The 12-month Weighted Anomaly Standardization Precipitation (WASP) index provides a simple visual means of extreme rainfall events for the past 12 months, whether they are drought or excess rainfall.
To compute the WASP index, monthly precipitation departures from the long-term (1981-2010) 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.
Clicking on the map will pop-up a local time series at the 0.05˚ gridbox clicked. Users can also choose to spatially average the WASP over Districts, Provinces or Countries, by clicking on the map or picking an administrative name in the drop-down list, once having chosen which level of administration. Note that the color bars are placed on the last month of the corresponding 12-month WASP, ie that the bar on Mar 2017 represents Apr 2016 - Mar 2017 12-monht WASP. Users can also focus the time series on a small time period by using the menu in the Control Bar.
Lyon, B., and A. G. Barnston.ENSO and the spatial extent of interannual precipitation extremes in tropical land areas.Journal of Climate, 200518:5095-5109.
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