Data:  CAMS_OPI monthly precipitation on a 2.5 x 2.5 deg. lat/lon grid
Base Period for anomalies:  1961-1990
Source:  Climate Prediction Center, NCEP

Description:  This index gives an estimate of the relative deficit or surplus of precipitation for different time intervals ranging from 1 to 12 months. 

WASP is an acronym for the "Weighted Anomaly Standardized Precipitation" index and is based solely on monthly precipitation data.  To compute the index, monthly precipitation departures from the long-term average are obtained and then standardized by dividing by the standard deviation of monthly precipitation. The standardized monthly anomalies are then weighted by multiplying by the fraction of the average annual precipitation for the given month.  These weighted anomalies are then summed over varying time periods - here, 3, 6, 9 and 12 months.  On the plots, the value of the given WASP index has itself been standardized. 

For the WASP index, shading starts at +/- 1.0 with green shades indicating unusually wet conditions and brown unusually dry, respectively. Regions with an annual average precipitation of less than 0.2 mm/day have been "masked" from the plot. 

References: 

Lyon, B., and A. G. Barnston, 2005: ENSO and the Spatial Extent of Interannual Precipitation Extremes in Tropical Land Areas. Journal of Climate, 18, 5095-5109.

Lyon, B., 2004: The strength of El Niño and the spatial extent of tropical drought. Geophys. Res. Lett., 31, L21204.

NCEP is an acronym for the US National Centers for Environmental Prediction.