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Volume 18, issue 11 | Copyright
Atmos. Chem. Phys., 18, 8439-8452, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 15 Jun 2018

Research article | 15 Jun 2018

Dynamical response of Mediterranean precipitation to greenhouse gases and aerosols

Tao Tang1, Drew Shindell1, Bjørn H. Samset2, Oliviér Boucher3, Piers M. Forster4, Øivind Hodnebrog2, Gunnar Myhre2, Jana Sillmann2, Apostolos Voulgarakis5, Timothy Andrews6, Gregory Faluvegi7,8, Dagmar Fläschner9, Trond Iversen10, Matthew Kasoar5, Viatcheslav Kharin11, Alf Kirkevåg10, Jean-Francois Lamarque12, Dirk Olivié10, Thomas Richardson4, Camilla W. Stjern2, and Toshihiko Takemura13 Tao Tang et al.
  • 1Nicholas School of the Environment, Duke University, Durham, USA
  • 2CICERO Center for International Climate and Environmental Research, Oslo, Norway
  • 3Institute Pierre-Simon Laplace, Université Pierre et Marie Curie/CNRS, Paris, France
  • 4University of Leeds, Leeds, UK
  • 5Imperial College London, London, UK
  • 6Met Office Hadley Centre, Exeter, UK
  • 7Columbia University, New York, USA
  • 8NASA Goddard Institute for Space Studies, New York, USA
  • 9Max-Planck-Institut für Meteorologie, Hamburg, Germany
  • 10Norwegian Meteorological Institute, Oslo, Norway
  • 11Canadian Centre for Climate Modelling and Analysis, Victoria, BC, Canada
  • 12National Center for Atmospheric Research, Boulder, USA
  • 13Kyushu University, Fukuoka, Japan

Abstract. Atmospheric aerosols and greenhouse gases affect cloud properties, radiative balance and, thus, the hydrological cycle. Observations show that precipitation has decreased in the Mediterranean since the beginning of the 20th century, and many studies have investigated possible mechanisms. So far, however, the effects of aerosol forcing on Mediterranean precipitation remain largely unknown. Here we compare the modeled dynamical response of Mediterranean precipitation to individual forcing agents in a set of global climate models (GCMs). Our analyses show that both greenhouse gases and aerosols can cause drying in the Mediterranean and that precipitation is more sensitive to black carbon (BC) forcing than to well-mixed greenhouse gases (WMGHGs) or sulfate aerosol. In addition to local heating, BC appears to reduce precipitation by causing an enhanced positive sea level pressure (SLP) pattern similar to the North Atlantic Oscillation–Arctic Oscillation, characterized by higher SLP at midlatitudes and lower SLP at high latitudes. WMGHGs cause a similar SLP change, and both are associated with a northward diversion of the jet stream and storm tracks, reducing precipitation in the Mediterranean while increasing precipitation in northern Europe. Though the applied forcings were much larger, if forcings are scaled to those of the historical period of 1901–2010, roughly one-third (31±17%) of the precipitation decrease would be attributable to global BC forcing with the remainder largely attributable to WMGHGs, whereas global scattering sulfate aerosols would have negligible impacts. Aerosol–cloud interactions appear to have minimal impacts on Mediterranean precipitation in these models, at least in part because many simulations did not fully include such processes; these merit further study. The findings from this study suggest that future BC and WMGHG emissions may significantly affect regional water resources, agricultural practices, ecosystems and the economy in the Mediterranean region.

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