Aerosol indirect effects in tropical shipping corridors: global modeling corroborates results from observations

We use data from simulations performed with the global aerosol-climate model ECHAM5-HAM to challenge the proposition obtained from observations that shipping emissions do not have a significant effect on water clouds over tropical oceans (Peters et al. (2011), P11). For this purpose we sample the model data according to the approach presented in P11, i.e. an approach aimed at separating "clean" from "polluted" oceanic regions based on i) the location of main shipping routes and ii) wind direction. We use data from the model simulations presented in Peters et al. (2012) where we found the upper estimate of aerosol indirect effects (AIEs) from shipping emissions to be -0.32 ± 0.01 Wm-2, i.e. about half of the previous upper estimate but still of substantial magnitude compared to the most recent multi-model estimate of total AIEs (-0.7 ± 0.5 Wm-2, Quaas et al. (2009)). The model data is complemented by additional sensitivity experiments, including one in which a the shipping emissions are scaled by a factor of 10 (AIE: -1.39 Wm-2). Our analysis shows that the boundary layer aerosol population indeed shows substantial changes in across corridor direction for the experiments yielding the highest AIEs. These changes are manifested in both aerosol numbers and CCN concentrations. However, we find the resulting effect on cloud properties to be non-significant, i.e. across-corridor profiles of cloud properties are very similar among all simulations. These results thus confirm our findings from observations (P11) and give room for speculations regarding implications for climate. On the one hand, these results highlight the effectiveness of compensating mechanisms on micro- and macroscopic scales, buffering possible aerosol induced cloud changes in shallow clouds over tropical oceans. On the other hand, even higher global AIEs from shipping emissions cannot be excluded as even such substantial perturbations as -1.39 Wm-2 are not discernible when using a sampling approach lending itself to satellite observations.