Allen Climate Dynamics Center AC/DC

To date, very few studies have focused on dust and sea salt cloud interactions, particularly the semidirect effect (SDE) that results from changes in column temperature and moisture. Here, we isolate the SDE using several climate models driven by semiempirical dust and sea salt direct radiative effects. The global annual mean SDE varies from 0.01 to 0.10 W/m2, with the bulk of the signal coming from an increase in shortwave radiation. This is consistent with decreases in low cloud over ocean due to cloud burn‐off and reductions in midlevel cloud over land due to atmospheric stabilization and decreased convection. Overall, longwave effects weaken the positive SDE but with opposing effects over land and sea. High cloud is reduced over land but enhanced over sea. We conclude that dust and sea salt likely exert a global mean warming effect through cloud rapid adjustments.

Through its northward heat transport and carbon sequestration, the Atlantic Meridional Overturning Circulation (AMOC) is an important component of the climate system. Paleoclimate reconstructions suggest past abrupt climate change is linked to AMOC variability. More recent historical data and
modeling studies suggest a long-term AMOC weakening in response to anthropogenic factors, particularly greenhouse gases (GHGs). Anthropogenic aerosols exert a net negative radiative forcing that is about 40% as large as the positive forcing exerted by GHGs. A future decline in aerosol emissions will thus reinforce the positive forcing due to continued GHGs increases, particularly over the Northern Hemisphere, and this may impact the AMOC. Here, we use the fully-coupled Community Earth System Model version 1.2 driven by the Representative Concentration Pathway 4.5 to examine the impact of 21st century anthropogenic aerosol emission reductions on the AMOC. The ensemble mean response to all external forcing is a 30% AMOC weakening; 12% of AMOC weakening is due to aerosols and 18% is due to GHGs. Thus, aerosols are responsible for 40% of the total weakening. Moreover, AMOC weakening is largest during the first few decades of the 21st century, which is almost entirely caused by the decrease in aerosol. This response is associated with weakening of the North Atlantic jet stream, anomalous surface easterlies, and a decrease in latent and sensible surface heat fluxes at the North Atlantic deep convection regions. The decrease in surface heat fluxes is associated with a decrease in surface density flux and stabilization of the water column, which in turn reduces the rate of sinking water and weakens the AMOC. These results are consistent with currently available CMIP6 simulations, and suggest future aerosol reductions will significantly weaken of the AMOC.

Many climate models simulate an increase in anthropogenic aerosol species in response to warming, particularly over the Northern Hemisphere mid-latitudes during June, July and August. Recently, it has been argued that this increase in anthropogenic aerosols can be linked to a decrease in wet removal associated with reduced precipitation, but the mechanisms remain uncertain. Here, using a state-of-the-art climate model (the Community Atmosphere Model version 5), we expand on this notion to demonstrate that the enhanced aerosol burden and hydrological changes are related to a robust climate change phenomenon—the land–sea warming contrast. Enhanced land warming is associated with continental reductions in lower-tropospheric humidity that drive decreases in low clouds—particularly large scale (stratus) clouds—which, in turn, lead to reduced large-scale precipitation and aerosol wet removal. Idealized model simulations further show that muting the land–sea warming contrast weakens these hydrological changes, thereby suppressing the aerosol increase. Moreover, idealized simulations that only feature land warming yield enhanced continental aridity and an increase in aerosol burden. Thus, unless anthropogenic emission reductions occur, our results add confidence that a warmer world will be associated with enhanced aerosol pollution.

Future California (CA) precipitation projections, including those from the most recent Climate Model Intercomparison Project (CMIP5), remain uncertain. This uncertainty is related to several factors, including relatively large internal climate variability, model shortcomings, and because CA lies within a transition zone, where mid-latitude regions are expected to become wetter and subtropical regions drier. Here, we use a multitude of models to show CA may receive more precipitation in the future under a business-as-usual scenario. The boreal winter season-when most of the CA precipitation increase occurs-is associated with robust changes in the mean circulation reminiscent of an El Niño teleconnection. Using idealized simulations with two different models, we further show that warming of tropical Pacific sea surface temperatures accounts for these changes. Models that better simulate the observed El Niño-CA precipitation teleconnection yield larger, and more consistent increases in CA precipitation through the twenty-first century.

The tropical belt has widened during the last several decades, and both internal variability and anthropogenic forcings have contributed. Although greenhouse gases (GHGs) and stratospheric ozone depletion have been implicated as the primary anthropogenic drivers of tropical expansion, the possible role of other drivers remains uncertain. Here, we analyze the tropical belt width response to idealized perturbations in multiple models. Our results show that absorbing black carbon (BC) aerosol drives tropical expansion and reflecting sulfate aerosol drives contraction. BC, especially from Asia, is more efficient per unit forcing than GHGs in driving tropical expansion, particularly in the Northern Hemisphere (NH). Tropical belt expansion (contraction) is related to an increase (decrease) in extratropical static stability induced by absorbing (scattering) aerosol.  Although a formal attribution is not possible, scaling the normalized expansion rates to the historical time period suggests BC is the largest driver of NH tropical widening.