This plot shows a time series of the country-averaged value of the 12-Month Weighted Anomaly Standardized Precipitation (WASP) index calculated using the latest version of the CMAP monthly precipitation dataset for a user-selected country in Africa.
To compute the WASP 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 a 12-month time period in this case, and this result is itself standardized.
The user is also able to select a reference year, placing a baseline on the plot corresponding to the WASP index value for the center of the selected year. For WASP index values above the baseline, the area between the index and the baseline value is shaded in green. For WASP index values below the baseline, the area between the index and the baseline value is shaded in brown.
The purpose of this tool is to provide a simple visual means of relating country-average precipitation to a reference year of interest.
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.