8.4.2 Pacific Decadal Variability
Recent work suggests that the Pacific Decadal Oscillation (PDO, see Chapters 3 and 9) is the North Pacific expression of a near-global ENSO-like pattern of variability called the Inter-decadal Pacific Oscillation or IPO (Power et al., 1999; Deser et al., 2004). The appearance of the IPO as the leading Empirical Orthogonal Function (EOF) of SST in AOGCMs that do not include inter-decadal variability in natural or external forcing indicates that the IPO is an internally generated, natural form of variability. Note, however, that some AOGCMs exhibit an El Niño-like response to global warming (Cubasch et al., 2001) that can take decades to emerge (Cai and Whetton, 2000). Therefore some, though certainly not all, of the variability seen in the IPO and PDO indices might be anthropogenic in origin (Shiogama et al., 2005). The IPO and PDO can be partially understood as the residual of random inter-decadal changes in ENSO activity (e.g., Power et al., 2006), with their spectra reddened (i.e., increasing energy at lower frequencies) by the integrating effect of the upper ocean mixed layer (Newman et al., 2003; Power and Colman, 2006) and the excitation of low frequency off-equatorial Rossby waves (Power and Colman, 2006). Some of the inter-decadal variability in the tropics also has an extratropical origin (e.g., Barnett et al., 1999; Hazeleger et al., 2001) and this might give the IPO a predictable component (Power et al., 2006).
Atmosphere-Ocean General Circulation Models do not seem to have difficulty in simulating IPO-like variability (e.g., Yeh and Kirtman, 2004; Meehl and Hu, 2006), even AOGCMs that are too coarse to properly resolve equatorially trapped waves important for ENSO dynamics. Some studies have provided objective measures of the realism of the modelled decadal variability. For example, Pierce et al. (2000) found that the ENSO-like decadal SST mode in the Pacific Ocean of their AOGCM had a pattern that gave a correlation of 0.56 with its observed counterpart. This compared with a correlation coefficient of 0.79 between the modelled and observed interannual ENSO mode. The reduced agreement on decadal time scales was attributed to lower than observed variability in the North Pacific subpolar gyre, over the southwest Pacific and along the western coast of North America. The latter was attributed to poor resolution of the coastal waveguide in the AOGCM. The importance of properly resolving coastally trapped waves in the context of simulating decadal variability in the Pacific has been raised in a number studies (e.g., Meehl and Hu, 2006). Finally, there has been little work evaluating the amplitude of Pacific decadal variability in AOGCMs. Manabe and Stouffer (1996) showed that the variability has roughly the right magnitude in their AOGCM, but a more detailed investigation using recent AOGCMs with a specific focus on IPO-like variability would be useful.