The western North Pacific Subtropical High (WNPSH) is a key atmospheric circulation system strongly influences weather and climate over the entire East and Southeast Asia. It determines the strength and position of the Mei-yu/Baiu/Changma Front and the trajectories of typhoon and western Pacific tropical cyclones.

How it will change in the future concerns the livelihood of many millions of people. The answer from state-of-the-art climate models is currently ambiguous. A total of 35 models participating in the Fifth Phase of Coupled Model Intercomparison Project (CMIP5) cannot agree on the sign of future changes.

A new research published in Nature Communications on June 4 led by researchers from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences, with collaborators from the Met Office Hadley Centre in the UK and Nanjing University, found that such uncertainties are mainly resulted from systematic biases in simulating the historical sea surface temperature from individual models.

With corrections using observed sea surface temperature, uncertainties can be reduced by 45%. Under the Representative Concentration Pathway (RCP) 8.5, a high greenhouse gas emission scenario, the models will tend to agree on a future intensification of the WNPSH, which implies stronger East Asian summer monsoon with increased rainfall but reduced typhoon landfalls over East Asia. In the meantime, it could also imply increased risk of heatwaves in the southern and eastern China.

"Uncertainties with climate models are there to stay, and even models are progressively improved. How to get the best information for climate adaptation from currently available model projections is an important research topic." Said Dr. CHEN Xiaolong from IAP, lead author of the study.

Dr. ZHOU Tianjun, executive director of the institute and a co-author, said, "This is an important step forward in predicting potential future changes of the WNPSH and its impacts. The link with future changes of the East Asia summer monsoon and western Pacific tropical cyclones needs further investigation."

The western North Pacific Subtropical High: A controlling circulation on location of Mei-yu Front and track of tropical storms (Image by CHEN Xiaolong)  

"Given the importance of the WNPSH, this is an important step towards the right direction. As the observed climate was only one realization of many possibilities, some uncertainties will remain," said Dr. WU Peili, a co-author from the Met Office Hadley Centre.

Another co-author, Prof. WANG Minghuai from Nanjing University added, "This work not only reduces the uncertainty in future WPNSH projection, but also identifies two major sources that lead to the projection uncertainty and therefore provides research directions for further improving the WPNSH simulations in climate models."

The study was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, the National Natural Science Foundation of China, the International Partnership Program of Chinese Academy of Sciences, and an international programme from the Chinese Academy of Science and the Newton Fund from the UK under CSSP-China.