Southern (low-latitude) East Asia has experienced a pronounced decline in March–April rainfall over recent decades while trends at mid-latitudes are weak, forming a latitudinal dipole that Ryu & Min (2022) attributed synergistically to greenhouse warming (~34%, via an anomalous anticyclone over the East China Sea and northwestward extension of the western North Pacific subtropical high, the semi-permanent high-pressure system that steers moisture transport toward East Asia) and anthropogenic aerosols (~17%). However, ERA5 vertical motion at 600 hPa does not clearly exhibit the overturning trends this thermal-circulation pathway requires, and a series of later studies has proposed additional or competing mechanisms — heat-induced overturning reinforced by Philippine Sea tropical convection, Tibetan-Plateau-deflected subtropical westerlies, poleward-shifting westerlies, reduced extratropical cyclone activity, and Pacific sea-surface-temperature thermal-contrast changes — without reconciling their relative contributions. The open problem is twofold and linked: quantitatively disentangle the competing mechanisms behind the observed spring rainfall dipole (including the role of internal decadal variability and the missing vertical-motion signature), and determine how the combination of continued greenhouse warming and projected near-future aerosol emission reductions will reshape the spring dipole and modulate drought and flood risk across East Asia.

Background

Each boreal spring (March–April–May), before the Asian summer monsoon onset, sustained rain — the so-called spring persistent rains, falling mainly from March to early May — occurs over southern China and subtropical East Asia and is a key water resource for the region. Over recent decades, observations and the ERA5 reanalysis (the European Centre for Medium-Range Weather Forecasts' fifth-generation global reanalysis, which assimilates observations into a numerical weather model to produce a physically consistent record of the atmosphere since 1979) show a marked downward trend of March–April rainfall over low-latitude East Asia, especially southern China, while rainfall trends at mid-latitudes are weak — a north–south 'latitudinal dipole' of spring rainfall trends.

Ryu and Min (2022) quantified the human influence on this dipole using the WRF-Chem regional model (the Weather Research and Forecasting model coupled with interactive atmospheric chemistry, allowing aerosol emissions to affect radiation, clouds, and precipitation) together with CMIP6 DAMIP simulations (the Detection and Attribution Model Intercomparison Project of the Coupled Model Intercomparison Project Phase 6, whose single-forcing experiments isolate the responses to greenhouse gases, anthropogenic aerosols, and natural forcing). They concluded that greenhouse warming heats high-latitude East Asia faster than the western North Pacific — partly through reduced spring snow cover — inducing an anomalous anticyclone over the East China Sea and a northwestward extension of the western North Pacific subtropical high (the semi-permanent high-pressure system governing summer-era moisture transport toward East Asia), which suppresses rainfall at low latitudes and mildly increases it at mid-latitudes. Anthropogenic aerosols (airborne particulate pollution that cools the surface and stabilizes the atmosphere by scattering and absorbing sunlight and by modifying clouds) reduce rainfall at both latitudes. The two forcings therefore act synergistically on the south, with greenhouse gases contributing roughly 34% and aerosols roughly 17% of the low-latitude spring drying, while their effects offset at mid-latitudes. In their analysis, low-latitude East Asia is southern China (22–28°N, 110–123°E), mid-latitudes are 30–35°N, 112–122°E, trends of March–April mean daily rainfall are computed over 1979–2020 from ERA5 and GPCP data, and the quoted contributions are percentages of the low-latitude March–April rainfall reduction. Several of the later studies cited below analyse different trend windows (for example February–April, or 1979–2023), so their quoted contributions are not directly comparable to this March–April, 1979–2020 baseline.

Two caveats motivated the open question. First, although ERA5 horizontal winds at 850 hPa show the expected northeasterly trends over southern China and the East China Sea, vertical motion at 600 hPa does not clearly show the ascent/descent pattern that the proposed anomalous thermal circulation (spanning Mongolia to the East China Sea) would require — suggesting that other mechanisms interfere. The leading alternative, from Li et al. (2016), is intensified springtime deep convection over the South China Sea and Philippine Sea, whose enhanced latent heat release drives a heat-induced anomalous overturning circulation that weakens moisture-flux convergence into southern China and dries it; what intensifies that convection was itself left unexplained. Second, because East Asian temperatures will keep rising while anthropogenic aerosol emissions are projected to decline sharply in the coming decades, the two anthropogenic forcings will pull the spring hydrological cycle in changing, partially opposing directions.

