Mechanisms behind the rapid rise of extreme heat discomfort days in South China

Extreme heat discomfort days (EHDDs) are increasing in frequency and intensity under global warming, raising growing concerns in subtropical regions such as South China. Using multi-decadal observational data, we identify a marked increase in the frequency and spatial extent of EHDDs since 2000, following an absence of significant trends in earlier decades. A convolutional neural network trained on sea level pressure and 500-hPa geopotential height fields successfully detects 92% of these events and reveals their dominant circulation features. The northwestward extension of the western North Pacific subtropical high induces an anomalous anticyclone over South China, weakening summer monsoon winds and suppressing ventilation. In the presence of high humidity, these stagnant and hot conditions exacerbate heat discomfort. Concurrently, a post-2000 strengthening of mid-latitude blocking high activity reinforces high-pressure anomalies over East Asia. The joint influence of intensified blocking highs and northwestward extension of the western North Pacific subtropical high drives the sharp rise in both frequency and intensity of regional EHDDs. These findings highlight the importance of large-scale circulation changes, in combination with long-term global warming, in amplifying extreme thermal stress under a warming climate.