Weighted Anomaly Standardized Precipitation Index (WASP)
Index name: Weighted Anomaly Standardized Precipitation Index (WASP).
Ease of use: Green.
Origins: Developed by B. Lyon to monitor precipitation in the tropical regions within 30 degrees of the equator.
Characteristics: Uses gridded monthly precipitation data on a 0.5° by 0.5° resolution, and is based on 12-month overlapping sums of weighted, standardized monthly precipitation anomalies.
Input parameters: Monthly precipitation and annual precipitation values.
Applications: Used mainly in wet tropical regions to monitor developing drought, taking into account the defined wet and dry periods in the climate regime. Can be used to monitor droughts that affect agriculture and other sectors.
Strengths: Using precipitation as a single input allows for simpler computations.
Weaknesses: Does not work so well in desert regions. Gridded precipitation data may be a challenge to obtain in an operational capacity.
Resources: The methods and calculations are provided and explained in the literature, http://iridl.ldeo.columbia.edu/maproom/Global/Precipitation/WASP_Indices.html
Reference: Lyon, B., 2004: The strength of El Niño and the spatial extent of tropical drought. Geophysical Research Letters, 31: L21204, doi:10.1029/2004GL020901.