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.

中文翻译：

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Double-ITCZ作为北半球地中海气候区未来降水的紧急限制

与地球上大多数其他地区不同，半干旱的地中海气候区冬季湿润，夏季干燥。为响应变暖，气候模型预测冬季美国西南部（USSW）的降水增加，而地中海盆地（MED）的降水减少，但存在明显的不确定性。使用多模型合奏，我们发现具有过大的热带双热带收敛带（ITCZ）偏差的模型往往会夸大USSW上的降水增加，而低估未来MED上的降水减少。这些关系分别归因于热带降水增加和变暖下大西洋子午向翻转环流的减弱较弱所驱动的大气环流变化。用观察值约束当前的双ITCZ，预计对USSW的润湿将保持不变，而对MED的干燥将增强32％。对于已经处于严重缺水状态的这些地区，未来降水的相对减少具有深远的社会和经济意义。