Africa (see Nicholson, 1989, for a brief overview)
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From page 29... ...
Consequently, this phenomenon is sometimes referred to as the Atlantic Tropical Dipole, al though the lack of a clear consensus on the actual dipole nature of the pattern leaves many simply referring to it as the decadal tropical Atlantic SST variability. This low-fre quency SST phenomenon shows concurrent anomalies in the rainfall over Brazil and northern Africa (Figure 3-4a)
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From page 30... ...
examined the multidecadal variability in the observational record of SLP, SST, and surface wind velocity in the North Atlantic basin, and found two examples of warm and cold epochs in the twentieth century; each of these epochs lasted more than a decade. Warm periods are characterized by positive SST anomalies around southern Greenland and negative anomalies along the northeastern U.S.
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From page 31... ...
(From Zhang et al., 1997; reprinted with permission of the American Meteorological Society.) 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 Time FIGURE 3-7 Time series of a cold-tongue index, corresponding to the SST pattern displayed in Figure 3-6.
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From page 32... ...
. ~ r V ~,11~ ~ 3 ~ 1900 1910 1920 1930 1940 1950 1960 t970 1980 1990 2000 Time FIGURE 3-9 Time series of the global-residual index, corresponding to the SST pattern displayed in Figure 3-8, which indicates that the ENSO-like pattern in that figure is associated primarily with decade-to-century-scale variability.
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From page 33... ...
dominated by dec-cen-scale variability (Figure 3-9~. The two components exhibit remarkably similar spatial signatures in global SST, SLP, and wind-stress fields, with the SST field in the residual pattern being less equatorially confined in the eastern Pacific than the interannual pattern, and having a larger extratropical signature in the North Pacific.
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From page 34... ...
Specifically, the mean anticyclonic motion of the ice pack should decrease, and the sea ice should now be more divergent than it was during the 1970s and early 1980s. Indeed, there have been two remarkable large-scale anomalies in Arctic sea ice in the past decade: the extraordinarily thin sea ice that was experienced by the SHEBA (Surface Heat Budget of the Arctic project; see Moritz and Perovich, 1996, for a description)
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From page 35... ...
THE ROLE OF DEC-CEN VARIABILITY IN GLOBAL WARMING It is clear that the global warming experienced over the past 20 years is distinguished by an enhanced warming in winter that was not evident in previous decades, dominated by a strong warming over Northern Hemisphere land, and compensated for to some degree by a lesser cooling over parts of the Northern Hemisphere oceans. Despite its decadal persistence, this pattern is consistent with the basic COWL pattern described above.
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From page 36... ...
Despite the uncertainties about their roles in anthropogenic global warming and natural climate change, they represent an obvious avenue through which coherent climate variations and change may be propagated globally. The IPCC Second Assessment (IPCC, 1996a)
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From page 37... ...
Is the residual warmingthat is, warming apart from the COWL contribution natural variability or anthropogenic warming? What is the relationship between the COWL pattern, greenhouse fingerprint, and natural climate patterns that is, how do the natural modes of the climate system respond to different changes in forcing, natural or anthropogenic?
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From page 38... ...
It is clear that a more comprehensive understanding of the variability of these patterns on decade-to-century time scales is absolutely essential if we are truly to distinguish ! between anthropogenic change and natural climate variability, or understand their interaction.
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