For large parts of eastern Asia, the summer in 2020 was the wettest summer for almost 60 years, with flooding in parts of China, Japan and South Korea.

The journal Advances in Atmospheric Sciences invited scientists from China, the UK, South Korea, the US and Japan to study on the east Asian flooding of 2020 and published a special issue consecrated to the event.

"Climate change will impact everyone. We're already seeing its fingerprints on the extreme weather around the world," said Dr. Robin Clark from the Met Office, one of the co-organizers of the special issue and co-author of the preface.

A key aspect of the summer is extremely unusual behavior of an anti-cyclone usually present in the sub-tropical west Pacific. "The anticyclone is a feature in every year, but in summer 2020, the periodic evolution of the western Pacific subtropical high is a key atmospheric circulation factor that led to the persistent extreme mei-yu season in 2020," said Prof. SUN Jianhua from the Institute of Atmospheric Physics of the Chinese Academy of Sciences, one of the co-organizers of the special issue.

Many of the scientists writing in the journal attributed this behavior to the Indian Ocean, which was much warmer than normal.

What's surprising about the 2020 summer was that it wasn't preceded by a major El Niño, the classic warming in the east Pacific which usually precedes the wettest summers in eastern Asia. However, a warming further west occurred in the central Pacific and rapidly cooled, which looks to have worked in tandem with the influence of the Indian Ocean on the behavior of the west Pacific anticyclone.

Seasonal prediction models appeared to have grasped the underlying effects of the ocean on the high pressure. "Despite great advances in recent decades, it is still a great challenge to accurately forecast extreme heavy precipitation events, including their spatial and temporal evolutions, coverage and intensity," said Prof. DONG Xiquan from the University of Arizona.

A contribution to the special issue by Dr. Clark and co-authors also highlighted how the rainfall levels seen in 2020 are something which society will have to prepare for in future years.

The studies and outcomes revealed in this special issue on the extreme rainfall in summer 2020 should contribute to a common understanding of extreme weather and rainfall during the Asian monsoon period over East Asia and the surrounding regions.

The cover of the special issue is a rough diagram of the circulation which occurred during the extraordinarily wet summer mei-yu of 2020 in eastern Asia. (Image by Advances in Atmospheric Sciences)