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

The study addresses why and how Pakistan’s unprecedented precipitation and the Yangtze River Valley’s prolonged heatwaves co-occurred during July–August 2022. Against a backdrop of rapid global warming that intensifies the hydrological cycle and increases extremes, concurrent and compound events have become more frequent and impactful. The 2022 boreal summer produced exceptional flooding in Pakistan and record-breaking, long-lived heat and drought across the YRV, causing severe socio-economic damages. Prior research points to the role of anomalous zonal flow over the subtropical Tibetan Plateau, spring TP thermal anomalies, teleconnections including Rossby wave trains, and the triple-dip La Niña with an enhanced equatorial western Pacific SST gradient in shaping circulation anomalies such as the EAWJ shift, SAH and WPSH expansions. Yet, despite evidence for large-scale linkages, the co-occurrence mechanisms and multi-scale energetic interactions underlying the simultaneous extremes had not been quantitatively and systematically assessed. This study aims to quantify intra- and inter-scale energetic processes that set favorable circulation regimes for concurrent extremes in 2022 and to distinguish dynamical differences between July and August.

Previous work has highlighted: (1) Tibetan Plateau thermal anomalies modulating meridional winds, shifting the East Asian westerly jet, and promoting the eastward extension of the South Asian High and westward extension of the Western Pacific Subtropical High; (2) Rossby wave–like teleconnections linking remote regional extremes, including patterns connecting Pakistan flooding and East Asian heatwaves (e.g., 2010, 2020); (3) the 2022 triple-dip La Niña and a strong equatorial western Pacific SST gradient displacing tropical convection and diabatic heating, reinforcing downstream wave trains and extending SAH over East Asia; (4) the role of amplified mid-latitude planetary waves in concurrent extremes across hemispheres. These studies suggest the 2022 events likely arose from large-scale wave patterns and tropical-extratropical coupling, but lacked a formal, quantitative decomposition of cross-scale energy interactions. The present work fills this gap using multiscale energetics based on the multiscale window transform (MWT).

Data: Station daily Tmax (1961–2022) from China Meteorological Administration (508 YRV stations after QC); CPC daily precipitation (0.5°×0.5°); ERA5 reanalysis (6-hourly and monthly winds, omega, geopotential height, temperature at 12 levels from 1000–100 hPa; 0.25°×0.25°; 1978–2022). Analysis region generally 20°–35°N, 60°–120°E; vertical integration 850–200 hPa. Multiscale energetics: Employed the multiscale window transform (MWT) to decompose fields into three orthogonal temporal scale windows: basic (>64 days, window 0), intraseasonal (8–64 days, window 1), and synoptic (<8 days, window 2). Using Liang’s multiscale energetics formalism, derived window-specific kinetic energy (K^θ) and available potential energy (A^θ) budgets including canonical transfers Γ^θ that quantify inter-window energy exchanges, buoyancy conversion b^θ between APE and KE, in-window flux divergences of KE and APE (∇·Q^θ_K, ∇·Q^θ_A), pressure work, and residuals representing friction/diabatic effects. Energetic terms were computed locally and integrated vertically (850–200 hPa) and over the spatial domain to diagnose intra- and inter-scale processes during July and August 2022, including KE flux divergence/convergence patterns and canonical transfers among windows. Numerical experiments: Conducted atmosphere-only simulations with CESM2/CAM5.1 at 1.9°×2.5° with 37 vertical levels. Control run used prescribed climatological boundary conditions (40-year run; last 35 years analyzed). Sensitivity run applied July–August 2022 SST anomalies over the tropical western–central Pacific (15°S–15°N, 100°E–140°W) to assess SST-forced modulation of large-scale circulation, moisture fluxes, and their divergences. Diagnostics: Hovmöller analyses of Tmax, precipitation, 588 gpm isolines; canonical transfer fields from intraseasonal to synoptic windows; synoptic-window KE flux divergence; temporal evolution of domain-integrated KE and APE processes to delineate event phases.

