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Twentieth-century Azores High expansion unprecedented in the past 1,200 years


The Azores High is a persistent atmospheric high-pressure ridge over the North Atlantic surrounded by anticyclonic winds that steer rain-bearing weather systems and modulate the oceanic moisture transport to Europe. The areal extent of the Azores High thereby affects precipitation across western Europe, especially during winter. Here we use observations and ensemble climate model simulations to show that winters with an extremely large Azores High are significantly more common in the industrial era (since ce 1850) than in pre-industrial times, resulting in anomalously dry conditions across the western Mediterranean, including the Iberian Peninsula. Simulations of the past millennium indicate that the industrial-era expansion of the Azores High is unprecedented throughout the past millennium (since ce 850), consistent with precipitation proxy evidence from Portugal. Azores High expansion emerges after ce 1850 and strengthens into the twentieth century, consistent with anthropogenically driven warming.

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Fig. 1: AHA and its recent expansion.
Fig. 2: AHA in observations and simulations since 1850.
Fig. 3: AHA and number of extreme events over the past millennium.
Fig. 4: Hydroclimate during winters with extremely large Azores High.

Data availability

Observational datasets are publicly accessible at NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their website at; and ERA-20C from ECMWF ( Climate model output is publicly available as follows: CESM LME ( is available through Earth System Grid (, PMIP3 data through Stable isotope data in Fig. 3d are available through NCEI at

Code availability

Code used to perform the analysis presented in this study is provided as part of the replication package. It is available at


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Use of the following datasets is gratefully acknowledged: Global Precipitation Climatology Center dataset by the German Weather Service (DWD), HadSLP2 from the UK Hadley Centre and NOAA–CIRES Twentieth Century Reanalysis, all provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their website at; ERA-20C from ECMWF. Support for the Twentieth Century Reanalysis Project version 2c dataset is provided by the US Department of Energy, Office of Science Biological and Environmental Research (BER) and by the National Oceanic and Atmospheric Administration Climate Program Office. We acknowledge the CESM1(CAM5) Last Millennium Ensemble Community Project and supercomputing resources provided by NSF/CISL/Yellowstone, as well as the World Climate Research Programme’s Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modelling groups for producing and making available their model output, with specific contributions from Beijing Climate Center’s Climate System Modelling Division, the National Center for Atmospheric Research, Institut Pierre Simon Laplace’s Climate Modelling Center, Max-Planck-Institut fur Meteorologie’s Modelling Division and the Atmosphere and Ocean Research Institute at the University of Tokyo. CMIP5 model output was provided by the WHOI CMIP5 Community Storage Server via their website: Cave research at Buraca Gloriosa conducted with the assistance of Associação de Estudos Subterrâneos e Defesa do Ambiente and Jonathan Haws. This work was supported by the US National Science Foundation (grants: AGS-1804528 to A.D.W.; AGS-1804635 to R.F.D.; AGS-1804132 to C.C.U.), Cornell College (to R.F.D.) and the Ocean Climate Change Institute and James E. and Barbara V. Moltz Fellowship for Climate-Related Research at WHOI (to C.C.U.).

Author information

Authors and Affiliations



N.C.-C. and C.C.U. conceived the study and designed the analyses. N.C.-C. conducted all the analyses. N.C.-C. and C.C.U. wrote the manuscript. D.L.T., A.D.W. and R.F.D. provided feedback on the analyses and manuscript. Y.A. and V.J.P. provided feedback on the manuscript.

Corresponding author

Correspondence to Caroline C. Ummenhofer.

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The authors declare no competing interests.

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Nature Geoscience thanks Pedro Sousa and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: James Super, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 Azores High Area in PMIP3 Models.

(a,c,e,g,i) Timeseries of Azores High Area (AHA) during the PMIP3 last millennium (grey) and historical (light red) simulations. Extremely large events are indicated as red triangles. Thick red line represents the number of extreme winters in the 100-year period surrounding each year. (b,d,f,h,j) Distribution of AHA during the last millennium (grey) and historical (red) simulations. Each row is calculated using output from the model indicated on the left. For more information on models see Methods: PMIP3 Models.

Extended Data Fig. 2 Extremely Large Azores High Area in PMIP3 Models.

(a) The number of winters with extremely large Azores High in the 100-year period surrounding each year for PMIP3 last millennium (grey) and historical (red) simulations. The Multi Model Ensemble average is shown for the last millennium (thick black) and historical (thick red) simulations. (b) The results of our Monte Carlo test for significance of extremely large Azores High winters (see Methods: Monte Carlo Validation) for the Multi Model Ensemble. The red vertical line shows the number of extremely large Azores High winters that occurred in the last 100 years of the historical simulation; this is significant to p = 0.02 (see Methods: Monte Carlo Validation). The models and simulations used are discussed in Methods: PMIP3 Models.

Extended Data Fig. 3 Hydroclimate during winters with extremely large Azores High in observations and models.

(a,c,e) Wintertime anomalies in precipitation (contours) and moisture transport in g kg–1 m s–1 (vectors) during winters with extremely large Azores High. (b,d,f) Wintertime anomalies in sea level pressure (contours) and moisture transport (vectors) during winters with extremely large Azores High. Each row was calculated using the dataset indicated to the left. Precipitation in (a) and (c) is based on the GPCC dataset. The multi-model ensemble was calculated using PMIP3 models interpolated onto a regular 2° by 2°grid.

Extended Data Fig. 4 Hydroclimate during winters with extremely large Azores High in individual models.

(a,c,e,g,i) Wintertime anomalies in precipitation (contours) and moisture transport in g kg1 m s1(vectors) during winters with extremely large Azores High. (b,d,f,h,j) Anomalies in sea level pressure (contours) and moisture transport (vectors) during winters with extremely large Azores High. Each row was calculated using output from the models indicated to the left. For information on models and simulations see Extended Data Table 2.

Extended Data Table 1 Summary of the observational and reanalysis products used in the study
Extended Data Table 2 Summary of the Paleo Model Intercomparison Project phase 3 (PMIP3) models and experiments used in the study

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Cresswell-Clay, N., Ummenhofer, C.C., Thatcher, D.L. et al. Twentieth-century Azores High expansion unprecedented in the past 1,200 years. Nat. Geosci. 15, 548–553 (2022).

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