Image by Kholifur Rohman/Shutterstock

In August 2021, rain fronts stagnated around Japan, causing persistent heavy rainfall and suggesting the return of the rainy season. Researchers from the University of Tsukuba determined the co-variation mechanism caused by the joint action of remote tropical and extratropical influences. This groundbreaking achievement marks the genesis of subtropical climate dynamics and shows promise in improving the accuracy of seasonal forecasts.

Tsukuba, Japan—In Japan, the summer season is characterized by stagnant rain fronts, causing persistent heavy rainfall. This phenomenon is reportedly associated with global-scale atmospheric and oceanic anomalies. Remote influences from the tropical and extratropical regions have been identified as the main causes, respectively. However, the link between the two causes remains unclear.

Researchers from University of Tsukuba elucidated the mechanism of the heavy rainfall in August 2021, which caused record-breaking natural disasters over a wide area extending from the western to eastern part of Japan, through computer simulations and other methods.

During the heavy rainfall, three high/low-pressure systems, namely the Okhotsk high, trough over the Korean Peninsula, and northwestern Pacific subtropical high, were aligned in a north-south direction and intense. The researchers analyzed the observation data, reanalysis data, cutoff low index, and numerical simulation results. The results revealed that the three systems were linked by a process through which a small cutoff low was concentrated near the Korean Peninsula due to an upper-tropospheric blocking high and a near-surface atmospheric flow induced by an upper-tropospheric anticyclone. In addition, cumulus convection within the tropics enhanced the northwestern Pacific subtropical high. The researchers concluded that the joint action of these tropical and extratropical variations led to a concentration of water vapor and updrafts around Japan, resulting in the persistent heavy rainfall.

The study elucidated the formation and maintenance mechanisms of the northwestern Pacific subtropical high, which played an important role in water-vapor transport, with a focus on wave dynamics in the tropics and quasigeostrophic dynamics in the extratropics. These results herald the dawn of subtropical climate dynamics. Deep investigations into the factors that lead to such large-scale atmospheric flows may contribute to a better understanding of extreme weather events and improving the accuracy of seasonal forecasts.

###
This work was supported by JSPS KAKENHI (grant number 21H00626).

Original Paper

Title of original paper:

Coherent amplification of the Okhotsk high, Korean trough, and northwestern Pacific subtropical high during heavy rainfall over Japan in August 2021

Journal:

Progress in Earth and Planetary Science

DOI:

10.1186/s40645-023-00598-4

Correspondence

Professor UEDA Hiroaki
Institute of Life and Environmental Sciences, University of Tsukuba

Professor HONDA Meiji
Faculty of Science, Niigata University

Professor TAKAYA Koutarou
Department of Astrophysics and Atmospheric Sciences, Faculty of Science, Kyoto Sangyo University

Related Link

Institute of Life and Environmental Sciences