Modeling the Interdecadal Variation of ENSO Teleconnections

The largest interannual variability in global climate is associated with the El Niño-Southern Oscillation (ENSO) cycle. Observed relationships between the Southern Oscillation Index (SOI) and local precipitation and temperature anomalies are now used in many parts of the world to forecast the impacts of ENSO variations. However, recent analyses of extended historical data sets have revealed there are also substantial interdecadal fluctuations in climate and circulation of the global atmosphere and ocean. Of particular interest is the Interdecadal Pacific Oscillation (IPO), which has its largest amplitude in the North Pacific but also affects sea surface temperatures (SSTs) in the eastern equatorial Pacific, and therefore has the potential to modulate the ENSO cycle and its teleconnections. Phase reversals of the IPO this century occurred around 1925, 1947 and 1977, and several recent studies provide observational evidence of coincident abrupt climate changes or altered SOI-climate relationships.

This paper describes an atmosphere general circulation model study that investigates atmospheric responses to jointly-varying SST patterns associated with canonical versions of ENSO and the IPO. Simulations were carried out at NIWA (New Zealand) with the U.K. Meteorological Office Unified Model (HadAM2b version). Four simulations, each of 20 years, covered the possible combinations of ENSO and IPO extremes, where a specified anomaly was superimposed on the 1961-1990 SST climatology. The model reproduces observed ENSO variations well in most places. Statistically significant differences between IPO extremes occur over substantial areas of the globe in mean sea level pressure, precipitation, wind and other fields. The “positive” IPO extreme (post-1977) strengthens El Niño climate patterns in many but not all places. The paper will describe the modulated ENSO patterns in detail, with particular attention to the Southern Hemisphere.