A41P-02
Easy Aerosol - a model intercomparison project to study aerosol-radiative interactions and their impact on regional climate

Thursday, 17 December 2015: 08:15
3010 (Moscone West)
Aiko Voigt, Lamont -Doherty Earth Observatory, Ocean and Climate Physics, Palisades, NY, United States, Sandrine Bony, Laboratoire de Météorologie Dynamique UPMC, Paris, France, Bjorn B Stevens, Max Planck Institute for Meteorology, Hamburg, Germany, Olivier Boucher, LMD, Paris Cedex 05, France, Brian Medeiros, National Center for Atmospheric Research, Boulder, CO, United States, Robert Pincus, University of Colorado at Boulder, Boulder, CO, United States, Zhili Wang, Institute of Atmospheric Composition, Chinese Academy of Meteorological Sciences, Beijing, China, Kai Zhang, Pacific Northwest National Laboratory, Richland, WA, United States, Anna Lewinschal, Department of Meteorology, Stockholm University, Stockholm, Sweden, Nicolas Bellouin, University of Reading, Department of Meteorology, Reading, United Kingdom and Young Min Yang, International Pacific Research Center, Honolulu, HI, United States
Abstract:
Recent studies illustrated the potential of aerosols to change the large-scale atmospheric circulation and precipitation patterns, but it remains unclear to what extent the proposed aerosol-induced changes reflect robust model behavior and are affected by the climate system's internal variability. "Easy Aerosol" addresses this question by subjecting nine comprehensive climate models with prescribed sea-surface temperatures (SSTs) to the same set of idealized "easy" aerosol perturbations.

The aerosol perturbations are designed based on a global aerosol climatology and mimic the gravest mode of the anthropogenic aerosol. They both scatter and absorb shortwave radiation, but to focus on direct radiative effects aerosol-cloud interactions are omitted. Each model contributes seven simulations. A clean control case with no aerosol-radiative effects is compared to six perturbed simulations with differing aerosol loading, zonal aerosol distributions, and SSTs. To estimate the role of internal variability, one of the models contributes a 5-member ensemble for each simulation.

When observed SSTs from years 1979-2005 are used, the aerosol leads to a local depression of precipitation at the Northern Hemisphere center of the aerosol and a northward shift of the intertropical convergence zone (ITCZ). This is consistent with the aerosol’s shortwave atmospheric heating and the fact that SSTs are fixed. Moreover, the Northern hemisphere mid-latitude jet shifts poleward in the annual and zonal-mean. Due to large natura variability, however, these signals only emerge in ensemble runs or if the aerosol optical depth is increased by a factor of five compared to the observed magnitude of the present-day anthropogenic aerosol. When SSTs are adapted to include the cooling effect of the aerosol, the ITCZ and the Northern hemisphere jet shift southward in the annual and zonal-mean. The models exhibit very similar precipitation and zonal wind changes in response to the SST change, showing that SSTs are a key factor for the circulation response. Yet, model differences in the surface and top-of-atmosphere energy balances due to evaporation and cloud-radiative effects imply that the models would show much more different responses if they were coupled to an interactive ocean.

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