Dimming over the oceans: Transient anthropogenic aerosol plumes in the twentieth century

Anthropogenic aerosols reduce incoming surface solar radiation (SSR), but the magnitude of this effect for reducing sea surface temperatures (SST) is still debated. Using simulations from the global climate model ECHAM5 with the Hamburg Aerosol Module and prescribed SSTs, we quantify anthropogenic aerosol dimming over sea surfaces by comparing ensembles, which only differ in anthropogenic aerosol emissions. We isolate the anthropogenic aerosol effect on SSR with sufficiently large ensemble sizes to provide statistically significant results. The following simulation results are obtained: Dimming plumes extend from their source regions with clear seasonality. The latter is predominantly shaped by atmospheric circulation, while interdecadal changes follow the gradual increase in anthropogenic aerosol emissions. Comparing the 1990s with the 1870s, on average, 9.4% (clear-sky SSR) or 15.4% (all-sky SSR) of the entire ocean surface was affected by anthropogenic aerosol dimming larger than -4 Wm(-2) (decadal mean). Comparing the same time periods, global average anthropogenic dimming over oceans is -2.3 Wm(-2) and -3.4 Wm(-2) for clear-sky and all-sky SSR, respectively. Surface dimming is hemispherically asymmetrical with stronger Northern Hemispheric dimming by 2.3 Wm(-2) and 4.5 Wm(-2) for clear-sky and all-sky SSR, respectively. Zonal average clear-sky dimming reaches its maximum (-5.5 Wm(-2)) near the equator. All-sky dimming peaks at 40 degrees N (-8 Wm(-2)) and is regionally larger than clear-sky dimming. Regionally, surface dimming can reach values up to 9.5 Wm(-2) (clear-sky) and 25 Wm(-2) (all-sky). Results are a contribution toward better quantifying spatially heterogeneous and time-dependent anthropogenic dimming effects on SSTs.