The Influence of Tropical Air-Sea Interaction on the Climate Impact of Aerosols: A Hierarchical Modeling Approach

Monday, May 12, 2014 - 07:00
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Aerosols have a significant impact on energy fluxes at the air-sea interface. In the tropics, the atmosphere-ocean system responds to changes in surface fluxes by altering horizontal gradients of the sea surface temperature (SST), which leads to changes in the surface winds. This results in coupled ocean-atmosphere feedbacks that can significantly alter the uncoupled response to aerosol effects. These feedbacks are particularly important in the tropical Pacific, where the coupled El Nino-Southern Oscillation (ENSO) phenomenon plays a dominant role in climate variability at seasonal-to-interannual time scales. One of the difficulties in using coupled general circulation models (CGCMs) to assess the influence of air-sea interaction on the climate impact of aerosols is that CGCMs often exhibit significant biases in their simulation of ENSO and other tropical phenomena. In this study, we use a hierarchical modeling approach to investigate the influence of tropical air-sea feedbacks on climate impacts of aerosols in the Community Earth System Model (CESM). We construct four different models by coupling the atmospheric component of CESM, the Community Atmospheric Model (CAM), to four different ocean models: (i) the Data Ocean Model (DOM; prescribed SST), (ii) Slab Ocean Model (SOM; thermodynamic coupling), (iii) Reduced Gravity Ocean Model (RGOM; thermodynamic and dynamic coupling), and (iv) the Parallel Ocean Program (POP; full ocean physics). These four models represent progressively increasing degree of coupling between the atmosphere and the ocean. The RGOM model, in particular, is tuned to produce a good simulation of ENSO and the associated tropical air-sea interaction, without being impacted by the climate drifts exhibited by fully-coupled GCMs. For each method of coupling, a pair of numerical experiments, including present day (year 2000) and preindustrial (year 1850) sulfate aerosol loading, were carried out. Our results indicate that the inclusion of air-sea interaction has large impacts on the spatial structure of the climate response induced by aerosols. The response patterns of temperature, precipitation, zonal winds, mean meridional circulation, radiative fluxes and cloud coverage vary depending upon the degree of atmosphere-ocean coupling. We present analyses of the regional response to sulfate aerosol forcing in the equatorial Pacific as well as the zonally-averaged response, including changes in the strength of the Hadley circulation and impact on the position of the ITCZ.