Unconventional cold vortex as precursor to historic early summer heatwaves in North China 2023

Heatwaves, characterized by prolonged periods of exceptionally high temperatures, pose a significant global concern due to their increasing frequency and intensity. These events severely impact ecosystems, public health, and societal well-being. Understanding the mechanisms driving heatwave formation and evolution is crucial for mitigation and preparedness. Heatwave occurrences involve complex interactions across various spatial and temporal scales. Long-term trends are primarily influenced by global greenhouse gas concentrations and regional anthropogenic effects, with urbanization significantly exacerbating urban heat island effects. Interdecadal variations are linked to large-scale climate modes like the Pacific Decadal Oscillation and Atlantic Multidecadal Oscillation, as well as Arctic sea ice loss. Interannual variations are modulated by ENSO, Indian Ocean sea surface temperature anomalies (SSTAs), soil moisture, and surface vegetation changes. Subseasonal influences include intraseasonal oscillations within monsoon systems, the North Atlantic Oscillation (NAO), and the Madden-Julian Oscillation. In boreal summer, extratropical heatwaves are often associated with quasi-stationary high-pressure systems or "heat domes", characterized by atmospheric blockings, jet stream meandering, and Rossby wave breaking. These can arise from internal atmospheric dynamics or local and remote diabatic heating anomalies. The summer of 2023 witnessed historic heatwaves in North China, exceeding 1.5°C above seasonal mean surface air temperature (SAT) anomalies with unprecedented intensity and duration. These heatwaves, occurring in early summer, deviated from typical "heat dome" patterns, prompting this study to investigate the underlying mechanisms, focusing on the unusual atmospheric circulation and the intricate interplay between atmospheric conditions and heatwaves on the subseasonal timescale.

Existing research extensively explores the multifaceted drivers of heatwaves, examining their long-term trends related to greenhouse gas increases and regional human activities (e.g., urbanization's urban heat island effect). Studies have also linked interdecadal heatwave variability to major climate modes like the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO), and to the decline of Arctic sea ice. The influence of interannual variability linked to El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD), soil moisture, and vegetation changes has been extensively documented. Subseasonal factors, including intraseasonal oscillations (ISOs), the NAO, and the Madden-Julian Oscillation (MJO), are known to impact heatwave patterns across various regions. Concerning atmospheric circulation, extratropical heatwaves are often linked to "heat dome" patterns, involving high-pressure systems, atmospheric blocking, jet stream meandering, Rossby wave breaking, and disruptions in westerly winds. These patterns can result from internal atmospheric dynamics or local/remote diabatic heating anomalies, with influences from ENSO events noted in various studies. However, the 2023 North China heatwaves differed from these typical patterns, necessitating a novel investigation.

The study employed a combination of observational data and numerical modeling to analyze the 2023 North China heatwaves. In-situ observational records from 334 stations in North China (36°-43°N, 113°-122°E) provided data on 2-m air temperature, rainfall, surface temperature, and 10-m wind speed. The ERA-5 reanalysis dataset offered high-resolution atmospheric circulation and thermal structure information. NASA Global Precipitation Measurement (GPM) data provided daily global precipitation, and NOAA's High-resolution Blended Analysis of Daily SSTs provided sea surface temperature data. The period covered 1981-2023 (with GPM and SST data starting later). Heatwave days were defined as days exceeding the 90th percentile of daily mean SAT (1991-2020). Regional heatwave days were identified when over 30% of stations met the criteria. The ECMWF real-time subseasonal-to-seasonal (S2S) forecasts (CY47R2), with 51 ensemble members and twice-weekly runs, were used for subseasonal predictability assessment. Surface sensible heat flux (SH) was calculated using the bulk aerodynamic method. Potential vorticity (PV) and wave activity flux were calculated using standard formulas. Anomalies were calculated as deviations from the 1991-2020 climatology. Numerical experiments using a Linear Baroclinic Model (LBM) were conducted to assess tropical-extratropical teleconnections by simulating diabatic heating or cooling in specific regions based on observations. Statistical analyses, including Pearson correlation and linear regression, were applied to evaluate relationships between variables.

