Changes of Precipitation and Drought Associated with Global Warming

Precipitation is associated with moist tropospheric ascent. An outstanding issue in climate change research is whether the precipitation changes associated with global warming are dictated mainly by a general increase of atmospheric humidity (“thermodynamic control”) or by changes in the statistics of large-scale tropospheric vertical velocity (“dynamic control”). Thermodynamic control is stronger and more robust than dynamic control in current climate models. Our investigation of observed precipitation changes over the last 70 years, however, tells a different story. At most locations around the globe, the observed changes in both the mean and extreme daily precipitation are much more consistent with changes in vertical velocity, i.e. with dynamic control. Most tellingly, and with important implications for drought, the observed changes in the probability of days of no precipitation are remarkably consistent with changes in the probability of days of descent. This aspect of the precipitation statistics is consistent with dynamic control, but is totally beyond thermodynamic control.

We also find that AMIP-style uncoupled atmospheric GCM simulations of the last 70 years with prescribed observed SSTs are reasonably able to capture the observed changes in precipitation and vertical velocity, and hence the dominance of dynamic control. The fact that fully coupled climate models, which predict the SSTs, do not correctly capture these changes suggests that the problem originates in their misrepresentation of the SSTs, especially tropical SSTs. Specifically, climate models continue to underestimate the spatial variation of tropical SST trends, and hence the magnitude of changes in the mean circulation and stormtracks, leading to an underestimation of the dynamic control of precipitation that in reality is stronger than the thermodynamic control. This issue has large implications for our confidence in regional climate predictions around the globe.