Subtropical drying under greenhouse gas-induced warming

Precipitation increases under increasing greenhouse gases (GHGs) over the globe, except many subtropical areas where it decreases. Several mechanisms have been proposed to explain this subtropical drying, which increases the risk of drought over subtropical land areas but was considered as a temporary response to increased GHGs by a recent study. Here climate simulations by different models under different forcing scenarios, including three multi-millennium simulations, are analyzed to examine the changes in the boundaries, area and mean precipitation of the subtropical dry zones, defined as the areas with annual-mean precipitation (P) below 2.5 mm/day. Results show that dry-zone mean P decreases under all forcing scenarios, over all time periods and persists into new equilibrium states. After the initial transient period, the northern and southern dry-zone boundaries of the Northern Hemisphere shift poleward and equatorward respectively, while those of the Southern Hemisphere mainly shift equatorward. During the initial transient period, the dry-zone boundaries expand both equatorward and poleward, consistent with previous studies. Dry-zone areas of both hemispheres increase. In contrast, mean precipitation averaged over subtropical subsidence zones may increase due to increased water vapor and weak drying over areas with weak subsidence. Increased subtropical subsidence and decreased subtropical precipitation are associated with increased equator-subtropical sea surface temperature gradients, which may lead to increased dry-zone area. Particularly, the P decreases over the subtropical dry zones result mainly from the enhanced drying effect due to increased vertical gradient of water vapor (dq/dz), with additional drying from increased subsidence, but offset by the wetting effect of increased water vapor. The dq/dz change results from tropospheric warming that persists throughout all stages of GHG-induced warming, which explains why the subtropical drying is a permanent response to GHG increases.