Abstract
Abstract. The unintended climatic implications of aerosol and precursor emission reductions implemented to protect public health are poorly understood. We investigate the precipitation response to regional changes in aerosol emissions using three coupled chemistry–climate models: NOAA Geophysical Fluid Dynamics Laboratory Coupled Model 3 (GFDL-CM3), NCAR Community Earth System Model (CESM1), and NASA Goddard Institute for Space Studies ModelE2 (GISS-E2). Our approach contrasts a long present-day control simulation from each model (up to 400 years with perpetual year 2000 or 2005 emissions) with 14 individual aerosol emissions perturbation simulations (160–240 years each). We perturb emissions of sulfur dioxide and/or carbonaceous aerosol within six world regions and assess the significance of precipitation responses relative to internal variability determined by the control simulation and across the models. Global and regional precipitation mostly increases when we reduce regional aerosol emissions in the models, with the strongest responses occurring for sulfur dioxide emissions reductions from Europe and the United States. Precipitation responses to aerosol emissions reductions are largest in the tropics and project onto the El Niño–Southern Oscillation (ENSO). Regressing precipitation onto an Indo-Pacific zonal sea level pressure gradient index (a proxy for ENSO) indicates that the ENSO component of the precipitation response to regional aerosol removal can be as large as 20 % of the total simulated response. Precipitation increases in the Sahel in response to aerosol reductions in remote regions because an anomalous interhemispheric temperature gradient alters the position of the Intertropical Convergence Zone (ITCZ). This mechanism holds across multiple aerosol reduction simulations and models.
Highlights
Understanding the regional climate consequences of aerosols is of growing importance as emissions of aerosols and their precursors are projected to decline in most regions over the coming decades due to policies enacted to protect human health from the negative effects of air pollution (Rao et al, 2017; van Vuuren et al, 2011)
Our results in GFDL-CM3 are consistent with findings in CAM3 that show a positive North Atlantic Oscillation (NAO)-like response to increasing aerosols (Allen and Sherwood, 2011) and results from the Precipitation Driver Response Model Intercomparison Project (PDRMIP) models that showed a northward shift of the storm track over the North Atlantic and drying over the Mediterranean in response to black carbon (BC) (Tang et al, 2018)
We conduct a series of 14 aerosol emissions perturbation simulations (160–240 years each) in which we perturb emissions of sulfur dioxide and/or carbonaceous aerosol within six world regions relative to a long present-day control simulation in three coupled chemistry–climate models: NOAA Geophysical Fluid Dynamics Laboratory Coupled Model 3 (GFDL-CM3), NCAR Community Earth System Model (CESM1), and NASA Goddard Institute for Space Studies ModelE2 (GISS-E2)
Summary
Understanding the regional climate consequences of aerosols is of growing importance as emissions of aerosols and their precursors are projected to decline in most regions over the coming decades due to policies enacted to protect human health from the negative effects of air pollution (Rao et al, 2017; van Vuuren et al, 2011). Global emissions of anthropogenic aerosols and their precursors, including sulfur dioxide (SO2, precursor to sulfate aerosol), black carbon (BC), and organic carbon aerosol (OA), peaked in the 1970s and have been declining for the last few decades (Klimont et al, 2013; Smith et al, 2011; Smith and Bond, 2014). Westervelt et al (2017) began this process by simulating the precipitation response to the complete removal of US anthropogenic SO2 emissions in three coupled chemistry– climate models and found statistically significant increases in Sahel rainfall in multiple models. We use our multimodel regional aerosol perturbation framework to focus on the Sahel and Mediterranean precipitation responses in detail in addition to our more general analysis of precipitation responses around the globe
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