Do anthropogenic aerosols enhance or suppress the surface cloud forcing in the Arctic?

Earlier studies suggest that aerosol-cloud interactions may have contributed to the increase in surface air temperature recently observed in the Arctic. While those studies focused on longwave effects of strong pollution events around Barrow, Alaska, we use a global climate model (CAM-Oslo) to study the annual and seasonal net radiative effect of aerosol-cloud interactions over the entire Arctic region. The model is validated against and adjusted to match observations from the Surface Heat Budget of the Arctic Ocean campaign along with measuring stations within the Arctic region. Several sensitivity experiments were conducted which included changes in both cloud properties and aerosol concentrations. Results show that the longwave indirect effect at the surface lies between 0.10 and 0.85 W/m(2) averaged annually north of 71 degrees N, while the shortwave indirect effect lies between -1.29 W/m(2) and -0.52 W/m(2). Due to longwave dominance in winter, 6 of 11 simulations give a positive change in net cloud forcing between October and May (-0.16 to 0.29 W/m(2)), while the change in forcing averaged over the summer months is negative for all model simulations (from -2.63 to -0.23 W/m(2)). The annually averaged change in net cloud forcing at the surface is negative in 10 of 11 simulations, lying between -0.98 and 0.12 W/m(2). In conclusion, our results point to a small decrease in the surface radiative flux due to the aerosol indirect effect in the Arctic, but these estimates are subject to uncertainties in the frequency of thin clouds and biases in the estimated cloud cover.