An Origin of North American Monsoon Retreat Biases in Climate Models

Abstract

Abstract Many coupled climate models suffer from a late retreat bias in North American monsoon (NAM) simulations, which is manifested by overestimated precipitation in October. The overestimated precipitation has long been attributed to the negative sea surface temperature (SST) biases in the tropical Atlantic and insufficient model resolution to resolve mesoscale features. However, we found little correlation between CMIP6 model resolutions and the simulated NAM retreat-season precipitation in October. Instead, we showed that tropical eastern North Pacific SST biases and the associated large-scale circulation biases play a dominant role in inducing the retreat-season biases, with SST biases in other ocean basins playing a secondary role. As revealed by simulations using a hierarchy of models, the positive SST biases in the tropical eastern North Pacific enhance local convection and lead to positive diabatic heating biases throughout the troposphere; the diabatic heating biases generate a Matsuno–Gill type of response that strengthens the subtropical high over the North Atlantic and weakens the subtropical high over the North Pacific, enhancing the low-level northward moisture transport from the tropics to the NAM region. The conclusion is robust across phase 6 of CMIP (CMIP6) models. The precipitation seasonality in the NAM region is used to constrain future projection. The “good” CMIP6 models project that the timing of the NAM peak season remains the same, but the peak-season precipitation is reduced and monsoon retreat is delayed, while the “poor” CMIP6 models project a delayed monsoon peak season with slightly enhanced peak-season precipitation. Both model groups project a drier dry season in the NAM region.