PNNL

The Science

Mediterranean climate regions feature low annual precipitation with an outstanding seasonal cycle of mild, wet winters and hot, dry summers. Climate models generally project wetter winters for the U.S. Southwest and drier winters for the Mediterranean Basin under global warming, but the models exhibit marked uncertainty. Scientists discovered a strong relationship between the common model bias that shows a double Intertropical Convergence Zone (ITCZ) in present-day simulations and future precipitation changes over the U.S. Southwest and Mediterranean Basin. Correcting this double-ITCZ bias reduced the projected rainfall in the U.S. Southwest and amplified projected dryness in the Mediterranean Basin.

The Impact

Changes in winter precipitation have profound implications for water resources and crop production in the U.S. Southwest and Mediterranean Basin, areas already under severe water stress. However, large uncertainties in model projections have invited challenges to their use in creating infrastructure and resource management plans. Understanding how model biases affect future projections enhances the use of climate projections to support long-term decision-making. The relative reduction in future winter precipitation projected after the bias correction indicates reduced spring runoff and increased spring temperatures, both of which increase the likelihood of future water stress and wildfires in the studied regions.

Summary

This study analyzed 37 climate models from the fifth phase of the Coupled Model Intercomparison Project and revealed a statistically significant relationship between the double-ITCZ bias in present-day simulations and future projections of winter precipitation in the U.S. Southwest and Mediterranean Basin. More specifically, models with excessive double-ITCZ biases tend to exaggerate the future precipitation increase over the U.S. Southwest and understate the precipitation decrease over the Mediterranean Basin. Well-established physical processes, driven by increased tropical rainfall and weaker slowdown of the Atlantic Meridional Overturning Circulation under warming, underpin the correlation through atmospheric circulation changes. The tropical pathway over the Pacific Ocean connects the double-ITCZ bias to the U.S. Southwest precipitation changes, while the high-latitude pathway over the Atlantic Ocean primarily affects the Mediterranean Basin. The mechanistic and statistically significant connections between the double-ITCZ bias and future model projections support the use of the double-ITCZ as a constraint when projecting future precipitation changes. Constraining a model by the observed double-ITCZ index lowers the projected winter precipitation increase to no change over the U.S. Southwest and intensifies the projected decrease by 32% over the Mediterranean Basin under global warming.

PNNL Contact

L. Ruby Leung, Pacific Northwest National Laboratory, Ruby.Leung@pnnl.gov 

Funding

All authors were supported by the U.S. Department of Energy’s Office of Science Biological and Environmental Research through the Regional and Global Model Analysis and Multisector Dynamics program areas.