Simulated changes due to global warming in the variability of precipitation, and their interpretation using a gamma-distributed stochastic model

The standard deviation of monthly mean precipitation S generally increases as the global model warms, with the global mean S in 2071-2100 in January (July) being 9.0% (11.5%) larger than in 1961-1990. Decreases in some subtropical locations occur, particularly where mean precipitation decreases. The global pattern of overdispersion is largely unchanged, however, and the changes in S can be related to those in the stochastic model parameters. Much of the increase in S is associated with increases in the scale parameter of the gamma distribution of wet-day amounts. Changes in C, which is unaffected by this parameter, are generally small. Increases in C in several subtropical bands and over northern midlatitude land in July are related to a decreased frequency of precipitation, and (to a lesser degree) changes in the gamma shape parameter. Some potential applications of the results to downscaling are discussed, and illustrated using observed rainfall from southeast Australia.