Climate Change 2001:
Working Group I: The Scientific Basis
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9.3.5 Changes in Variability

The capability of models to simulate the large-scale variability of climate, such as the El Niño-Southern Oscillation (ENSO) (a major source of global interannual variability) has improved substantially in recent years, with an increase in the number and quality of coupled ocean-atmosphere models (Chapter 8) and with the running of multi-century experiments and multi-member ensembles of integrations for a given climate forcing (Section 9.2). There have been a number of studies that have considered changes in interannual variability under climate change (e.g., Knutson and Manabe, 1994; Knutson et al., 1997; Tett et al. 1997; Timmermann et al. 1999; Boer et al. 2000b; Collins, 2000a,b). Other studies have looked at intra-seasonal variability in coupled models and the simulation of changes in mid-latitude storm tracks (e.g., Carnell et al. 1996; Lunkeit et al., 1996; Carnell and Senior, 1998; Ulbrich and Christoph, 1999), tropical cyclones (Bengtsson et al., 1996; Henderson-Sellers et al., 1998; Knutson et al., 1998; Krishnamurti et al., 1998; Royer et al., 1998) or blocking anticyclones (Lupo et al., 1997; Zhang and Wang, 1997; Carnell and Senior, 1998). The results from these models must still be treated with caution as they cannot capture the full complexity of these structures, due in part to the coarse resolution in both the atmosphere and oceans of the majority of the models used (Chapter 8).

An expanding area of research since the SAR is the consideration of whether climate change may be realised as preferred modes of non-linear naturally occurring atmospheric circulation patterns, or so-called weather regimes as proposed by Palmer (1999). Recent work (e.g., Hurrell 1995, 1996; Thompson and Wallace 1998; Corti et al., 1999) has suggested that the observed warming over the last few decades may be manifest as a change in frequency of these naturally preferred patterns (Chapters 2 and 7) and there is now considerable interest in testing the ability of climate models to simulate such weather regimes (Chapter 8) and to see whether the greenhouse gas forced runs suggest shifts in the residence time or transitions between such regimes on long time-scales. There are now several multi-ensemble simulations using scenarios of time-evolving forcing and multi-century experiments with stabilised forcing, which may help to separate the noise of decadal variability from the signal of climate change.

In this section, changes in variability (defined as the deviation from some mean value) will be considered on different time-scales (intra-seasonal, interannual, and decadal and longer). Particular attention will be given to changes in naturally occurring modes of variability such as ENSO, the Arctic Oscillation (AO; and its more spatially restricted counterpart, the North Atlantic Oscillation, NAO) and the Antarctic Oscillation (AAO) etc.

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