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Climate Dynamics

, Volume 39, Issue 3–4, pp 709–727 | Cite as

Observed and simulated impacts of the summer NAO in Europe: implications for projected drying in the Mediterranean region

  • Ileana BladéEmail author
  • Brant Liebmann
  • Didac Fortuny
  • Geert Jan van Oldenborgh
Article

Abstract

Climate models predict substantial summer precipitation reductions in Europe and the Mediterranean region in the twenty-first century, but the extent to which these models correctly represent the mechanisms of summertime precipitation in this region is uncertain. Here an analysis is conducted to compare the observed and simulated impacts of the dominant large-scale driver of summer rainfall variability in Europe and the Mediterranean, the summer North Atlantic Oscillation (SNAO). The SNAO is defined as the leading mode of July–August sea level pressure variability in the North Atlantic sector. Although the SNAO is weaker and confined to northern latitudes compared to its winter counterpart, with a southern lobe located over the UK, it significantly affects precipitation in the Mediterranean, particularly Italy and the Balkans (correlations of up to 0.6). During high SNAO summers, when strong anticyclonic conditions and suppressed precipitation prevail over the UK, the Mediterranean region instead is anomalously wet. This enhanced precipitation is related to the presence of a strong upper-level trough over the Balkans—part of a hemispheric pattern of anomalies that develops in association with the SNAO—that leads to mid-level cooling and increased potential instability. Neither this downstream extension nor the surface influence of the SNAO is captured in the two CMIP3 models examined (HadCM3 and GFDL-CM2.1), with weak or non-existent correlations between the SNAO and Mediterranean precipitation. Because these models also predict a strong upward SNAO trend in the future, the error in their representation of the SNAO surface signature impacts the projected precipitation trends. In particular, the attendant increase in precipitation that, based on observations, should occur in the Mediterranean and offset some of the non-SNAO related drying does not occur. Furthermore, the fact that neither the observed SNAO nor summer precipitation in Europe/Mediterranean region exhibits any significant trend so far (for either the full century or the recent half of the record) does not increase our confidence in these model projections.

Keywords

Summer NAO Summer Mediterranean precipitation CMIP3 models Projected drying NAO teleconnections Model projections Observed precipitation trends 

Notes

Acknowledgments

We thank Pablo Zurita for his comments on the manuscript and Nate Mantua, Javier García-Serrano and Vicent Altava for useful discussion of our work. We also thank Ricardo Trigo and an anonymous reviewer for their useful suggestions, which have helped clarify several points in the manuscript. Roy Mendelssohn kindly supplied the SST data. DF was supported by grant Consolider 2007-CSD2007-00050. The work was funded by grant CGL2009-06944 of the Spanish MICINN. We acknowledge the modeling groups, the Program for Climate Model Diagnosis and Intercomparison (PCMDI) and the WCRP's Working Group on Coupled Modelling (WGCM) for their roles in making available the WCRP CMIP3 multi-model dataset. Support of this dataset is provided by the Office of Science, U.S. Department of Energy.

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Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  • Ileana Bladé
    • 1
    Email author
  • Brant Liebmann
    • 2
  • Didac Fortuny
    • 1
  • Geert Jan van Oldenborgh
    • 3
  1. 1.Facultat de Física, Departament d’Astronomia i MeteorologiaUniversitat de BarcelonaBarcelonaSpain
  2. 2.NOAA/Earth System Research LaboratoryCIRES Climate Diagnostics CenterBoulderUSA
  3. 3.Royal Dutch Meteorological InstituteDe BiltNetherlands

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