3.6.3 Pacific Decadal Variability
Decadal to inter-decadal variability of the atmospheric circulation is most prominent in the North Pacific, where fluctuations in the strength of the winter Aleutian Low pressure system co-vary with North Pacific SST in the PDO. These are linked to decadal variations in atmospheric circulation, SST and ocean circulation throughout the whole Pacific Basin in the Inter-decadal Pacific Oscillation (IPO; Trenberth and Hurrell, 1994; Gershunov and Barnett, 1998; Folland et al., 2002; McPhaden and Zhang, 2002; Deser et al., 2004). Key measures of Pacific decadal variability are the North Pacific Index (NPI; Trenberth and Hurrell, 1994), PDO index (Mantua et al., 1997) and the IPO index (Power et al., 1999b; Folland et al., 2002; see Figures 3.28 and 3.29). Modulation of ENSO by the PDO significantly modifies regional teleconnections around the Pacific Basin (Power et al., 1999b; Salinger et al., 2001), and affects the evolution of the global mean climate.
The PDO/IPO has been described as a long-lived El Niño-like pattern of Indo-Pacific climate variability (Knutson and Manabe, 1998; Evans et al., 2001; Deser et al., 2004; Linsley et al., 2004) or as a low-frequency residual of ENSO variability on multi-decadal time scales (Newman et al., 2003). Indeed, the symmetry of the SST anomaly pattern between the NH and SH may be a reflection of common tropical forcing. However, Folland et al. (2002) showed that the IPO significantly affects the movement of the South Pacific Convergence Zone in a way independent of ENSO (see also Deser et al., 2004). Other results indicate that the extratropical phenomena are generic components of the PDO (Deser et al., 1996, 1999, 2003; Gu and Philander, 1997). The extratropics may also contribute to the tropical SST changes via an ‘atmospheric bridge’, confounding the simple interpretation of a tropical origin (Barnett et al., 1999; Vimont et al., 2001).
The inter-decadal time scale of tropical Indo-Pacific SST variability is likely due to oceanic processes. Extratropical ocean influences are also likely to play a role as changes in the ocean gyre evolve and heat anomalies are subducted and re-emerge (Deser et al., 1996, 1999, 2003; Gu and Philander, 1997). It is also possible that there is no well-defined coupled ocean-atmosphere ‘mode’ of variability in the Pacific on decadal to inter-decadal time scales, since instrumental records are too short to provide a robust assessment and palaeoclimate records conflict regarding time scales (Biondi et al., 2001; Gedalof et al., 2002). Schneider and Cornuelle (2005) suggested that the PDO is not itself a mode of variability but is a blend of three phenomena. They showed that the observed PDO pattern and evolution can be recovered from a reconstruction of North Pacific SST anomalies based on a first order autoregressive model and forcing by variability of the Aleutian low, ENSO and oceanic zonal advection in the Kuroshio-Oyashio Extension. The latter results from oceanic Rossby waves that are forced by North Pacific Ekman pumping. The SST response patterns to these processes are not completely independent, but they determine the spatial characteristics of the PDO. Under this hypothesis, the key physical variables for measuring Pacific climate variability are ENSO and NPI (Aleutian Low) indices, rather than the PDO index.
Figure 3.29 (top) shows a time series of the NPI for 1900 to 2005 (Deser et al., 2004). There is substantial low-frequency variability, with extended periods of predominantly high values indicative of a weakened circulation (1900–1924 and 1947–1976) and predominantly low values indicative of a strengthened circulation (1925–1946 and 1977–2005). The well-known decrease in pressure from 1976 to 1977 is analogous to transitions that occurred from 1946 to 1947 and from 1924 to 1925, and these earlier changes were also associated with SST fluctuations in the tropical Indian (Figure 3.29, lower) and Pacific Oceans although not in the upwelling zone of the equatorial eastern Pacific (Minobe, 1997; Deser et al., 2004). In addition, the NPI exhibits variability on shorter time scales, interpreted in part as a bi-decadal rhythm (Minobe, 1999).
Figure 3.29. (Top) Time series of the NPI (sea level pressure during December through March averaged over the North Pacific, 30°N to 65°N, 160°E to 140°W) from 1900 to 2005 expressed as normalised departures from the long-term mean (each tick mark on the ordinate represents two standard deviations, or 5.5 hPa). This record reflects the strength of the winter Aleutian Low pressure system, with positive (negative) values indicative of a weak (strong) Aleutian Low. The bars give the winter series and the smooth black curves show decadal variations (see Appendix 3.A). Values were updated and extended to earlier decades from Trenberth and Hurrell (1994). (Bottom) As above but for SSTs averaged over the tropical Indian Ocean (10°S–20°N, 50°E –125°E; each tick mark represents two standard deviations, or 0.36°C). This record has been inverted to facilitate comparison with the top panel. The dashed vertical lines mark years of transition in the Aleutian Low record (1925, 1947, 1977). Updated from Deser et al. (2004).
There is observational and modelling evidence (Pierce, 2001; Schneider and Cornuelle, 2005) suggesting the PDO/IPO does not excite the climate shifts in the Pacific area, but they share the same forcing. The 1976–1977 climate shift in the Pacific, associated with a phase change in the PDO from negative to positive, was associated with significant changes in ENSO evolution (Trenberth and Stepaniak, 2001) and with changes in ENSO teleconnections and links to precipitation and surface temperatures over North and South America, Asia and Australia (Trenberth, 1990; Trenberth and Hurrell, 1994; Power et al., 1999a; Salinger et al., 2001; Mantua and Hare, 2002; Minobe and Nakanowatari, 2002; Trenberth et al., 2002b; Deser et al., 2004; Marengo, 2004). Schneider and Cornuelle (2005) added extra credence to the hypothesis that the 1976–1977 climate shift is of tropical origin.