The study revealed a unique sequence of events leading to the 2023 heatwaves: 1. **Unconventional Cold Vortex:** An upper-tropospheric cold vortex formed over North China in early June, contrasting with typical "heat dome" patterns associated with heatwaves. 2. **Suppressed Indian Summer Monsoon:** The zonal sea surface temperature anomaly (SSTA) gradient in the Indo-Pacific warm pool suppressed Indian summer monsoon convection. 3. **Tropical-Extratropical Teleconnection:** The suppressed monsoon triggered a tropical-extratropical teleconnection, resulting in the cold vortex over North China. 4. **Air-Land Interaction:** The cold vortex cooled the troposphere, increasing air-land thermal contrast and enhancing sensible heating. Reduced rainfall led to drier soil, further augmenting sensible heating and surface temperature. 5. **Heatwave Development:** The enhanced sensible heating led to the extraordinary heatwaves from mid-June to July, advancing the seasonal peak by a month. 6. **Cold Vortex Dissipation:** The subsequent lower-level warming destabilized the air column, eventually mitigating the upper-level cold vortex. 7. **ECMWF S2S Forecasts:** The ECMWF real-time S2S forecasts successfully captured the air-land feedback mechanisms during both the cold vortex and heatwave stages, although they underestimated the heatwave intensity due to biases in soil moisture anomaly representation. 8. **Statistical Significance:** Significant negative correlations were found between the North China Vortex Index (NCVI) and Indian summer monsoon rainfall, and positive correlations between NCVI during the cold vortex stage and SAT during the heatwave stage in both S2S forecasts and historical data (1981-2023). 9. **Zonal SSTA Gradient:** The zonal SSTA gradient in the Indo-Pacific warm pool showed strong correlation with ISM rainfall deficiency in the cold vortex stage. 10. **El Niño's Role:** While El Niño development coincided with the events, its direct contribution was less significant compared to the influence of the zonal SSTA gradient. The study highlighted a distinctive "bottom-up" warming process, in contrast to the "top-down" mechanism of conventional heat domes, leading to more intense heatwaves. Composite analysis showed higher SAT anomalies in years with preceding cold vortices compared to those with upper-level heat dome patterns.

The findings highlight a previously less understood mechanism for heatwave formation in North China, characterized by an initial cold vortex followed by intense warming due to air-land interactions. This contrasts with the more commonly studied "heat dome" scenario. The study's success in predicting the heatwave using ECMWF S2S forecasts, despite underestimation of intensity, indicates the potential for subseasonal forecasting improvements by addressing biases in soil moisture representation. The strong connection between the zonal SSTA gradient in the Indo-Pacific warm pool, suppressed Indian monsoon, and the subsequent cold vortex underscores the importance of tropical-extratropical teleconnections in shaping extratropical weather events. Future research should explore the generality of this "cold vortex-heatwave" mechanism in other extratropical regions and investigate how its prevalence might change with continued global warming. The identified biases in the S2S forecasts also provide avenues for model improvement.

This study revealed a novel mechanism for extreme early summer heatwaves in North China, characterized by an initial upper-tropospheric cold vortex acting as a precursor. The process involves a complex interplay between tropical-extratropical teleconnections, suppressed monsoon rainfall, air-land interactions, and soil moisture feedback. The findings demonstrate subseasonal predictability potential using the ECMWF S2S forecasts, although improvements are needed in representing soil moisture. Future research should explore the generality of this mechanism and investigate its potential changes in a warming climate.

The study's reliance on a single year's data limits the generalizability of findings, necessitating further research across multiple years to confirm the robustness of the identified mechanism. The ECMWF S2S forecast model's underestimation of heatwave intensity, due to biases in soil moisture, points to a need for more accurate representation of land-atmosphere interactions in future models. The focus on North China may limit direct applicability to other regions, although the broader implications for tropical-extratropical teleconnections are likely relevant elsewhere.