Climate Models Exaggerate the Enhanced Double‐ITCZ in the Warming Tropical Pacific Due To Preexisting Precipitation Bias

Abstract Future greenhouse warming projections from the CMIP6 multi‐model ensemble indicate increased precipitation in the southeastern Pacific Ocean and a reduction in interhemispheric precipitation asymmetry, suggesting an enhanced double‐Intertropical Convergence Zone (ITCZ) in the tropical Pacific. However, these models exhibit a persistent annual‐mean double‐ITCZ bias in preindustrial and historical simulations, casting doubt on their projections. Indeed, models with the smallest preexisting double‐ITCZ bias—aside from a few outliers—typically show the least enhancement of the double‐ITCZ feature under global warming. To investigate this further, we use CESM2 and conduct flux‐adjusted simulations that nearly eliminate the precipitation bias in preindustrial experiments, followed by CO 2 ‐doubling simulations. Our bias‐reduced simulations show a much smaller reduction in the precipitation asymmetry index and only half the increase in rainfall in the southeastern Pacific compared to the original model. We conclude that climate models likely overestimate the enhanced double‐ITCZ feature under global warming due to the preexisting precipitation bias.