- Global Weighted Anomaly Standardized Precipitation indexes, and
- Global Standardized Precipitation Indexes
Rasmus had discussed the potential for global warming to have been implicated in the current drought in the Amazon, and I had asked the underlying question of how much of a precipitation shortfall there had been. It’s a key question in understanding drought, because much of what we think of today as “drought” is in fact a relatively modest shortfall in precipitation, combined with substantial population growth and therefore huge increased water demands.
You can see that, by both indices cited by Lyon, the shortfall over the past year in the Amazon is substantial.
(FYI, SPI for the United States is available on the Western Regional Climate Center web site, sliced up a number of different useful ways.)