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

Abstract

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 ∘ 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.

Mixed‐layer ocean responses to anthropogenic aerosol dimming from 1870 to 2000

“…Thus, the MLO fraction of ocean area affected is lower for clear‐sky SSR and about five times as large for all‐sky SSR. Applying the same −4 W m −2 criterion to atmosphere only simulations of the 1990s, Dallafior et al [] found much more similar fractions, 9.4% for clear sky and 15.4% for all sky. This may point to significant, MLO‐related feedback processes.…”

“…According to this analysis, most prominent oceanic regions susceptible to SSR changes caused by anthropogenic aerosols are located in the northern midlatitudes and extratropics, i.e., North Pacific, and the northern extratropical Pacific and Atlantic with sensitivities of tsurf toward all-sky SSR between 0.2 and 0.7 K W m −2 . For all-sky SSR, plume regions collocated with those identified by Dallafior et al [2015] show strong linear relationship with tsurf, namely, off the West African Coast, the Indian Ocean, and off East Asia. To a lesser extent, this also holds true for clear-sky SSR, where the plume regions with strong linearity are less extended than in all-sky SSR.…”

“…ALL generally shows larger fractions of ocean area affected and even a reduction in affected areas after the 1960 equilibria of AOD, tsurf, and all-sky SSR. Dallafior et al [2015] previously estimated the fraction of ocean area affected by anthropogenic aerosol dimming from transient atmosphere-only experiments with EHCAM5-HAM using a more strict criterion; i.e., an area was considered anthropogenically affected by SSR dimming if the 1990s average SSR was more than −4 W m −2 below the preindustrial (1870s) reference. For comparison, we derived the shares of ocean area affected by a dimming stronger than −4 W m −2 for the ALL experiment of the present data set.…”

“…This makes sense considering that greenhouse gases mainly affect long-wave fluxes while SSR changes are of secondary importance. If only aerosols are included (AERO, Figure 5b), blue areas are seen in regions where anthropogenic aerosol plumes as defined in Dallafior et al [2015] are encountered, e.g., off west coast of Africa, East Asia, and South America. Note that some regions also encounter larger back-of-envelope tsurf response than actually calculated, which are not classified as plume regions, e.g., around Australia.…”

Contrasting the Effects of the 1850–1975 Increase in Sulphate Aerosols from North America and Europe on the Atlantic in the CESM

“…In addition to the studies focussing on the AMV, others found a longer-term impact of anthropogenic aerosols on downward surface solar radiation over the Atlantic and consequently SSTs (Dallafior et al, 2015), affecting the interhemispheric SST gradient and thereby causing a shift of the Atlantic Intertropical Convergence Zone (ITCZ; e.g., Chang et al, 2011;Hwang et al, 2013). Long-term anthropogenic aerosol forcing has furthermore been suggested to have strengthened the AMOC (Cowan & Cai, 2013;Delworth & Dixon, 2006;Menary et al, 2013) and to have delayed ocean heat content increase and associated sea level rise in response to greenhouse gas (GHG) forcing (e.g., Delworth et al, 2005) during the twentieth century.…”