Interdecadal Pacific Oscillation and South Pacific climate

The Interdecadal Pacific Oscillation (IPO) has been shown to be associated with decadal climate variability over parts of the Pacific Basin, and to modulate interannual El Nino-Southern Oscillation (ENSO)-related climate variability over Australia. Three phases of the IPO have been identified during the 20th century: a positive phase (1922-1944), a negative phase (1946-1977) and another positive phase (1978-1998). Climate data are analysed for the two most recent periods to describe the influence of the IPO on decadal climate trends and interannual modulation of ENSO teleconnections throughout the South West Pacific region (from the equator to 55degreesS, and 150degreesE to 140degreesW). Data coverage was insufficient to include the earliest period in the analysis. Mean sea level pressure (SLP) in the region west of 170degreesW increased for the most recent positive IPO period, compared with the previous negative phase. SLP decreased to the east of 170degreesW, with generally more southerly quarter geostrophic flow over the region. Annual surface temperature increased significantly southwest of the South Pacific Convergence Zone (SPCZ) at a rate similar to the average Southern Hemisphere warming. Northwest of the SPCZ temperature increases were less, and northeast of the SPCZ more than the hemispheric warming in surface temperature. Increases of annual precipitation of 30% or more occurred northeast of the SPCZ, with smaller decreases to the southwest, associated with a movement in the mean location of the SPCZ northeastwards. The IPO modulates teleconnections with ENSO in a complex way, strengthening relationships in some areas and weakening them in others. For New Zealand, there is a consistent bias towards stronger teleconnections for the positive IPO period. These results demonstrate that the IPO is a significant source of climate variation on decadal time scales throughout the South West Pacific region, on a background which includes global mean surface temperature increases. The IPO also modulates interannual ENSO climate variability over the region. Copyright (C) 2001 Royal Meteorological Society.