[Text Source: Curtis, S. and R. Adler, 2000: ENSO indexes based on patterns of satellite-derived precipitation. J. Climate, 13, 2786-2793 and http://trmm.gsfc.nasa.gov/ESPIsummary.html
A summary of the GPCP precipitation-based ENSO indices (EI, LI, ESPI)
The goal in constructing a precipitation-based measure of ENSO was to estimate the gradient ... of rainfall anomalies across the Pacific basin and ensure a good relationship with SST- and pressure-based indices. Areas were selected that represent the Maritime Continent (MC) (10° N – 10°S; 90° E – 150° E) and central to eastern Pacific (P) (10° N – 10° S; 160° E – 100° W). These regions capture the largest precipitation anomalies associated with the interannual variations of the Walker circulation and contain the largest correlations between GPCP and Nino 3.4 and SOI. Within P and MC the absolute magnitude of the largest correlation is over +0.6.
Because of the spatially varying nature of rainfall, it was decided to use a moving block average which would capture the strongest zonal gradients within the equatorial Pacific. This procedure is unlike many fixed area average indices, and allows for a realistic meridional component of the precipitation gradient and migration of the ascending and descending branches of the Walker circulation.
Additional Information: http://trmm.gsfc.nasa.gov/ESPIsummary.html