Westen et al. (2025) Changing European hydroclimate under a collapsed AMOC in the Community Earth System Model

Identification

• Journal: Hydrology and earth system sciences
• Year: 2025
• Date: 2025-11-21
• Authors: René M. van Westen, Karin van der Wiel, Swinda K. J. Falkena, Frank Selten
• DOI: 10.5194/hess-29-6607-2025

Research Groups

• Royal Netherlands Meteorological Institute (KNMI), De Bilt, the Netherlands
• Institute for Marine and Atmospheric research Utrecht, Department of Physics, Utrecht University, Utrecht, the Netherlands

Short Summary

This study investigates the impact of Atlantic Meridional Overturning Circulation (AMOC) collapse, combined with climate change scenarios, on the European hydroclimate using the Community Earth System Model (CESM). It finds that AMOC collapse exacerbates projected drying trends and drought extremes across Europe, primarily through reduced precipitation and increased potential evapotranspiration under higher radiative forcing.

Objective

• To quantitatively assess how the balance between precipitation and potential evapotranspiration in Europe changes under various Atlantic Meridional Overturning Circulation (AMOC) regimes (including collapse scenarios) and different anthropogenic climate change pathways.
• To test the hypothesis that projected changes in hydrological extremes are exacerbated under an AMOC collapse.

Study Configuration

• Spatial Scale: European continent, with specific analyses for the Netherlands, South Sweden, and North Spain. Global climate patterns (Northern/Southern Hemisphere, tropical rain bands) are also referenced for context.
• Temporal Scale: Analysis periods of 100 years for statistical equilibrium, with simulations integrated for 500 years. Daily and monthly frequencies for data analysis, focusing on the growing season (April–September) and annual hydroclimatic conditions for the (far) future (beyond 2100).

Methodology and Data

• Models used: Fully-coupled Community Earth System Model (CESM, version 1.0.5) with horizontal resolutions of 1° for ocean/sea ice and 2° for atmosphere/land components.
• Data sources:
  • Eight unique AMOC scenarios simulated with CESM, combining different AMOC strengths (on/off states) and radiative forcing conditions (pre-industrial, RCP4.5, RCP8.5).
  • ERA5 reanalysis data (hourly and monthly averaged) for comparison, validation of PET calculations, and establishing reference climatologies.

Main Results

• Drier hydroclimatic conditions are generally expected across Europe under an AMOC collapse.
• Under pre-industrial conditions, AMOC collapse primarily drives drier conditions through reduced precipitation rates across Europe.
• In scenarios combining AMOC collapse with increased radiative forcing (RCP4.5, RCP8.5), higher potential evapotranspiration (PET) rates, driven by increased atmospheric temperatures, further exacerbate shifts towards increased seasonal drought extremes.
• AMOC collapse significantly enhances the already projected shifts to a drier European summer climate under standard climate change scenarios.
• Most European land surfaces show an increase in dry season intensity across all AMOC collapse and climate change scenarios, with the largest increases observed under high emission (RCP8.5) scenarios.
• Temperature changes (near-surface air temperature and dew-point temperature) are the dominant drivers of PET changes, with net surface radiation also contributing.
• All AMOC collapse and climate change scenarios lead to higher probabilities of drought (SPEI-6 ≤ -1) and lower probabilities of wet conditions (SPEI-6 ≥ 1) compared to their respective AMOC-on reference scenarios.
• Atmospheric circulation regimes during the growing season remain largely robust in their spatial patterns across different AMOC scenarios, though small displacements in pressure maxima and variations in regime frequency occur.

Contributions

• Provides a novel quantitative assessment of the balance between precipitation and potential evapotranspiration changes in Europe under different AMOC regimes, including the occurrence of droughts, which was previously unstudied in this detail.
• Identifies the dominant physical drivers (reduced precipitation vs. increased potential evapotranspiration) of hydroclimatic changes under various combinations of AMOC strength and radiative forcing.
• Quantifies the exacerbating effects of an AMOC collapse on projected increases in European dry season intensity and drought extremes under anthropogenic climate change.
• Highlights the critical importance of considering potential AMOC changes in future climate change projections for European hydroclimate, given the significant societal and ecological impacts.

Funding

• NWO-SURF project 2024.013 (PI: Dijkstra)
• ERC-AdG project TAOC (project 101055096; PI: Dijkstra)
• EC Horizon Europe project OptimESM “Optimal High Resolution Earth System Models for Exploring Future Climate Changes” under Grant 101081193
• Dutch Research Council (NWO) Vici project to A. S. von der Heydt (project number VI.C.202.081, NWO Talent programme)

Citation

@article{Westen2025Changing,
  author = {Westen, René M. van and Wiel, Karin van der and Falkena, Swinda K. J. and Selten, Frank},
  title = {Changing European hydroclimate under a collapsed AMOC in the Community Earth System Model},
  journal = {Hydrology and earth system sciences},
  year = {2025},
  doi = {10.5194/hess-29-6607-2025},
  url = {https://doi.org/10.5194/hess-29-6607-2025}
}