[4] Folland et al., 2002 defined an index to describe the Interdecadal Pacific oscillation (IPO) with sea surface temperature and
night marine air temperature to determine how the SPCZ varies with the IPO.
Rayner, N. A.; Parker, D. E.; Horton, E. B.; Folland, C. K.; Alexander, L. V.; Rowell, D. P.; Kent, E. C.; Kaplan, A. (2003) Global analyses of sea surface temperature, sea ice, and
night marine air temperature since the late nineteenth century J. Geophys.