Twenty-First-Century Multimodel Subtropical Precipitation Declines Are Mostly Midlatitude Shifts

Declines in subtropical precipitation are a robust response to modeled twenty-first-century global warming. Two suggested mechanisms are the "dry-get-drier" intensification of existing subtropical dry zones due to the thermodynamic increase in vapor transport and the poleward expansion of these same dry zones due to poleward shifts in the modeled general circulation. Here, subtropical drying in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report multimodel archive is compared to each of these two mechanisms. Each model's particular, biased, and seasonally and zonally varying mean state is considered relative to the location of that model's predicted changes, and these relationships are recorded in a common framework that can be compared across models. The models have a strong tendency to reduce precipitation along the subtropical flanks of their existing midlatitude cyclonic precipitation belts. This broad result agrees with the poleward expansion mechanism and with a poleward storm-track shift in particular. In contrast, the models have no clear tendency to reduce precipitation in the central nor equatorward portions of their subtropical dry zones, implying that the thermodynamic mechanism is broadly unimportant for the precipitation reductions. This is unlike the response of precipitation minus evaporation, which robustly declines in large portions of these regions, especially over the oceans. The models also tend to increase precipitation in their wet deep tropical areas, but this is not as robust as the above reduction in the subtropical midlatitudes. High-latitude precipitation increases are the most robust precipitation changes of all in this framework.