The tropical oceanic forcing on the occurrence of the heavy rain event of July 2020 in East Asia

Abstract:

In late-June to mid-July of 2020, eastern China, South Korea, and Japan suffered from continuous heavy rainfall and related natural disasters. The floods of the Yangtze River in China and Kumagawa River in Japan killed more than 140 and 60 people, respectively. Using the atmospheric reanalysis data, we identified that the widespread heavy rainfall and floods are associated with enhanced moisture transport bands called atmospheric rivers (ARs). ARs are major weather events causing flash floods and landslides over the west coasts of Europe and North America. We found that ARs also often cause heavy rainfall and induce huge socio-economic damages in East Asia. The enhanced AR activity in this year should be related to the sea surface temperature anomaly in the tropical Pacific and Indian Ocean. The warmed Indian Ocean in 2020 favored eastward propagating Kelvin waves and resultant anomalous anticyclone over the subtropical western North Pacific. The sustained subtropical anticyclone helps the continuous moist advection from the Indian Ocean and the South China Sea to eastern China, South Korea and Japan. In addition, the developing La Niña in the tropical Pacific also facilitate the southwesterly moist flow to East Asia via an anomalous subtropical anticyclone. The two teleconnection patterns found in this year are consistent with the dominant patterns related to the anomalous East-Asian AR activity obtained from the 60-yr historical data. Using ensemble model simulations, we also found that global warming increases the occurrence frequency of ARs over East Asia due to the increase of water vapor content. In addition to the thermodynamic effect, the warmed Indian Ocean under global warming also enhances the AR activity over mid-latitude East Asia. Based on outputs from downscaling experiments using a regional climate model, we confirmed that the global warming and the warmed Indian Ocean result in a great increase in the occurrence of extreme rainfall associated with ARs.