Decadal-to-interdecadal variability of the atmospheric circulation is most prominent in the North Pacific, where fluctuations in the strength of the wintertime Aleutian Low (AL) pressure system co-vary with North Pacific sea surface temperatures (SST) in the Pacific Decadal Oscillation (PDO). ttese are linked to decadal variations in atmospheric circulation, SST and ocean circulation throughout the whole Pacific Basin in the IPO. ttis is an 'ENSO-like' feature of the climate system that operates on time scales of several decades, tte main centre of action in SST is in the north Pacific centred near the Date Line at 40°N. ttree phases of the IPO have been identified during the 20th century: a positive phase (1922-1946), a negative phase (1947-1976) and the most recent positive phase (1977-1998). tte IPO has been shown to be a significant source of decadal climate variation throughout the South Pacific (Salinger et al. 2001), New Zealand and Australia (Power et al. 1999)
Folland et al. (2002) showed that the IPO significantly affects the movement of the South Pacific Convergence Zone (SPCZ) in a way independent of ENSO. tte South Pacific Convergence Zone (SPCZ) is one of the most significant features of the subtropical Southern Hemisphere climate (Kiladis et al. 1989; Vincent and Dias 1999; Vincent 1994). It is characterized by a quasi-permanent band of low-level convergence, enhanced cloudiness and precipitation originating from the west Pacific warm pool in the Indonesian region and trending south-east towards French Polynesia, tte SPCZ when it is active during the November - April South Pacific wet season produces a good proportion of the precipitation. In the negative phase of the IPO the SPCZ is displaced southwest producing a drying in climates to the north east, and increases in precipitation to the southwest, tte east and north of New Zealand becomes wetter, whilst the west and south drier. In Australia rainfall tends to increase in eastern Australia, tte positive phase reverses the climate anomalies: the SPCZ is displaced north east increasing precipitation to the northeast and making the subtropical southwest Pacific drier, tte north and east of New Zealand becomes drier, whilst the south and west wetter. Similarly eastern Australia tends to be drier.
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