9.5.3 Atmospheric Circulation Changes
Natural low-frequency variability of the climate system is dominated by a small number of large-scale circulation patterns such as ENSO, the Pacific Decadal Oscillation (PDO), and the NAM and Southern Annular Mode (SAM) (Section 3.6 and Box 3.4). The impact of these modes on terrestrial climate on annual to decadal time scales can be profound, but the extent to which they can be excited or altered by external forcing remains uncertain. While some modes might be expected to change as a result of anthropogenic effects such as the enhanced greenhouse effect, there is little a priori expectation about the direction or magnitude of such changes.
9.5.3.1 El Niño-Southern Oscillation/Pacific Decadal Oscillation
The El Niño-Southern Oscillation is the leading mode of variability in the tropical Pacific, and it has impacts on climate around the globe (Section 3.6.2). There have been multi-decadal oscillations in the ENSO index (conventionally defined as a mean SST anomaly in the eastern equatorial Pacific) throughout the 20th century, with more intense El Niño events since the late 1970s, which may reflect in part a mean warming of the eastern equatorial Pacific (Mendelssohn et al., 2005). Model projections of future climate change generally show a mean state shift towards more El-Niño-like conditions, with enhanced warming in the eastern tropical Pacific and a weakened Walker Circulation (Section 10.3.5.3); there is some evidence that such a weakening has been observed over the past 140 years (Vecchi et al., 2006). While some simulations of the response to anthropogenic influence have shown an increase in ENSO variability in response to greenhouse gas increases (Timmermann, 1999; Timmermann et al., 1999; Collins, 2000b), others have shown no change (e.g., Collins, 2000a) or a decrease in variability (Knutson et al., 1997). A recent survey of the simulated response to atmospheric CO2 doubling in 15 MMD AOGCMs (Merryfield, 2006) finds that three of the models exhibited significant increases in ENSO variability, five exhibited significant decreases and seven exhibited no significant change. Thus, as yet there is no detectable change in ENSO variability in the observations, and no consistent picture of how it might be expected to change in response to anthropogenic forcing (Section 10.3.5.3).
Decadal variability in the North Pacific is characterised by variations in the strength of the Aleutian Low coupled to changes in North Pacific SST (Sections 3.6.3 and 8.4.2). The leading mode of decadal variability in the North Pacific is usually referred to as the PDO, and has a spatial structure in the atmosphere and upper North Pacific Ocean similar to the pattern that is associated with ENSO. One recent study showed a consistent tendency towards the positive phase of the PDO in observations and simulations with the MIROC model that included anthropogenic forcing (Shiogama et al., 2005), although differences between the observed and simulated PDO patterns, and the lack of additional studies, limit confidence in these findings.