Wintertime hydro-dynamic response to anthropogenic aerosol along the
west coast of North America
Abstract
Several studies show anthropogenic aerosols (AAs) can perturb
large-scale atmospheric circulation, as well as precipitation associated
with the tropical rain belt and monsoons of the Northern Hemisphere,
including the South and East Asian monsoons, as well as the West African
monsoon. In the Northern Hemisphere mid-latitudes, however, the impact
of AAs on regional climate and precipitation remains uncertain. This
work investigates the influence of AAs on wintertime precipitation along
the west coast of North America using the newest climate models from the
Coupled Model Intercomparison Project phase 6 (CMIP6). Over much of the
20th century, particularly from 1930-1975 when U.S. and European AA
emissions rapidly increased, models show a robust wintertime
precipitation dipole pattern along the Pacific Coast, with wetting
southward of 40°N and drying to the north. This dipole pattern is
consistent with a deepening of the Aleutian Low and a southeastward
shift of the east Pacific jet stream, which in turn is associated with a
southeastward displacement of the region of maximum baroclinicity. These
changes in baroclinicity are related to strong aerosol-induced
tropospheric cooling that maximizes at 40°N over the east Pacific. This
strong cooling is consistent with aerosol-induced decreases in
top-of-the-atmosphere radiation and changes in clouds, including
decreases in cloud top effective radius and increases in cloud cover.
Consistently, in response to 21st-century AA reductions an opposite
hydro-dynamic dipole pattern occurs, including drying southward of 40°N
and wetting to the north. Although uncertainties remain, including
possible overestimation of aerosol forcing, our results suggest aerosols
can perturb precipitation along the west coast of North America.
However, the dipole pattern under AA forcing is partially offset by
GHGs, resulting in a muted dipole response in all-forcing simulations.