Abstract
Abstract. Climate–aerosol interactions in India are studied by employing the global climate–aerosol model ECHAM5-HAM and the GAINS inventory for anthropogenic aerosol emissions. Model validation is done for black carbon surface concentrations in Mukteshwar and for features of the monsoon circulation. Seasonal cycles and spatial distributions of radiative forcing and the temperature and rainfall responses are presented for different model setups. While total aerosol radiative forcing is strongest in the summer, anthropogenic forcing is considerably stronger in winter than in summer. Local seasonal temperature anomalies caused by aerosols are mostly negative with some exceptions, e.g., parts of northern India in March–May. Rainfall increases due to the elevated heat pump (EHP) mechanism and decreases due to solar dimming mechanisms (SDMs) and the relative strengths of these effects during different seasons and for different model setups are studied. Aerosol light absorption does increase rainfall in northern India, but effects due to solar dimming and circulation work to cancel the increase. The total aerosol effect on rainfall is negative for northern India in the months of June–August, but during March–May the effect is positive for most model setups. These differences between responses in different seasons might help converge the ongoing debate on the EHPs and SDMs. Due to the complexity of the problem and known or potential sources for error and bias, the results should be interpreted cautiously as they are completely dependent on how realistic the model is. Aerosol–rainfall correlations and anticorrelations are shown not to be a reliable sole argument for deducing causality.
Highlights
Aerosols have a significant impact on human health, the environment and, through their effects on regional climate, on agriculture and other aspects of society
black carbon (BC) and organic carbon (OC) concentrations are smaller than in the previously published simulations using the REAS emission inventory (Henriksson et al, 2011) due to different emissions and a slightly different model version
The Indian climate was simulated in a series of eleven simulations applying the aerosol–climate model ECHAM5-HAM and the GAINS emission inventory
Summary
Aerosols have a significant impact on human health, the environment and, through their effects on regional climate, on agriculture and other aspects of society. To account for effects of aerosol forcing on SSTs, two simulations were made by coupling the atmospheric model to a mixed-layer ocean model. The NOABS and NUDGE simulations included anthropogenic emissions from the GAINS inventory but differed from the MAIN simulation as follows: in NOABS, aerosol single-scattering albedo was set to 1, which eliminates aerosol absorption. The difference between simulations with and without anthropogenic aerosols represents only a part of their full climate effect when using fixed SSTs. The difference between the mixed-layer ocean simulations MAIN_MLO and ZERO_MLO represents a more full physical response. These simulations were similar to MAIN, ZERO, NOABS and SSTMODIF, respectively Conducting these simulations both with and without aerosol activation is useful for assessing which aspects of the climate response are robust to changes in model formulation. The first year of each simulation was discarded as a spin-up period when analyzing the results, and in the mixed-layer ocean simulations only the last 10 years were analyzed to allow the model to reach equilibrium
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