The impact of El Nino on East Asian climate under a warmer climate will be dominated by the change in El Nino decaying pace, according to a new paper published by a research team based in the Institute of Atmospheric Physics, Chinese Academy of Sciences, China.
The western North Pacific anomalous anticyclone (WNPAC) is a low-level atmospheric circulation system, linking up El Nino events with East Asian -western Pacific summer climate. The WNPAC can persist from El Nino mature phase in boreal winter to the upcoming summer, bringing abundant moisture to enhance the precipitation over East Asia. How the WNPAC will change in the future concerns millions of people living in the East Asian -western Pacific region, but the future change in the WNPAC under global warming is highly uncertain across climate models.
The study, which appears in Journal of Climate
online on 5th June, found that about 23% of the uncertainty in WNPAC projection is attributed to the El Nino amplitude change while the rest 77% is from non-amplitude change which is mainly related to the change in El Nino decaying pace, according to ZHOU Tianjun, the corresponding author of the paper.
ZHOU is a senior scientist at the Institute of Atmospheric Physics in the Chinese Academy of Sciences. He is also a professor at the University of Chinese Academy of Sciences (UCAS).
"To separately quantify the contributions of El Nino amplitude change and non-amplitude change from the total uncertainties, we have developed a new decomposition method. This decomposition method is based on large ensemble climate simulation. We have used the output of 40-member large ensemble from the Community Earth System Model Large Ensemble project", said WU Mingna, the first author of the study, who is a Ph. D student from the UCAS.
Schematic illustrates the relative contributions of change in different El Nino characteristics (i.e. amplitude and decaying pace) to the total uncertainties of the western North Pacific anomalous anticyclone (WNPAC) projection. D(0)JF(1) and JJA(1) represent El Nino mature winter and decaying summer. (Image by WU Mingna and CHEN Xiaolong)
"A larger El Nino amplitude can enhance the WNPAC through a stronger tropical Indian Ocean capacitor effect under a warmer climate, while a faster El Nino decaying pace can also enhance the WNPAC through descending Rossby waves in response to colder than normal sea surface temperature over the tropical central-eastern Pacific, and vice versa", explained Dr. CHEN Xiaolong, co-author of the study, and an associated professor at the Institute of Atmospheric Physics, Chinese Academy of Sciences.
"In addition to El Nino amplitude, more attention should be paid to the influence of other El Nino characteristics (i.e. El Nino decaying pace) in climate system. Our decomposition method can be used to diagnose the origin of uncertainty related to El Nino in climate projections, as well as the relevant mechanisms." Highlighted WU.
This work is jointly supported by National Natural Science Foundation of China under Grant Nos. 41988101 and 41605057 and National Key R&D Program of China under Grant 2020YFA0608904.
Wu, M., Zhou, T., & Chen, X. (2021). The source of uncertainty in projecting the anomalous western North Pacific anticyclone during El Nino–decaying summers. Journal of Climate, DOI: 10.1175/JCLI-D-20-0904.1. https://journals.ametsoc.org/view/journals/clim/aop/JCLI-D-20-0904.1/JCLI-D-20-0904.1.xml
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