Recent years have witnessed an increase in the frequency and intensity of heatwaves across East Asia. These heatwaves are often associated with high-pressure systems, leading to decreased cloud cover and increased solar radiation at the surface. The western Pacific subtropical High (WPSH) and the upper-troposphere South Asia high (SAH) are crucial in heatwave formation over the Yangtze River Valley (YRV). The westward expansion of the WPSH often coincides with the eastward expansion of the SAH, a coupling possibly caused by SAH-enhanced divergence and zonal temperature gradients. Tropical sea surface temperature (SST) forcing, particularly El Niño-Southern Oscillation (ENSO), plays a significant role in modulating the WPSH. La Niña, in particular, can enhance the WPSH. Extratropical atmospheric teleconnections, such as the Scandinavian pattern and the circumglobal teleconnection (CGT) pattern, also impact East Asia's summer climate. The CGT, often excited by Indian summer monsoon (ISM) rainfall, can displace the East Asia westerly jet northward, promoting the northeastward extension of the SAH and westward expansion of the WPSH. In 2022, the YRV experienced record-breaking heatwaves, exceeding climatology by four standard deviations. The resulting drought affected millions and caused billions of dollars in economic losses. This study aims to investigate the causes of these unprecedented heatwaves.

Existing literature highlights the roles of various factors in YRV heatwaves, including the WPSH and SAH's impact on descending motion and increased solar radiation. Studies have shown the influence of tropical SST forcing, particularly ENSO, on the WPSH, with La Niña events often enhancing the high-pressure system. The Indo-western Pacific Ocean capacitor effect and atmosphere-ocean interactions have been identified as important mechanisms. Extratropical teleconnections, like the Scandinavian pattern, Silk Road pattern, and CGT pattern, also affect East Asian summer climate. The CGT, linked to ISM rainfall, plays a significant role in displacing the westerly jet and enhancing both the SAH and WPSH. While previous studies have explored these influences individually, the interaction of these factors in driving extreme events remains a subject of ongoing research.

This study employed a combination of observational analysis and numerical modeling. Observational data included daily 2-m air temperature from Chinese meteorological stations, atmospheric circulation data from ERA5 reanalysis, diabatic heating rates from JRA-55, rainfall data from GPCP, and SST data from ERSST. Heatwave days (HWDs) were defined as days when daily maximum temperature exceeded the 90th percentile based on a 5-day window. Statistical tools used included EOF analysis, SVD analysis, correlation analysis, regression analysis, and wave activity flux calculations. Numerical experiments were performed using the Community Atmospheric Model (CAM) version 6.0. Three sensitivity experiments were conducted: EXP_SST (forced by anomalous SST in the tropical western-central Pacific), EXP_Q (forced by anomalous diabatic heating over Pakistan and northwest India), and EXP_Q_SST (forced by both SST and diabatic heating anomalies). A 1000-year CESM2 pre-industrial control experiment was also conducted to assess the synchronization among CGT, La Niña, Pakistan rainfall, and YRV heatwaves in a coupled-modeling environment without anthropogenic forcing.

The 2022 YRV heatwaves were characterized by a barotropic high-pressure system, with positive 500-hPa geopotential height anomalies extending from the western North Pacific (WNP) to northern India. The WPSH showed a pronounced westward expansion, associated with a low-level WNP anticyclone. The SAH ridge extended eastward. While the 2022 triple-dip La Niña strengthened the zonal SST gradient in the equatorial western Pacific, suppressing convection and enhancing the WPSH, it alone was insufficient to explain the extreme heatwaves. The unprecedented Pakistan rainfall, exceeding historical records, played a crucial role. This rainfall promoted downstream Rossby wave trains, resulting in an enhanced CGT pattern, displacing the westerly jet and further extending the SAH eastward. The Pakistan rainfall and YRV SAT showed a strong correlation both interannually and intraseasonally. The relationship between Pakistan rainfall and YRV heatwaves remained significant even after removing the effect of ENSO. Numerical experiments using CAM6.0 confirmed the crucial role of Pakistan rainfall in triggering the intense CGT and enhancing the SAH and WPSH. La Niña contributed by strengthening the low-level WPSH. A 1000-year CESM2 pre-industrial control experiment reinforced the necessity of intense Pakistan rainfall for CGT formation and YRV heatwaves; La Niña alone was insufficient. The model simulations largely support the findings from observational data, though there are some minor discrepancies which may be attributed to model limitations such as northward displacement of the simulated subtropical westerly jet.

This study demonstrates that the unprecedented Pakistan floods played a leading role in the 2022 record-breaking YRV heatwaves, a finding beyond the traditional focus on La Niña's dominant role. The extreme westward expansion of the WPSH and the eastward stretch of the SAH were key factors. The triple-dip La Niña exhibited unusual intensification and westward movement, contributing to WPSH enhancement. The Pakistan rainfall, through diabatic heating, strengthened the CGT, and further enhanced the SAH and WPSH. Numerical simulations supported the leading role of the Pakistan rainfall and the contribution of the intensified La Niña. The study's findings provide critical insights into the complex interplay of factors contributing to extreme heat events in East Asia, improving the understanding and prediction of future events. Further investigation is needed to fully explain the extreme Pakistan rainfall, although a combination of potential factors is hypothesized, including positive feedbacks between rainfall and the CGT, the influence of North Atlantic Oscillation (NAO), and the role of atmospheric rivers originating from the Arabian Sea.

This study provides compelling evidence that unprecedented Pakistan floods, in conjunction with the triple-dip La Niña, were the primary drivers of the record-breaking 2022 YRV heatwaves. The findings highlight the need to consider the teleconnections between seemingly distant events when assessing extreme weather phenomena. Future research should focus on a more in-depth exploration of the mechanisms behind the extreme Pakistan rainfall, including the role of NAO and atmospheric rivers, and a detailed analysis of the similarities and differences in the causes and mechanisms of heatwaves over the Tibetan Plateau and YRV.

The study's reliance on reanalysis data might introduce uncertainties. The model simulations, while supportive, show some discrepancies with observations, potentially due to model limitations in representing the subtropical westerly jet. The focus on the 2022 event limits the generalizability of findings to other years or regions. Further research is needed to fully elucidate the complex interactions and understand the long-term trends and implications of these findings.