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

Abstract

In August 2021, rain front stagnation in Japan resulted in prolonged and disastrous rainfall across the entire country. During the heavy rainfall period, the large-scale atmospheric field over the East Asian–western North Pacific region was characterized by meridional tripolar circulation anomalies: the Okhotsk high (OH), the trough over the Korean Peninsula (Korean trough), and the northwestern Pacific subtropical high (NWPSH). Simultaneously, tropical convective activity was enhanced over the eastern Indian Ocean and suppressed over the tropical western–central Pacific. This study investigates the dynamic mechanism of linkage of the extratropical tripolar anomalies and the effects of tropical convective modulation using a reanalysis dataset, a cutoff low detection scheme, the potential vorticity inversion method, and numerical experiments. Upper-tropospheric blocking over eastern Siberia connected to the surface OH is conducive to the stagnation of synoptic depressions, including cutoff lows and troughs, over the Korean Peninsula, contributing to the development and maintenance of the quasi-stationary Korean trough. Rossby waves emanating from the Korean trough excite an anticyclonic anomaly over the northwestern Pacific. This upper-level anomalous anticyclone acts to enhance the surface NWPSH through zonal heat transport, accompanied by a northward tilting structure with height. Simultaneously, the tropical intraseasonal oscillation is amplified over the Indo–western Pacific Ocean sector under the negative-phase Indian Ocean dipole and multi-year La Niña conditions. The combination of enhanced convection over the eastern Indian Ocean and suppressed convection across the tropical western–central Pacific reinforces the NWPSH. The anomalous circulation associated with the extratropical tripolar pattern and concurrent tropical heat forcing causes more moisture transport, convergence, and anomalous ascent, which contribute to heavy rainfall in Japan. These results suggest that the dynamically correlated amplification of tropical and extratropical circulation anomalies plays a crucial role in precipitation variability in East Asia.