The recent wave of torrential rainfall and severe flooding has been driven by an unusual combination of atmospheric circulation patterns, including an intensifying subtropical high-pressure system, against a backdrop of global warming, meteorologists said.

Heavy rain has continued to batter large parts of the country during the early flood season. Since the start of the season on Apr 1, average national precipitation had reached 110.1 millimeters by May 22, about 18.6 percent above the seasonal norm, according to the National Climate Center.

Sun Mingyang, an engineer at the center, said extreme rainfall events have been widespread across both northern and southern China. A total of 76 national-level meteorological stations recorded daily precipitation that broke historical springtime records.

Among them, Enping in Guangdong province recorded 597.7 mm of rainfall in a single day, while Jingzhou in Hubei province, Shangyou in Jiangxi province and Guigang in Guangxi Zhuang autonomous region also exceeded historical daily rainfall records.

Southern China has been repeatedly hit by rainstorms, flash flooding and severe convective weather, including thunderstorms and strong winds, Sun said.

Meteorologists said the recent rainfall was mainly caused by the combined influence of several major atmospheric systems.

Gao Hui, chief forecaster at the National Climate Center, said the western Pacific subtropical high, a major weather system influencing East Asia, has remained unusually strong and positioned farther west and north than normal over the past week.

"The system's western flank has continuously transported warm and moist air from tropical oceans into inland China, helping fuel heavy rainfall," Gao said.

Since mid-May, the northern boundary of the subtropical high has extended to around or north of 27 degrees latitude, roughly corresponding to its typical position in June rather than May, he said.

The moisture transport associated with the system has already reached provinces including Hunan and Hubei. At the same time, active cold air masses moving southward from midlatitude regions have collided with warm and humid air, triggering intense precipitation, he added.

Forecasters also said the South China Sea summer monsoon is approaching its onset stage and is expected to set in soon.

Normally, within two weeks of the monsoon's onset, stronger southwesterly airflows carry abundant moisture from the tropical Indian Ocean and the South China Sea into East Asia, significantly increasing convective rainstorms south of the Yangtze River.

Sheng Jie, chief forecaster at the National Meteorological Center, said southwesterly warm and humid airflows have strengthened since mid-May, supplying eastern and central China with abundant moisture needed for extreme rainfall.

Meanwhile, long-term warming trends and the development of El Nino have created background conditions more conducive to extreme rainfall.

Chen Lijuan, chief forecaster at the National Climate Center, said sea surface temperatures in the central and eastern equatorial Pacific have continued to rise, with warming conditions associated with El Nino emerging in May.

"The tropical atmosphere has responded rapidly to the ocean warming, which favors strengthening of the western Pacific subtropical high and creates favorable moisture conditions for increased rainfall in southern China," Chen said.