The index is constructed from the first 2 principal components (PCs) of the band pass filtered 30-90 day 850 hPa 5N-5S zonal wind from the National Centers for Environmental Prediction / National Center for Atmospheric Research reanalysis. PC1 is correlated with PC2 at 0.6 at a lag 12 days, and the index is the sum of PC2 and the value of PC1 12 days later (Maloney and Kiehl 2002). The index captures the MJO variabilty less well during ENSO warm episodes.

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