Response of the current climate to land-ocean contrasts in parameterized cumulus entrainment

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.