Since 2022 the mechanism literature has proliferated rather than converged: proposed drivers now also include changes in the zonal sea-surface-temperature contrast across the Pacific (Li et al. 2022, DOI: 10.1175/JCLI-D-21-0891.1), a weakened western Pacific teleconnection pattern — a recurring large-scale atmospheric pressure-anomaly pattern over the western North Pacific — north of 26°N (Chen et al. 2023, DOI: 10.1002/joc.8090), weakened subtropical westerlies deflected by the Tibetan Plateau (Zeng et al. 2024, DOI: 10.1029/2024GL109795), dynamic moisture divergence driven by Indo-Pacific warm-pool warming including natural Pacific decadal variability (Kim et al. 2024, DOI: 10.1088/1748-9326/ad8245), poleward-shifting westerlies whose weakened southern branch over the Tibetan Plateau may account for more than half of the February–April drying (He 2025, DOI: 10.1175/JCLI-D-24-0177.1), and reduced spring extratropical cyclone activity (Chen et al. 2026, DOI: 10.1029/2025JD045731). Meanwhile, CMIP6 HighResMIP (the High Resolution Model Intercomparison Project) evaluations show that most current coupled models fail to reproduce the observed spring drying trend (Zi et al. 2026, DOI: 10.1007/s00382-026-08151-x), single-forcing attribution exists for summer but not spring rainfall over southern China (Xu et al. 2024, DOI: 10.1029/2023JD040331), and future-projection work has so far addressed the annual dry–wet contrast reversal driven by aerosol reductions (Zhao et al. 2025, DOI: 10.1029/2024GL114527) and April–June extreme precipitation at global warming levels (Sheng et al. 2025, DOI: 10.1007/s00382-025-07644-5) rather than the spring dipole itself.

Problem Statement

Two linked questions arise from Ryu & Min (2022). (1) Mechanism disentanglement for the observed spring drying: the ERA5 850-hPa wind trends over southern China and the East China Sea are consistent with the proposed greenhouse-warming-induced anomalous anticyclonic (thermal) circulation spanning Mongolia to the East China Sea, yet the ERA5 vertical motion at 600 hPa does not clearly exhibit the associated upward/downward trends. What are the relative contributions of the greenhouse-warming-induced thermal-circulation pathway, the heat-induced anomalous overturning circulation reinforced by tropical convection over the Philippine Sea, and other proposed pathways (Tibetan-Plateau-deflected subtropical westerlies, poleward-shifting westerlies, extratropical cyclone changes, Pacific sea-surface-temperature thermal-contrast changes), including the role of decadal internal variability, in producing the observed latitudinal dipole of March–April rainfall trends over East Asia (latitude bands, trend baseline, and contribution metric as defined in Background) — and why do the observed 600-hPa vertical-motion trends not clearly exhibit the associated thermal-circulation signature? (2) Near-future combined forcing: as East Asian temperatures continue to rise while anthropogenic aerosol emissions are projected to decrease, how will the combination of stronger greenhouse warming and reduced aerosol loading reshape the latitudinal dipole of spring rainfall trends over East Asia, and how will it modulate the risk of spring droughts at low latitudes and floods at mid-latitudes? Ryu & Min (2022) note that regional aerosol effects on temperature, circulation and precipitation are sensitive to model uncertainties.

References

  1. Ryu, Young-Hee; Min, Seung-Ki (2022). Greenhouse warming and anthropogenic aerosols synergistically reduce springtime rainfall in low-latitude East Asia. npj Climate and Atmospheric Science 5:69. DOI: 10.1038/s41612-022-00295-x. https://doi.org/10.1038/s41612-022-00295-x
  2. Li, Zhenning; Yang, Song; He, Bian; Hu, Chundi (2016). Intensified Springtime Deep Convection over the South China Sea and the Philippine Sea Dries Southern China. Scientific Reports 6:30470. DOI: 10.1038/srep30470. https://doi.org/10.1038/srep30470
  3. Li, Yana; Zhou, Wen; Yang, Song; Zhang, Ruhua; Cheung, Ho-Nam; Zhang, Yue (2022). Recent Early-Spring Drying Trend over Southern China Associated with Changes in the Zonal Thermal Contrast over the Pacific. Journal of Climate 35:6487–6498. DOI: 10.1175/JCLI-D-21-0891.1. https://doi.org/10.1175/JCLI-D-21-0891.1
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  10. Zhao, Min; Xie, Yongkun; Shi, Jinsen; Bao, Zhongrui; Mi, Jiaqin; Su, Zifan (2025). Reversed Future Drying-Wetting Precipitation Patterns Over the Northwestern and Southeastern East Asia Driven by Reduced Aerosol Emissions. Geophysical Research Letters 52:e2024GL114527. DOI: 10.1029/2024GL114527. https://doi.org/10.1029/2024GL114527
  11. Sheng, Bosi; Dong, Buwen; Wang, Haolin; Zhang, Mingming; Liu, Yang; Li, Qingxiang (2025). Extremely persistent precipitation events during April–June 2022 in the southern China: projected changes at different global warming levels and associated physical processes. Climate Dynamics 63:217. DOI: 10.1007/s00382-025-07644-5. https://doi.org/10.1007/s00382-025-07644-5
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