Climate models

Latest CMIP6 climate models overestimate future Afro-Asian monsoon rainfall and runoff

The shadings and percentages in the subplots are the fractions of area that will experience a significant increase in precipitation (left) and runoff (right) in the unconstrained (blue) and constrained (red) projections. Credit: IAP

Climate projections are essential for adaptation and mitigation planning. Results from the latest cycle of the Coupled Model Intercomparison Project, Phase 6 (CMIP6) have been widely used in climate projections.

However, a subset of CMIP6 models are “too warm” and the projected warming due to greenhouse gases is excessive. Previously, it was difficult to know how to solve the “hot model” problem on a regional scale.

The latest CMIP6 climate models tend to overestimate future Afro-Asian Summer Monsoon (AfroASM) rainfall and runoff due to current biases in warming patterns, according to a research team from the Institute of Atmospheric Physics. (IAP) of the Chinese Academy of Sciences. Limiting the biases, however, the increase in precipitation is 70% of the raw projection.

The study will be published today (May 10, 2022) in the journal Nature Communication.

AfroASM includes the West African monsoon, the South Asian monsoon, and the East Asian monsoon.

The research team identified the primary mode of variability among CMIP6 models in projecting future changes in AfroASM rainfall. They found that the projection uncertainty was related to the current interhemispheric thermal contrast (ITC) bias. Since the large-scale monsoon circulation is driven by ITC due to wet static energy gradients, models with greater ITC tendency over the last thirty years tend to project more d increases in precipitation.

Since most CMIP6 models tend to overestimate current ICT trends, the team corrected the raw projection by designing an emergent constraint technique. The increase in precipitation in the constrained projection is about 70% of the ensemble mean of the CMIP6 models. The land area with a significant increase in precipitation is about 57% of the gross projection.

The research team then extended their analysis to runoff, which is a mirror of potential water availability. In the constrained projection, about 27% of the land area in the AfroASM region will experience a significant increase in potential water availability, or about 66% of the raw projection. Regionally, the impact of the observation constraint is most pronounced in the West African monsoon region where the fraction of land area with increased water availability is about 55% of the gross projection.

This study provides a solution to tackle the problem of the “hot model” on a regional scale. The emergent constraint technique reported in the study is based on the physical link between a modeled but observable variable at present and a projected variable in the future climate system.

“This technique is useful to correct the bias of CMIP6 models and ultimately increase the reliability of rainfall projection in the Afro-Asian summer monsoon region. The underlying physical mechanism is the impact of equilibrium climate sensitivity on interhemispheric thermal contrast in historical and future time periods,” said Dr. ZHOU Tianjun of IAP, corresponding author of the study.

“Smaller increases in rainfall and runoff are likely to reduce flood risk, while posing a challenge for future water resource management,” said CHEN Ziming, a Ph.D. student at the University of Chinese Academy of Sciences, first author of the study.

Reference: “Observational Constrained Projection of Afro-Asian Monsoon Rainfall” by Ziming Chen, Tianjun Zhou, Xiaolong Chen, Wenxia Zhang, Lixia Zhang, Mingna Wu, and Liwei Zou, May 10, 2022, Nature Communication.
DOI: 10.1038/s41467-022-30106-z

The study was supported by the National Key Research and Development Program of China and the National Natural Science Foundation of China.