Robust Changes in the Hadley Circulation and Global Dryness Under CO2 Warming from CMIP5 Model Projections

In this paper, we investigate changes in the Hadley Circulation (HC) and their connections to increased global dryness under CO2 warming from CMIP5 model projections. We find a strengthening of the HC manifested in a “deep-tropics squeeze” (DTS), i.e., a deepening and narrowing of the convective zone, enhanced ascent, increased high clouds, suppressed low clouds, and a rise of the level of maximum meridional mass outflow in the upper troposphere (200-100 hPa) of the deep tropics. The DTS induces atmospheric moisture divergence, reduces tropospheric relative humidity in the tropics and subtropics, in conjunction with a widening of the subsiding branches of the HC, resulting in increased frequency of dry events in preferred geographic locations worldwide. Among various water cycle parameters examined, global dryness is found to have the highest signal-to-noise ratio, and arguably could be among early warning signals of global water cycle changes under global warming. Our results provide the physical bases for inferring that greenhouse warming is likely to contribute to the observed trend of prolonged droughts worldwide in recent decades.