The influence of tropical air-sea interaction on the climate impact of aerosols: a hierarchical modeling approach
Weichun Hsieh, R. Saravanan, P. Chang, and S. Mahajan
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. 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), (i) Slab Ocean Model (SOM; thermodynamic coupling), (iii) Reduced Gravity Ocean Model (RGOM; dynamic coupling), and (iv) the Parallel Ocean Program (POP; full ocean model). These four models represent progressively increasing degree of coupling between the atmosphere and the ocean. For each method of coupling, a pair of numerical experiments, including present day (year 2000) and preindustrial (year 1850) aerosol loading, were carried out. In addition, historical simulation results with CESM from the Coupled Model Intercomparison Project Phase 5 (CMIP5) project were also analyzed. For present day conditions, we considered two additional cases, (i) where all aerosols components remain the same as the present day experiment except that preindustrial sulfate concentrations are used (denoted as the sulfate experiment, SO42-) and (ii) where only black carbon (BC) is changed to preindustrial levels. Our preliminary 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. Our simulations also indicate that the inclusion (or exclusion) of various aerosol components can generate a nonlinear response which is not equivalent to superposing the responses to individual aerosol perturbations. This further complicates the analysis and quantification of aerosol impacts on climate. An approximated moisture budget equation will be analyzed in order to understand the physical mechanism of precipitation changes induced by aerosol perturbations and its modulation by atmosphere-ocean interaction.