Recently, ‘ENSO-like’ spatial patterns in the climate system, which operate on decadal to multi-decadal time-scales, have been identified. This lower-frequency SST variability is less equatorially confined in the central and eastern Pacific, and relatively more prominent over the extra-tropics, especially the north-west Pacific, and has a similar counterpart in night marine air temperatures (Tanimoto et al., 1993; Folland et al., 1999a; Allan, 2000). The corresponding sea level pressure (SLP) signature is also strongest over the North Pacific, and its December-February counterpart in the mid-troposphere more closely resembles the Pacific-North America (PNA) pattern (Zhang et al., 1997b; Livezey and Smith, 1999). There is ambiguity about whether inter-decadal Pacific-wide features are independent of global warming. In the longer Folland et al. (1999) analyses since 1911 they appear to be largely independent, but in the Livezey and Smith analysis of more recent SST data they are an integral part of a global warming signal. Using a different method of analysis of data since 1901, Moron et al. (1998) find a global warming signal whose pattern in the Pacific is intermediate between these two analyses.

The PDO of Mantua et al. (1997), with lower-frequency variations in the leading North Pacific SST pattern, may be related to the same Pacific-wide features, and parallels the dominant pattern of North Pacific SLP variability. The relationship is such that cooler than average SSTs occur during periods of lower than average SLP over the central North Pacific and vice versa. Recently, the IPO, a Pacific basin-wide feature, has been described, which includes low-frequency variations in climate over the North Pacific (Power et al., 1998, 1999; Folland et al., 1999a). The time-series of this feature is broadly similar to the inter-decadal part of the North Pacific PDO index of Mantua et al. (1997). The IPO may be a Pacific-wide manifestation of the PDO and seems to be part of a continuous spectrum of low-frequency modulation of ENSO, and so may be partly stochastic. When the IPO is in a positive phase, SST over a large area of the south-west Pacific is cold, as is SST over the extra-tropical north-west Pacific. SST over the central tropical Pacific is warm but less obviously warm over the equatorial far eastern Pacific unlike ENSO. Warmth also extends into the tropical west Pacific, unlike the situation on the ENSO time-scale.

The IPO shows three major phases this century: positive from 1922 to 1946 and from 1978 to at least 1998, with a negative phase between 1947 and 1976. Arguably, the structure of this pattern, nearly symmetrical about the equator and only subtly different from ENSO, is a strong indication of the importance of the tropical Pacific for many remote climates on all time-scales (Garreaud and Battisti, 1999). Power et al. (1999) showed that the two phases of the IPO appear to modulate year-to-year ENSO precipitation variability over Australia. Salinger and Mullan (1999) showed that prominent sub-bidecadal climate variations in New Zealand, identified in the temperature signal by Folland and Salinger (1995), are related to a SST pattern like the IPO. The IPO is a significant source of decadal climate variation throughout the South Pacific, and modulates ENSO climate variability in this region (Salinger et al., 2001). Similarly, the PDO (and likely the IPO) may play a key role in modulating ENSO teleconnections across North America on inter-decadal time-scales (Gershunov and Barnett, 1998; Livezey and Smith, 1999).

A simple and robust index of climate variability over the North Pacific is the area-weighted mean SLP, averaged over most of the extra-tropical North Pacific Ocean, of Trenberth and Hurrell (1994). A general reduction in SLP after about 1976 has been particularly evident during the winter half (November to March) of many of these years. This is characterised by a deeper-than-normal Aleutian low pressure system, accompanied by stronger-than-normal westerly winds across the central North Pacific and enhanced southerly to south-westerly flow along the west coast of North America, as reviewed in the SAR (Figure 3.17). Consequently, there have been increases in surface air temperature and SST over much of western North America and the eastern North Pacific, respectively, over the past two decades, especially in winter, but decreases in SST, or only modest warming, over parts of the central extra-tropical North Pacific (Figure 2.10). Numerous studies have suggested that the mid-1970s changes in the atmospheric and oceanic circulation may reflect one or more low-frequency variations over the North Pacific, one being the PDO (Kawamura, 1994; Latif and Barnett, 1994; Mann and Park, 1994, 1996; Deser and Blackmon 1995; Zhang et al., 1997b; White and Cayan, 1998; Enfield and Mestas-Nuñez, 1999).

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