Relationships between amplified quasi-stationary waves and humid-heat extremes in the Yangtze River Delta region

High humidity combined with high temperature decreases the human body's capability to dissipate heat, causing serious health issues. While increased attention has been paid to changes in humid-heat extremes under the current climate and in the future, the causes of near-surface humid-heat extremes over the Yangtze River Delta region are still unclear. Here we investigate the relationships between quasi-stationary waves (QSWs), which are atmospheric Rossby waves with phase speed close to zero, and humid-heat extremes over the Yangtze River Delta region during the summer from 1959 to 2021. Additionally, the potential physical processes that link them are also explored. We find that the QSWs with wavenumber 1–8 present different vertical structures: Wave 1–2 are baroclinic, while Wave 3–8 are barotropic. Wave 1, 3, 5, 6, 7 and 8 show phase-locking behaviour. When the amplitudes of Wave 1, 3, 5 and 6 are higher, the frequency of humid-heat extremes near the surface at specific locations tends to be higher than the norm. The high-amplitude waves in different wavenumbers modulate near-surface temperature and/or humidity in various ways, ultimately resulting in anomalously intensive humid-heat extremes.