Abstract:The western Pacific subtropical high (WPSH) is a key large-scale circulation system of the East Asian summer monsoon and exerts a strong influence on summer climate and extreme weather events over East Asia,including severe flooding and prolonged heatwaves. The seasonal evolution of the WPSH is characterized by distinct northward jumps separated by quasi-stationary stages. El Niño,a dominant mode of interannual variability in the tropical Pacific,plays an important role in modulating the WPSH,particularly during the summer following its peak,mainly through the generation of a western North Pacific anomalous anticyclone (WNPAAC). Therefore,understanding the subseasonal variability of the WPSH and its meridional displacement during quasi-stationary periods in El Niño decaying years is important for improving seasonal climate prediction and disaster mitigation.
This study systematically investigates the north-south displacement of the WPSH during its early-summer quasi-stationary stage (defined as the period between the first and second northward jumps) in El Niño decaying years. The associated dynamical mechanisms and their impacts on precipitation anomalies over China are also examined. Daily ERA5 reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF),including geopotential height,wind,precipitation,and sea surface temperature (SST),are analyzed for the period 1958—2020. Twenty El Niño events are identified using the Niño3.4 index. To quantify the meridional position of the WPSH,a ridge line index is constructed. A Butterworth filter is applied to remove decadal-scale variability and isolate higher-frequency signals. Based on a ±0.5 standard deviation threshold of the filtered ridge line index,El Niño decaying years are classified into a northward-shifted group (6 years) and a southward-shifted group (9 years). Composite analysis and the Gill-Matsuno theoretical framework are then used to diagnose circulation differences and the dynamical forcing associated with SST anomalies.
The results reveal pronounced differences in atmospheric circulation,precipitation patterns,and SST forcing between the two groups. In the northward-shifted group,the WPSH is stronger than the climatological mean and exhibits a marked westward extension and northward displacement. This pattern is associated with a strong and extensive WNPAAC in the lower and middle troposphere that extends north of 35°N. As a result,enhanced moisture transport toward higher latitudes leads to above-normal precipitation north of the Yangtze River and below-normal precipitation over South China. In contrast,in the southward-shifted group,the WPSH is weaker and located farther southwest. The WNPAAC is confined to lower latitudes and is accompanied by a Pacific-Japan (P-J) or East Asia-Pacific (EAP) teleconnection pattern featuring a cyclonic anomaly over the Sea of Japan. This configuration limits northward moisture transport,resulting in increased precipitation south of the Yangtze River and reduced rainfall to the north.
Further analysis indicates that these differences are closely related to distinct patterns of tropical SST anomalies. In the northward-shifted group,significant warming in the Tropical North Atlantic (TNA) plays a dominant role by suppressing convection over the Pacific and inducing a broad WNPAAC that extends northward. In contrast,in the southward-shifted group,pronounced warming in the North Indian Ocean (NIO) excites an eastward-propagating Kelvin wave,producing low-latitude easterly anomalies and suppressing convection in the tropical western Pacific. This process generates a more southward-confined WNPAAC and consequently maintains the WPSH in a more southern position.
Overall,the meridional displacement of the WPSH during the early-summer quasi-stationary stage in El Niño decaying years is primarily controlled by the latitudinal position of the WNPAAC. The contrasting influences of TNA warming and NIO warming highlight the important role of tropical air-sea interactions in shaping the SST-convection-circulation linkage that modulates subseasonal variability of the East Asian summer monsoon. These findings improve our understanding of the mechanisms governing WPSH variability and may contribute to better prediction of East Asian summer precipitation. Future work should examine the meridional displacement of the WPSH during the subsequent quasi-stationary stage following the second northward jump,which corresponds to the North China rainy season,to further improve the predictability of summer climate extremes.
