The IPO was introduced by Power et al (1999) as the second EOF of SST in the Pacific Ocean. Here is defined similarly to van Oldenborgh et al (2012) as the first EOF of detrended low-pass filtered SST over 50°S-50°N, 100°E-70°W. The detrending is done by subtracting the regression on the IIASA CO2-equivalent series. The default low-pass filer is a 4-yr running mean, although 1-yr and 9-yr versions are also available.

The resulting patterns are very similar to the decadal component of ENSO and the PDO, as far as I know the question whether there is independent physics beyond the low-frequency component of ENSO is not settled. The pattern is much wider than the interannual ENSO pattern in the meridional direction. It differs from the Pacific Decadal Oscillation (PDO) in that it is symmetric about the equator (as far as there are observations in the Southern Hemisphere).

• Power S, Casey T, Folland C, Colman A, Mehta V (1999) Interdecadal modulation of the impact of ENSO on Australia. Clim Dyn 15:319-324. doi:10.1007/s003820050284
• van Oldenborgh G J, Doblas Reyes F J, Wouters B, Hazeleger W (2012) Decadal prediction skill in a multi-model ensemble. Clim Dyn 38:1263-1280, doi:10.1007/s00382-012-1313-4, PDF