Aerosols at the Poles: An AeroCom Phase II multi-model evaluation

The role of atmospheric aerosols in the changing Polar climate is not well understood and aerosols are poorly constrained in models. Aerosols from anthropogenic and natural sources reach the Polar Regions through long-range transport. Here we compare aerosol optical depth (AOD) from simulations with 16 global aerosol models from the AeroCom phase II model inter-comparison project with available observations at both Poles. We show that the annual mean multi-model median is representative of the observations in Arctic, but that the inter-model spread is large. We also document the geographical distribution and seasonal cycle of the AOD for the individual aerosol species; black carbon from fossil fuel and biomass burning, sulfate, organic aerosols, dust and sea-salt. A subset of models also represent nitrate and secondary organic aerosols. The models produce a median annual mean AOD of 0.07 in the Arctic. The models also predict a noteworthy aerosol transport to the Antarctic with a resulting AOD varying between 0.01-0.02. The models have estimated the shortwave anthropogenic radiative forcing contributions to the direct aerosol effect. The Arctic modeled annual mean direct aerosol effect is slightly negative (-0.12 W m^-2), dominated by a positive black carbon contribution during spring and a negative sulfate contribution during summer.