Influences of Quasi-Biweekly Oscillation on the Southeastern China Heat Waves during Different ENSO Episodes

Abstract Heat waves pose severe threats to human health and socioeconomic systems, and accurate short–medium-range forecasting is crucial for implementing effective mitigation strategies. During summer, the heat waves in southeastern China are significantly modulated by the 10–30-day quasi-biweekly oscillations (QBWOs) from the tropical northwestern Pacific. Accompanying the stepwise northwestward evolution of the QBWO dry phase, the heat wave day frequency (HWDF) in South China (SC) and middle reaches of the Yangtze River (MYR) increases by nearly 40% in phases 7–8 and 1–2, respectively. However, the above QBWO–HWDF relationship changes dramatically and exhibits distinction between the MYR and the SC in an El Niño–Southern Oscillation (ENSO) cycle. In the MYR (SC), the QBWO induces a larger HWDF perturbation during El Niño developing (decaying) summers which is more than twice that during El Niño decaying (developing) summers in phases 1–2 (7–8). This difference regarding El Niño evolution is primarily due to the change of QBWO temperature anomalies in the MYR, while in the SC, it is attributed to changes in both QBWO temperature anomalies and summer mean temperature. Further examination indicates that the QBWO change between El Niño’s developing and decaying summers is likely related to the variation of mean vertical wind shear, which behaves differently between the MYR and the SC. The analysis of the HWDF difference between La Niña’s decaying and developing summers is similar to that between El Niño’s developing and decaying summers. The observed QBWO–HWDF relationship, particularly the varying quantitative statistics across different episodes of an ENSO cycle, establishes a benchmark for future operational heatwave prediction in southeastern China. Significance Statement Accurate forecasting of heat waves within short-to-medium ranges is vital for human health and socioeconomic systems. This study endeavors to conduct a quantitative statistical analysis of the relationship between atmospheric quasi-biweekly oscillations and heat waves in southeastern China, along with their year-to-year variations within an El Niño–Southern Oscillation cycle. Our results indicate a pronounced increase in heat waves in southeastern China during the dry phase of quasi-biweekly oscillations. Moreover, their quantitative relationship varies remarkably and exhibits regional distinctions within an El Niño–Southern Oscillation cycle. The year-to-year variations are attributed to changes in both quasi-biweekly oscillations and summer mean temperatures. Our findings hold the potential to enhance operational heat wave forecasting in southeastern China through the monitoring and prediction of tropical quasi-biweekly oscillations.