An origin of North American monsoon retreat biases in climate models

Most state-of-the-art coupled climate models suffer from a late retreat bias in the 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, including complex terrains, local land-sea contrast, and Gulf of California (GoC) surges. However, we found little correlation between CMIP6 model resolutions and the simulated NAM retreat-season precipitation. 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 (i.e., the subtropical eastern Pacific and the tropical North Atlantic) 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 locally throughout the troposphere; the diabatic heating biases generate a Matsuno-Gill type 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 CMIP6 models. The precipitation seasonality in the NAM region is also used to constrain future projection. The “good” CMIP6 models project a drier monsoon peak season and a delayed monsoon retreat while the “poor” CMIP6 models project enhanced monsoon precipitation. All model groups project a drier dry season in the NAM region.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Atmospheric Sciences

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Ye, Jiacheng

Contributors dc:contributor

• Wang, Zhuo
• Yang, Fanglin
• Dominguez, Francina

• North American monsoon
• Climate modeling

Statement dc:rights

• Copyright 2022 Jiacheng Ye

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/116125

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F116125