Contrasting Climate Responses to the Scattering and Absorbing Features of Anthropogenic Aerosol Forcings

Abstract

Anthropogenic aerosols comprise optically scattering and absorbing particles, with the principal concentrations being in the Northern Hemisphere, yielding negative and positive global mean radiative forcings, respectively. Aerosols also influence cloud albedo, yielding additional negative radiative forcings. Climate responses to a comprehensive set of isolated aerosol forcing simulations are investigated in a coupled atmosphere–ocean framework, forced by preindustrial to present-day aerosol-induced radiative perturbations. Atmospheric and oceanic climate responses (including precipitation, atmospheric circulation, atmospheric and oceanic heat transport, sea surface temperature, and salinity) to negative and positive particulate forcings are consistently anticorrelated. The striking effects include distinct patterns of changes north and south of the equator that are governed by the sign of the aerosol forcing and its initiation of an interhemispheric forcing asymmetry. The presence of opposing signs of the forcings between the aerosol scatterers and absorbers, and the resulting contrast in climate responses, thus dilutes the individual effects of aerosol types on influencing global and regional climate conditions. The aerosol-induced changes in the variables also have a distinct fingerprint when compared to the responses of the more globally uniform and interhemispherically symmetric well-mixed greenhouse gas forcing. The significance of employing a full ocean model is demonstrated in this study by the ability to partition how individual aerosols influence atmospheric and oceanic conditions separately.

Comparing multiple model-derived aerosol optical properties to spatially collocated ground-based and satellite measurements

“…Radiative forcings are particularly sensitive to vertical distributions of aerosols due to the relative location of clouds, attenuation of insolation, and relative humidity (e.g., Haywood and Ramaswamy, 1998;Ocko et al, 2012;Samset et al, 2013). The vertical profile of absorbing aerosols, in particular, has a strong bearing on the hydrological cycle (Ming et al, 2010;Ocko et al, 2014).…”

“…Because the horizontal and vertical distributions of anthropogenic scattering and absorbing aerosols dominate a suite of climate responses to the forcings Donner et al, 2011;Naik et al, 2013;Ocko et al, 2014), it is critical to improve model performance of aerosol optical properties. Here we show that comparing multiple modelsimulated aerosol properties -from two prominent, related climate models with vastly different aerosol treatmentsto available datasets from spatially collocated ground-based and satellite instruments is important for determining model biases.…”

“…This is incredibly important and useful for climate model evaluation because aerosol radiative effects are extremely sensitive to elevation (e.g., Ocko et al, 2014). Satellite measurements are further valuable because they have a broad spatial coverage.…”

“…Atmospheric aerosols are important for Earth's climate because they are comprised of optically scattering and absorbing particles that can also serve as cloud condensation nuclei (e.g., Boucher et al, 2013). Aerosols' capabilities in either reflecting energy out to space through the scattering of sunlight, trapping additional energy in the Earth system through absorption of sunlight and longwave radiation, reducing insolation at the surface, or modifying cloud properties can significantly alter Earth's radiation budget and influence climate conditions (e.g., Ocko et al, 2014).…”

“…Further, sources of anthropogenic aerosols are mainly in the Northern Hemisphere, leading to a meridional asymmetry in distributions and aerosol forcings across the two hemispheres. Aerosols' perturbation of the energy balance, specifically in the Northern Hemisphere, has been shown to influence large-scale circulation as well as local climate (Bollasina et al, 2011;Ocko et al, 2014).…”