Abrupt climate changes observed in late August over central Japan between 1983 and 1984

Interdecadal climate changes occurring during the latter half of August (LA) in central Japan are described, and the associated changes in rainfall, typhoon tracks, and circulation patterns over East Asia and the western North Pacific (WNP) regions are investigated. Since 1984, rates of sunshine and temperature have increased, while rainfall has decreased significantly during LA in central Japan. In contrast, rainfall over the Southwest Islands, northern part of Taiwan, and in the south of the middle and lower reaches of the Yangtze River in China has increased. Changes in the positions and tracks of typhoons are responsible for these changes. Prior to 1983, many typhoons approached the central-western part of Japan during LA, while after 1984, most typhoons were deflected away from Japan and moved northwestward to Taiwan and China. The North Pacific subtropical high during LA extended more westward after 1984, which affected the interdecadal changes in sunshine, temperature, rainfall, and typhoon tracks, not only in central Japan, but also over East Asia and the WNP regions

Autumn rainfall increasing trend in south central Vietnam and its association with changes in Vietnam’s East Sea surface temperature

Certain parts of Southeast Asia, such as central Vietnam, experience heavy rainfall in the boreal autumn from September to December (SOND). The 52-year SOND rainfall over Vietnam from 1961 to 2012 shows increasing trends over the south central region (SR). Along the central coastal regions and SR, SOND rainfall as well as heavy rainfall indices, such as the number of heavy rainfall days, have increased significantly since the late 1980s and early 1990s. In contrast, a decreasing trend is observed in stations located north of 17°N. Tropical cyclone-induced rainfall exhibits an increasing trend over the SR. The increasing trend of SOND rainfall is associated with the recent sea surface temperature (SST) warming after the late 1980s over the South Vietnam East Sea (SVES). Owing to the recent SST warming and grand La Niña-like pattern after the 1990s, the SVES surface temperature has increased by 0.8–1.2 °C over the period 1961–2012, leading to enhanced moisture flux convergence over the SR. Moreover, the SVES warming strengthens the anomalous northeasterly winds that affect the SR. Consequently, SR has become more prone to deep convection and heavy rainfall events

Observation of daytime changes in boundary layer on a clear and weak-wind summer day in western suburban of Tokyo

Temperature, relative humidity, and wind conditions were observed on a clear, hot, summer day on 6 August 2019, 07:30–15:30 local time (LT), near the surface and in the boundary layer at the Minami-Osawa Campus of Tokyo Metropolitan University in the western suburb of Tokyo, Japan. Vertical wind profiles were obtained by pilot-balloon observations (PBOs) and unmanned aerial vehicle (UAV) observations. PBOs and surface wind observations revealed a southerly wind in the lower boundary layer from 11:00 LT, which became stronger during 12:30– 15:30 LT. Temperature plateaued at 11:00–13:00 LT near the surface, whereas a sharp peak was observed by the UAV in the surface boundary layer at 11:00 LT. Water-vapor mixing ratios were higher in the afternoon than in the morning, with heavy cloud cover after 14:30 LT. Comparison with PBO data indicated that wind-speed estimations based on UAV flight-attitude information are problematic

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