Zhang et al. (2025) Non-stationary influence of the North Atlantic Oscillation on summer precipitation in the Central-Eastern Himalayas

Identification

• Journal: npj Climate and Atmospheric Science
• Year: 2025
• Date: 2025-12-13
• Authors: Qiang Zhang, Xuelong Chen, Yaoming Ma, Dianbin Cao, Y. F. Lyu, Shuai Hu, Yi-Ting Yang, Zeyong Hu, Xin Xu, Sunil Subba
• DOI: 10.1038/s41612-025-01268-6

Research Groups

• Land-Atmosphere Interaction and its Climatic Effects Group, State Key Laboratory of Tibetan Plateau Earth System, Environment and Resources (TPESER), Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, China
• Key Laboratory of Land Surface Process and Climate Change in Cold and Arid Regions, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, China
• Key Laboratory of Earth System Numerical Modeling and Application, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
• Department of Environmental Engineering, Beijing Key Laboratory of Resource-oriented Treatment of Industrial Pollutants, School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing, China

Short Summary

This study reveals the non-stationary influence of the Summer North Atlantic Oscillation (SNAO) on summer precipitation in the Central-Eastern Himalayas (CEH), demonstrating a "weak–strong–weak" evolution of this linkage primarily modulated by the interaction of SNAO-driven circulation anomalies with the Himalayan topography.

Objective

• To investigate the critical influencing factors and mechanisms in the interannual variability of summer precipitation in the Central-Eastern Himalayas (CEH).
• To reveal the difficulties in developing reliable models for predicting summer precipitation in the CEH.
• To suggest potential strategies for the climate community to enhance precipitation prediction accuracy in the CEH.

Study Configuration

• Spatial Scale: Central-Eastern Himalayas (CEH), defined by the region outlined in Liu, et al. (2022).
• Temporal Scale: 1985–2019 (35 years), analyzed in three phases: Phase 1 (1985–1993), Phase 2 (1994–2010), and Phase 3 (2011–2019).

Methodology and Data

• Models used:
  • First Institute of Oceanography Earth System version 2 model (FGOALS-f3-L) from the Global Monsoons Model Intercomparison Project (GMMIP) of CMIP6, used for control (Ctrl) and no-topography (NoT) experiments.
• Data sources:
  • Rain gauges: Monthly precipitation records from 59 stations in the CEH provided by the Department of Hydrology and Meteorology, Government of Nepal (DHM).
  • Gridded precipitation products: Global Precipitation Climatology Centre (GPCC; 0.5° × 0.5° resolution), Asian Precipitation-Highly Resolved Observational Data Integration Towards Evaluation (APHRO; 0.25° × 0.25° resolution), Climatic Research Unit (CRU; 0.5° × 0.5° resolution).
  • Reanalysis: European Centre for Medium-Range Weather Forecasts reanalysis version 5 (ERA5; 0.25° × 0.25° resolution) for precipitation, atmospheric circulation, vertical velocity, and NAO index calculation.
  • Climate indices: Niño 3.4 index (NOAA Climate Prediction Center), Scandinavian pattern (SCA) index (NOAA Climate Prediction Center).

Main Results

• The interannual variability of CEH summer precipitation (1985–2019) is influenced by the El Niño–Southern Oscillation (ENSO) with a sustained negative association, and by the Summer North Atlantic Oscillation (SNAO) with a non-stationary effect.
• The SNAO–CEH precipitation linkage exhibits a "weak–strong–weak" evolution, with significant shifts identified in 1994 and 2010.
• During Phase 2 (1994–2010), the SNAO–CEH precipitation correlation is strong and positive (Pearson correlation coefficient = 0.75, p < 0.05), surpassing the ENSO–CEH correlation (Pearson correlation coefficient = -0.57, p < 0.05).
• In Phase 2, a positive SNAO triggers a 500 hPa anomalous anticyclone south of the CEH, which channels low-level winds towards the Himalayan foothills. This airflow is mechanically lifted by the topography, generating significant upward vertical motion and leading to positive precipitation anomalies.
• Atmospheric moisture budget analysis confirms that vertical moisture advection, primarily its dynamic component, is the main contributor to the positive precipitation anomalies in Phase 2.
• Topography-removal experiments (NoT) using the FGOALS-f3-L model demonstrate that without the Himalayan topography, the observed positive precipitation anomalies in Phase 2 disappear, replaced by a dipole-like precipitation distribution, confirming the crucial role of topography.
• The non-stationary nature of the SNAO–CEH precipitation linkage is attributed to quasi-decadal meridional shifts of the SNAO-induced anomalous anticyclone south of the CEH and the varying resemblance of the SNAO-related circulation to the Silk Road Pattern (SRP).
• In Phase 2, the SNAO-related 200 hPa meridional wind anomalies closely match the SRP typical pattern (spatial correlation coefficient = 0.93, p < 0.01), facilitating stronger downstream impacts and terrain-circulation coupling.
• Interdecadal variability of the 200 hPa zonal wind (U200) at the Asian jet entrance (30–45°N, 0–30°E) is strongly correlated with the SNAO–CEH precipitation relationship (Pearson correlation coefficient = 0.95, p < 0.01), suggesting that a stronger jet entrance in Phase 2 enhances the projection and amplification of SNAO-generated anomalies onto the SRP.
• The February Scandinavian pattern (SCA) index shows a strong anti-correlation with subsequent jet-entrance U200 strength (Pearson correlation coefficient = -0.76, p < 0.01), indicating a potential lagged influence via an ocean-atmosphere bridge.

Contributions

• Provides the first comprehensive analysis of the non-stationary influence of the Summer North Atlantic Oscillation (SNAO) on summer precipitation in the Central-Eastern Himalayas (CEH).
• Identifies the critical role of Himalayan topography in modulating the SNAO's impact on CEH precipitation through mechanical forcing, a key mechanism previously underexplored for this region.
• Explains the observed "weak–strong–weak" evolution of the SNAO–CEH precipitation linkage by linking it to meridional shifts of the SNAO-induced anticyclone and the varying projection of SNAO-related circulation onto the Silk Road Pattern (SRP).
• Highlights the interdecadal variability of the Asian jet entrance as a crucial background factor modulating the efficiency of SNAO-SRP coupling.
• Offers insights for enhancing precipitation prediction accuracy in the CEH by emphasizing the dynamic links between large-scale climate modes, atmospheric circulation, and regional topography.
• Underscores the need for denser in-situ precipitation measurements and systematic multi-dataset evaluation in the CEH to better constrain the non-stationary SNAO–CEH linkage.

Funding

• National Natural Science Foundation of China (Grant No. U2442213)
• Second Tibetan Plateau Scientific Expedition and Research (STEP) Program (Grant No. 2019QZKK0105)
• China Scholarship Council (Grant No. 202504910249)

Citation

@article{Zhang2025Nonstationary,
  author = {Zhang, Qiang and Chen, Xuelong and Ma, Yaoming and Cao, Dianbin and Lyu, Y. F. and Hu, Shuai and Yang, Yi-Ting and Hu, Zeyong and Xu, Xin and Subba, Sunil},
  title = {Non-stationary influence of the North Atlantic Oscillation on summer precipitation in the Central-Eastern Himalayas},
  journal = {npj Climate and Atmospheric Science},
  year = {2025},
  doi = {10.1038/s41612-025-01268-6},
  url = {https://doi.org/10.1038/s41612-025-01268-6}
}