Double‐ITCZ as an Emergent Constraint for Future Precipitation Over Mediterranean Climate Regions in the North Hemisphere

The semiarid Mediterranean climate regions feature wet winter and dry summer, distinct from most other regions on Earth. In response to warming, climate models project increased precipitation in US Southwest (USSW) and decreased in Mediterranean basin (MED) during winter, but with marked uncertainty. Using a multimodel ensemble, we found that models with excessive double Intertropical Convergence Zone (ITCZ) biases tend to exaggerate the precipitation increase over USSW and understate the precipitation decrease over MED in the future. These relationships are attributed to the atmospheric circulation changes driven by the increased tropical rainfall and the weaker slowdown of the Atlantic Meridional Overturning Circulation under warming, respectively. Constraining the present‐day double‐ITCZ with observations, the projected wetting over USSW is reduced to no change and the drying over MED is intensified by 32%. A relative reduction in future precipitation has profound societal and economic implications for these regions already under severe water stress. Plain Language Summary Winter precipitation has a dominant influence on water supply year‐round for densely populated Mediterranean climate regions, thence its future changes have profound societal and economic implications for water resources and crop production in these regions already under severe water stress. Climate models generally projected a wetter winter for the US Southwest and a drier winter for the Mediterranean basin under global warming. However, both projections are significantly influenced by a common model bias featuring a double Intertropical Convergence Zone in the current climate. By correcting the model bias, the projected future precipitation is further reduced in both regions, which would likely reduce spring runoff and increase spring temperature, increasing the likelihood of wildfires over California in the future. Key Points Double‐Intertropical Convergence Zone (ITCZ) can be used as an emergent constraint for future winter precipitation changes in US Southwest and Mediterranean Basin The emergent constraint for the two regions originates from atmospheric circulation response to tropical rainfall and Atlantic Meridional Overturning Circulation, respectively Constrained by the observed double‐ITCZ, future wetting in US Southwest is reduced and drying in Mediterranean Basin is intensified