The Role of Interannual ENSO Events in Decadal Timescale Transitions of the Interdecadal Pacifc Oscillation
To increase the credibility of initialized decadal climate predictions, there must be an improved understanding of the processes involved with modes of variability that could contribute to skillful predictions. One candidate could involve the build-up of decadal timescale upper ocean heat content in the off-equatorial western tropical Pacific that could then be triggered by interannual El Niño/Southern Oscillation (ENSO) events to contribute to decadal timescale transitions of tropical Pacific SSTs to the opposite phase of the Interdecadal Pacific Oscillation (IPO). This can be viewed as a corollary to subseasonal westerly wind burst events contributing to El Niño interannual timescale transitions.
A long pre-industrial control run with CESM1 (to tie into the CESM1 Decadal Prediction Large Ensemble) is analyzed to show that there is a greater chance of ENSO activity to contribute to an IPO transition when off-equatorial western Pacific Ocean heat content reaches either a maximum (for El Niño to contribute to a transition to positive IPO with prolonged above-normal SSTs across the tropical Pacific) or minimum (for La Niña to contribute to a transition to negative IPO) as seen in observations. These conditions then sustain the decadal-timescale transition to positive IPO (with the opposite sign for transition to negative IPO), with implications for improved decadal climate predictions.
With the improved understanding of these processes, initialized predictions that capture the off-equatorial western Pacific Ocean heat content anomalies could provide improved predictions of subsequent ENSO activity and associated transitions of decadal-timescale SST anomalies associated with the IPO and thus improve the credibility of those predictions.