ESPI is calcuated with precipitation anomalies from two areas (Figure 1) in the Pacific Ocean and quantifies the strength of ENSO, in terms of regional rainfall changes. A detailed summary of the methodology can be found at http://trmm.gsfc.nasa.gov/ESPIsummary.html . Time series of the bimonthly El Niņo Index (EI), La Niņa Index (LI), and ENSO Precipitation Index (ESPI) are shown in Figure 2 .

Figure 1 (click to enlarge)	Figure 2 (click to enlarge)