Title: Cnnecting regional aerosol emissions reductions to mean and extreme
temperature and precipitation responses

Presenter: Dan Westervelt

The unintended climatic implications of aerosol and precursor emission reductions
implemented to protect public health are not well understood. We investigate the
mean and extreme 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 fourteen individual aerosol
emissions perturbation simulations (160-240 years each). We perturb emissions of
sulfur dioxide (SO2) and/or carbonaceous aerosol within six world regions and
assess the significance of both local and remote precipitation responses relative
to internal variability determined by the control simulation and across the
models. Global and regional mean 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. Mean
precipitation responses to aerosol emissions reductions are largest in the
tropics and project onto the El Niño-Southern Oscillation (ENSO). 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. We find significant increases in both
the amount and frequency of extreme precipitation in response to removal of
regional SO2 emissions, especially in active monsoon regions such as South Asia
and West Africa. In these same regions, removal of regional black carbon
emissions tends to cause extreme drying. These findings imply a major role for
health-driven reductions in aerosol emissions in future projections of
hydroclimate.