We show that the Interdecadal Pacific Oscillation first named in 1999 and currently being much investigated in Australia and New Zealand, is likely to be the Pacific-wide manifestation of the better known Pacific Decadal Oscillation (PDO). Thus the PDO may be an incomplete description of a much wider climatic phenomenon. Both phenomena changed phase near the time of the Pacific “climatic shift” around 1976 and which apparently influenced global temperatures. The IPO SST pattern, which differs in several ways from the canonical ENSO pattern, has just as much variability in the South as the North Pacific and gives the impression of being quasi-symmetric about the equator. Although it remains unclear to what extent either phenomenon is physically separate from ENSO fluctuations, the IPO was shown a few years ago to modulate high frequency ENSO rainfall teleconnections to Australia in an apparently rather strong way. Recent work using the HadAM3 model forced with the Hadley Centres HADISST data set provides confirmation that an SST pattern like that of the IPO modulates ENSO influences on rainfall over Australia in a broadly similar way. The implication is that the state of the IPO, a decadal to multidecadal phenomenon, may indeed be important to long-range forecasts of ENSO effects on Australian rainfall a season or a few seasons ahead. As a result Australian farming and hydrological interests have recently taken a quite close interest in the IPO.

On the wider scale, we show recently published evidence using analysis of variance methods that the IPO modulated the South Pacific Convergence Zone in a manner independently of that of ENSO during the twentieth century. We also show how global rainfall influences due to ENSO may change as we progress from the ENSO time scale, through the decadal time scale and into the interdecadal time scale. Such results have recently resulted in the use of the IPO idea in paleoclimate studies. Finally, we mention work being carried out in the Hadley Centre using the HadCM3 coupled model to investigate the mechanisms of IPO and try to determine if the IPO is at least partly a physically separate oceanographic phenomenon from ENSO or hardly more than its stochastic low frequency envelope.