From the Cover: Robust Hadley Circulation changes and increasing global dryness due to CO2 warming from CMIP5 model projections

In this paper, we investigate changes in the Hadley Circulation (HC) and their connections to increased global dryness (suppressed rainfall and reduced tropospheric relative humidity) under CO₂ warming from Coupled Model Intercomparison Project Phase 5 (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 and 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. Our results provide a physical basis for inferring that greenhouse warming is likely to contribute to the observed prolonged droughts worldwide in recent decades.The Hadley Circulation (HC), the zonally averaged meridional overturning motion connecting the tropics and midlatitude, is a key component of the global atmospheric general circulation. How the HC has been or will be changed as a result of global warming has tremendous societal implications on changes in weather and climate patterns, especially the occurrences of severe floods and droughts around the world (1, 2). Recent studies have suggested that the global balance requirement for water vapor and precipitation weakens the tropical circulation in a warmer climate (3, 4). So far, the most robust signal of weakening of tropical circulation from models appears to come from the Walker circulation but not from the HC, possibly because of the large internal variability in the latter (5, 6). Observations based on reanalysis data have shown weak signals of increasing, decreasing, or no change in HC strength in recent decades, with large uncertainties depending on the data source and the period of analyses (7–10). Meanwhile, studies have also shown that even though water vapor is increased almost everywhere as global temperature rises, increased dryness (lack of rainfall and reduced surface relative humidity) is found in observations and in model projections, especially in many land regions around the world (11–13). Reduction in midtropospheric relative humidity and clouds in the subtropics and midlatitude under global warming have also been noted in models and observations, suggesting the importance of cloud feedback and circulation changes (14–16). Even though robust global warming signals have been found in changing rainfall characteristics (2, 17, 18), in the widening of the subtropics, and in the relative contributions of circulation and surface warming to tropical rainfall from climate model projections and observations (19–24), the dynamical linkages between HC changes and global patterns of moistening and drying have yet to be identified and understood. In this paper, we aim at establishing a baseline understanding of the dynamics of changes in the HC and relationships with increased global dryness based on monthly outputs from 33 Coupled Model Intercomparison Project Phase 5 (CMIP5) 140-y projection experiments under a scenario of a prescribed 1% per year CO₂ emission increase. The baseline developed here hopefully will provide guidance for future observational studies in the detection and attribution of climate change signals in atmospheric circulation and in the assessment of risk of global droughts. Consistent with previous studies (3, 4, 17, 24), we find that under the prescribed emission scenario, global rainfall increases at a muted rate of 1.5 ± 0.1% K⁻¹, much slower than that for saturated water vapor as governed by the Clausius−Clapeyron relationship (∼6.5% K⁻¹). In the following, the responses of various quantities related to the climatology and anomaly of the HC, rainfall, tropical convection, global dryness, and their interrelationships are discussed. For definitions of climatology and anomaly, see Methods and Materials.