Multiscale interaction underlying 2022 concurrent extreme precipitation in Pakistan and heatwave in Yangtze River Valley

The Earth's rising temperature is disrupting the atmospheric energy budget, intensifying the water cycle and leading to increased extreme precipitation and heatwaves. Spatiotemporal connections between extreme events are also strengthening, resulting in concurrent or consecutive events that severely impact the environment. The boreal summer of 2022 witnessed unprecedented concurrent extremes in South and East Asia, with Pakistan experiencing devastating floods and the Yangtze River Valley suffering a mega heatwave. These events caused widespread damage and underscore the need to understand their physical mechanisms. Previous research has pointed towards anomalous zonal flow over the Tibetan Plateau, spring thermal conditions, and the northward migration of the East Asian westerly jet as contributing factors. Atmospheric teleconnections and the triple-dip La Niña phenomenon of 2022 have also been implicated. However, a systematic quantification of the co-occurrence mechanisms of these extremes was lacking. This study aims to address this gap by utilizing advanced techniques to analyze the multiscale interactions and driving mechanisms behind the 2022 concurrent extreme events in Pakistan and the Yangtze River Valley.

Existing literature highlights the increasing frequency and intensity of extreme weather events globally, including extreme precipitation and heatwaves. Studies have explored the individual mechanisms driving these events, such as anomalous zonal flow over the Tibetan Plateau, the influence of spring thermal conditions on meridional winds, and the role of atmospheric teleconnections like Rossby waves. The impact of the triple-dip La Niña phenomenon of 2022 and its connection to the concurrent events in Pakistan and the Yangtze River Valley has also been investigated. However, there's a lack of comprehensive studies that quantify the multiscale interactions and the interplay between different scales of atmospheric dynamics in driving such concurrent extreme events. This study aims to fill this gap by providing a quantitative analysis of the inter-scale and intra-scale interactions.

This study employed the advanced multiscale window transform (MWT), a scale-window-based method localized in space and time, to analyze the probability of simultaneous extreme precipitation in Pakistan and mega heatwaves in the Yangtze River Valley. The MWT decomposes the function space into windows of different scales (basic flow, intraseasonal, and synoptic). Multiscale energetics analysis, based on MWT, was used to assess the role of atmospheric circulation in creating favorable conditions for extreme events. The analysis focused on the kinetic energy (KE) and available potential energy (APE) within each scale window and the canonical transfers of energy between scales. The study also examined the daily variation of domain-averaged precipitation in Pakistan and temperature in the Yangtze River Valley. Atmospheric conditions, including the Western Pacific Subtropical High (WPSH), were examined. The study also used numerical experiments with the Community Atmospheric Model version 6.0 (CAM5.1) to investigate the influence of sea surface temperature (SST) anomalies in the tropical western-central Pacific on large-scale circulation. The datasets used included daily maximum temperature data from the National Meteorological Information Center of the China Meteorological Administration, daily precipitation data from the Climate Prediction Center (CPC) of NOAA, and ERA5 reanalysis data from the European Centre for Medium-Range Weather Forecasts. The multiscale KE and APE equations were used to analyze energy transfers and conversions between different scales.

The study found that in July 2022, the basic flow scale window lost APE, which was transferred to the intraseasonal and synoptic scales. Inter-scale dynamic processes and barotropic instability of the basic flow scale maintained the concurrent extremes. In August, the eruptive synoptic-scale kinetic energy convergence provided dynamic conditions for sinking motion in the YRV and advection to PAK from the Indian Ocean. The interaction between high- and low-frequency processes drove atmospheric circulation in the summer, but the high-frequency process in August was vital for the extreme events. The tropical western-central Pacific heat source was identified as a key driver for localized repetitive bursts of energy. Analysis of multiscale energy cycles showed that in July, the basic flow window mainly lost APE through canonical transfers to both the intraseasonal and synoptic scales. A large amount of APE was converted to KE, primarily driven by the basic flow scale. The intensified zonal easterlies contributed to barotropic instability, amplifying the Rossby wave and enhancing low-frequency easterlies. In August, the APE canonical transfer persisted but with lower intensity. A positive APE influx in the intraseasonal-scale window and the positive APE canonical transfer from the intraseasonal to the synoptic scale underscored the dynamics of low-frequency barotropic instability. Notably, a notable influx of KE within the synoptic-scale window was observed, with intensified convergence in southern PAK, corresponding to extreme precipitation. The numerical experiments showed that the mid-latitude wave train originates from the eastern North Atlantic, traversing Eurasia, and extending toward the Indian subcontinent and Eastern Asia, linking extreme precipitation in PAK and mega-heatwaves in the YRV. In July, two intermittent concurrent extreme events were primarily driven by baroclinic instability from the basic flow scale window, linked with the westward shift of the WPSH. In August, the WPSH showed a high-frequency westward expansion, and extreme precipitation in PAK showed three westward-moving processes linked to APE canonical transfer and KE flux convergence.

The findings demonstrate the critical role of multiscale interactions in driving the concurrent extreme events in Pakistan and the Yangtze River Valley in 2022. The study highlights the importance of considering both low-frequency and high-frequency processes, as well as the interplay between different scales of atmospheric dynamics. The significant role of the tropical western-central Pacific heat source underscores the influence of remote forcing on regional extreme events. The results contribute to a better understanding of the complex mechanisms governing the occurrence of concurrent extreme events and have implications for improving climate prediction and mitigation strategies. The observed differences in dynamic processes between July and August highlight the non-stationary nature of these events and the need for considering temporal variations in future studies. The study’s quantitative analysis of multiscale interactions advances our understanding of the mechanisms driving such events, which is crucial for developing more effective prediction and mitigation strategies.

This study successfully quantified the multiscale interactions and driving mechanisms behind the unprecedented concurrent extreme events in Pakistan and the Yangtze River Valley in 2022. The findings highlight the crucial role of both low- and high-frequency processes and the interplay between different scales of atmospheric dynamics. Future research should focus on improving the representation of multiscale interactions in climate models to better predict and mitigate such events. Further investigation into the role of remote forcing and the specific interactions between different atmospheric components is also warranted.

The study relies on reanalysis data and model simulations, which have inherent limitations and uncertainties. While the MWT provides a powerful tool for multiscale analysis, the specific choice of scale windows might influence the results. The study primarily focuses on the atmospheric dynamics and does not incorporate other factors, such as land surface processes or human activities, which may also contribute to the extreme events. Further research could integrate these factors to develop a more complete understanding of the event.