Researchers from China University of Geosciences (Wuhan) and the National University of Defense Technology have made new progress in understanding the response of the Western Pacific Subtropical High (WPSH) to anthropogenic climate change.
The WPSH is a vital component of the East Asian summer monsoon system. Variations in its intensity, coverage and position strongly modulate summer rainfall and temperature over East Asia. In recent years, a growing number of extreme precipitation and heatwave events have been linked to anomalous WPSH activities.
However, substantial uncertainty remains in previous studies assessing long-term changes in the WPSH under global warming. The canonical WPSH is defined using a fixed geopotential-height metric, which is widely used in operational practice and research but may produce a spurious strengthening trend of WPSH because warming tends to thicken the troposphere column. To solve the limitations of the canonical definition, various studies have developed alternative WPSH metrics, either thermodynamic or dynamic. They often yield inconsistent trends and spatial coverages of WPSH under climate change, without a consensus on the WPSH response to warming.
To address this issue, this study develops a new WPSH definition based on thermodynamic–dynamic synergy. The method uses the joint probability distribution frequency (JPDF) of geopotential height and vertical velocity to identify a dynamically constrained geopotential-height threshold. By linking the WPSH boundary to its characteristic descent motion, the new JPDF metric gets rid of the warming-induced mean increase in geopotential height while retaining the physical connection of the WPSH with weather anomalies. In essence, the JPDF metric does not simply track a high-pressure region or an anticyclonic circulation; instead, it identifies the WPSH’s main body characterized by organized subsidence that is physically tied to East Asian summer rainfall and temperature anomalies.
The results show that the JPDF metric captures the linkage of the WPSH to East Asian weather anomalies, including upward motion associated with rainbands, as well as subsidence associated with high-temperature anomalies. Various WPSH indices derived from JPDF, including those describing its covering area, intensity, western ridge point, northwestward extension and land-area coverage, all show stronger correlations with East Asian precipitation and temperature anomalies.
Using CMIP6 projection experiments, the study projects robust increases in WPSH’s area and intensity under future warming, accompanied by a northwestward extension of the WPSH boundary and increased coverage over East Asian land areas. These changes appear robust since they are supported by strong inter-model consensus, especially under medium- and high-emission scenarios.
Furthermore, the present study projects that record-breaking WPSH events will become more frequent, stronger and longer-lasting on the synoptic scales. Strong WPSH cases may extend farther toward inland East Asia, and become more likely to combine with large-scale high-pressure systems over Eurasia, increasing the risk of record persistent heatwaves and intense monsoon rainfall.
This work yields a physically consistent framework for diagnosing WPSH-related weather anomalies and assessing long-term WPSH changes under anthropogenic warming, thereby providing important evidence for future extreme-weather attribution, climate-model evaluation and climate-risk assessment in East Asia.
National Science Review