Response of the current climate to land-ocean contrasts in parameterized
cumulus entrainment
Abstract
Cumulus entrainment substantially regulates the earth’s climate but
remains poorly constrained in global climate models. Recent studies have
shown that cumulus bulk entrainment (or dilution) is particularly
sensitive to continentality, with the entrainment rate in simulated
maritime cumuli nearly double that of continental cumuli. The present
study examines the impacts of such land–ocean entrainment contrasts on
the current climate using 21-year simulations with the Geophysical Fluid
Dynamics Laboratory’s (GFDL) High-Resolution Atmospheric Model (HIRAM).
In response to a 25% reduction in entrainment over land, precipitation
over tropical land regions increases by up to 40%. Along with directly
facilitating enhanced convective precipitation, this entrainment
reduction induces a positive soil moisture–precipitation feedback that
further enhances convective precipitation over land. A 25% entrainment
reduction over the oceans leads to more widespread modifications of
convection patterns, with the strongest signal in the tropical Pacific.
Deep convection shifts upstream (eastward) there, inducing enhanced
large-scale ascent over the central Pacific with compensating subsidence
and reduced humidity and precipitation over the western Pacific.
Land–ocean variations in entrainment project onto the Pacific Walker
circulation, with the 25% land reduction strengthening it by 4% and
the 25% ocean reduction weakening it by 14%. These changes are driven
by variations in convective and large-scale stratiform heating over the
Pacific. While reduced entrainment over land enhances diabatic heating
in the Maritime Continent to strengthen the Walker circulation, reduced
entrainment over the oceans decreases diabatic heating in the western
Pacific to weaken the Walker circulation.