- Event characterization: Pakistan experienced extraordinary precipitation in July–August 2022, with southern regions exceeding 10 mm day−1 above normal and multiple severe events leading to catastrophic flooding. The YRV endured an unprecedented 79-day heatwave since 1961, with daily Tmax ≥40 °C over ~1,029,000 km² and widespread drought lasting 20–30+ consecutive days. - July energetics and dynamics: The basic-flow window lost APE via canonical transfer to both intraseasonal and synoptic windows, indicating dominant inter-scale processes. Approximately 20% of total APE (~100.1×10^6 W) was converted to KE, largely driven by the basic flow (−91.03×10^6 W buoyancy conversion contribution), but a substantial portion of KE (~83.92×10^6 W) diverged through pressure work. Strengthened multiscale easterlies favored barotropic instability, amplifying Rossby wave amplitude and maintaining anomalous easterlies. Two intermittent concurrent extreme phases (early–late July) were initiated by explosive canonical APE transfers from the basic flow to intraseasonal and synoptic windows, indicative of baroclinic instability triggering heavy rainfall, while maintenance was aided by barotropic processes. - August energetics and dynamics: APE canonical transfer persisted but weakened (~10% of total APE converted). Positive APE influx emerged in the intraseasonal window, consistent with regional warming and as a driver for intermittent Pakistan precipitation. Crucially, synoptic-scale KE exhibited notable convergence, especially over southern Pakistan, supplying dynamical support for persistent subsidence over the YRV and moisture/energy advection toward Pakistan. KE convergence displayed three westward-shifting episodes that matched three westward-moving extreme precipitation events in Pakistan. Thus, August extremes were amplified by high-frequency (synoptic) intrascale energy in addition to inter-scale interactions, making August generally more extreme than July. - Inter-scale interaction and timing: July 3–28 featured baroclinic-triggered onsets with westward WPSH extensions and continuous Pakistan rainfall; however, in-window APE and KE fluxes often diverged, limiting event facilitation. In August, high-frequency continuous westward expansion of the WPSH coincided with synoptic KE convergence that sustained synoptic systems over both regions and aligned with precipitation extremes’ westward propagation. - SST-forced circulation: CAM5.1 experiments with tropical western–central Pacific SST anomalies reproduced a mid-latitude wave train originating near the eastern North Atlantic, traversing Eurasia to the Indian subcontinent and East Asia, linking Pakistan precipitation and YRV heatwaves. Simulations showed altered 500-hPa heights, 200-hPa winds, and vertically integrated moisture fluxes/divergence, supporting the role of tropical SST forcing in energizing the observed wave train and moisture transport. - Mechanistic pathway: Strong convection over Pakistan, aided by blocking and easterly anomalies and interaction of cold–dry high-latitude air with warm–moist monsoonal flow, excited upper-tropospheric divergence and negative Rossby waves, producing anticyclones over Iran and eastern Asia. The eastern anticyclone exhibited quasi-barotropic structure and anchored a downstream wave response over East Asia, enhancing YRV heat and dryness. Triple-dip La Niña contributed to an easterly anomaly and strengthened southerlies from the Arabian Sea, paralleling but exceeding mechanisms noted in 2010.

The findings quantitatively demonstrate that the 2022 concurrent extremes resulted from coupled multi-scale dynamics. In July, large inter-scale APE transfers from the basic flow energized intraseasonal and synoptic windows and, together with strengthened multiscale easterlies, fostered barotropic instability that maintained circulation anomalies, while baroclinic instability episodes initiated heavy Pakistan rainfall and concurrent YRV heat. In August, although low-frequency processes remained important, high-frequency synoptic processes became decisive: strong synoptic KE convergence sustained regional synoptic systems, aligned temporally and spatially with three westward-propagating precipitation episodes in Pakistan, and reinforced persistent heat over the YRV. The CAM sensitivity experiments attribute part of the circulation pattern to tropical western–central Pacific SST anomalies that helped excite and maintain a Eurasian wave train, modulate upper-level jets and highs (SAH, WPSH), and organize moisture fluxes. Overall, the multiscale energetics framework links external forcing (e.g., La Niña-related SST and tropical heating) with internal atmospheric instabilities (baroclinic and barotropic), clarifying how cross-scale transfers shape concurrent extremes and addressing the initial hypothesis that large-scale wave dynamics and multi-frequency interactions underpin the co-occurrence.

This study applies a multiscale window transform-based canonical transfer framework to the July–August 2022 concurrent Pakistan precipitation extremes and Yangtze River Valley mega-heatwaves, quantitatively resolving intra- and inter-scale energy interactions. The results show that July’s events were initiated by baroclinic instability via APE transfers from the basic flow and maintained by barotropic instability, whereas August’s more extreme events were critically reinforced by synoptic-scale KE convergence coupled with ongoing low-frequency interactions. CAM-based sensitivity experiments implicate tropical western–central Pacific SST anomalies in exciting a Eurasian wave train that links Pakistan and East Asia, and the analysis highlights a key heat source in the tropical western–central Pacific as a driver of localized energy bursts. The work emphasizes how external forcing interacts with internal atmospheric instabilities to produce concurrent extremes and provides a quantitative template to diagnose similar events under warming.