Global Precipitation Climatology Project (GPCP)

University of Maryland College Park
Earth System Science Interdisciplinary Center (ESSIC) and Cooperative Institute for Climate and Satellites (CICS)

ENSO Precipitation Index
Curtis and Adler (2000) introduced an index for the El Nino-Southern Oscillation (ENSO) based on precipitation from GPCP called the ENSO Precipitation Index (ESPI). The ESPI is based on the GPCP CDR up to the last 2 months, after which the ICDR estimates are used. The ESPI series is available for download here: ESPI.txt. Here is a plot of the ESPI from 1979 to present with the Nino 3.4 index overlaid:

Creation of the ESPI
The index is based on rainfall anomalies in two rectangular areas, one in the eastern tropical Pacific (10°S-10°N, 160°E-100°W) and the other over the Maritime Continent (10°S-10°N, 90°E-150°E). The first step of the procedure involves moving a 10° by 50° block around each box; the minimum and maximum values of all possible blocks is obtained for each box and these are combined to estimate an El Niño precipitation index (EI) and a La Niña precipitation index (LI) which are shown below in the plot below (after nomalization). The EI and LI are in turn combined to create the ESPI index. Finally, the ESPI index is normalized to have zero mean and unit standard deviation. A new climatology and normalization factors were calculated in August 2014 from GPCP v2.2 precipitation from 1979 to 2013 (35 full years) and these are used of the present calculation of the ESPI.

Curtis, S., and R. Adler, 2000: ENSO indices based on patterns of satellite-derived precipitation. J. Climate, 13, 2786-2793.