Years of research on La Niña and climate change have progressed

Recently, Jia Fan, an associate researcher at the Institute of Oceanography of the Chinese Academy of Sciences, together with scholars from China, Australia and the United States, has made important progress in the field of tropical air-sea interaction and climate change, and found for the first time that global warming will lead to an increase in the frequency of multi-year La Niña events. The research was published in Nature under the title “Increased occurrences of consecutive La Nina events under global warming.” At the same time, Nature presents it to media outlets around the world and invites experts in the field to publish commentary articles in Nature News & Views.     

The El Niño-Southern Oscillation (ENSO) is the strongest mode of interannual variation in the Earth’s climate system and has a significant impact on the Earth’s multisphere. Its warm phase El Niño usually matures rapidly and decays within a year, but cold phase La Niña tends to last more than two years and becomes a “multi-year La Niña”. Multi-year La Niña events like those in 2020-2022 will bring sustained and superimposed damage to the global climate, agro-ecological environment, and human economy and society, and significantly increase the risk of extreme weather disasters. How to change multi-year La Niña events under the future global warming scenario is one of the major frontier scientific issues in the field of climate change research.      

Using data from multi-model multi-scenario trials in CMIP6, the study found for the first time that the frequency of multi-year La Niña events will increase significantly in the 21st century after global warming (Figure 1). Strong El Niño is an important factor in the production of La Niña over the years. Although global warming will lead to frequent strong El Niño events, the study shows a greater increase in the frequency of multi-year La Niña than the former, suggesting that future El Niños can more effectively trigger multi-year La Niña. Further, this efficiency stems from the enhanced interaction between the tropics and the outside tropics caused by changes in mean SST in the Pacific background. After global warming, two ocean regions have significantly stronger rates of warming (Figure 2). The first is the subtropical northeast Pacific, which makes it easier for the same intensity of tropical extratropical atmospheric disturbances to induce a more northerly “North Pacific meridional mode” response in the region; The second is the equatorial eastern Pacific Ocean, where an El Niño warm SST anomaly of the same intensity can trigger stronger tropical outer atmospheric disturbances. The combination of the two results in more and wider meridional modal responses in the North Pacific Ocean in the spring of La Niña in the first year, resulting in a more northerly position of the out-equatorial easterly anomaly, and further weakening of heat transport to the equator (the “ocean charging” process), which is conducive to the maintenance of La Niña’s cold SST anomaly into the second year, and eventually manifests itself as more frequent multi-year La Niña events. The study suggests that climate disasters associated with multi-year La Niña may occur more frequently in the future, highlighting the need and urgency to curb greenhouse gas emissions.Figure 1. The frequency of multi-year La Niña events increases significantly under future warming scenarios modeled by climate models

Figure 2. (a) Changes in SST, wind field and sea surface pressure in the Pacific background after global warming; (b) Following global warming, the response of the meridional mode of the North Pacific in the following spring to the warm El Niño SST anomaly in the tropical eastern Pacific Ocean of the previous winter is enhanced.

This study is a new idea and perspective to explore the process and mechanism of tropical-extratropical interaction under global warming, which is of great significance for exploring the diversity of ENSO and its future changes, which is helpful to improve the prediction level of ENSO in the future, and provides a scientific basis for the international community to effectively respond to climate change and formulate climate policies. (Source: Institute of Oceanology, Chinese Academy of Sciences)

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