Sensitivity of the Climate Response to the Altitude of Black Carbon in the Northern Subtropics in an Aquaplanet GCM

Abstract

This study explores the dependence of the climate response on the altitude of black carbon in the northern subtropics by employing an atmospheric general circulation model coupled to an aquaplanet mixed layer ocean, with a focus on the pattern changes in the temperature, hydrological cycle, and large-scale circulation. Black carbon added below or within the subtropical low-level clouds tends to suppress convection, which reduces the low cloud amount, resulting in a positive cloud radiative forcing. The warmer northern subtropics then induce a northward shift of the intertropical convergence zone (ITCZ) and a poleward expansion of the descending branch of the northern Hadley cell. As the black carbon-induced local warming is amplified by clouds and is advected by the anomalous Hadley circulation, the entire globe gets warmer. In contrast, black carbon added near the surface increases the buoyancy of air parcels to enhance convection, leading to an increase in the subtropical low cloud amount and a negative cloud radiative forcing. The temperature increase remains local to where black carbon is added and elsewhere decreases, so that the ITCZ is shifted southward and the descending branch of the northern Hadley cell contracts equatorward. Consistent with previous studies, the authors demonstrate that the climate response to black carbon is highly sensitive to the vertical distribution of black carbon relative to clouds; hence, models have to accurately compute the vertical transport of black carbon to enhance their skill in simulating the climatic effects of black carbon.

Contrasting Response of Precipitation to Aerosol Perturbation in the Tropics and Extratropics Explained by Energy Budget Considerations

“…The vertical distribution of aerosol is based on the kernel of Euler's β function (see details in Stevens et al, 2017) and is mostly confined to below 5 km. Differences in the vertical placement of the plume may cause different response of the climate system and general circulation (Ban-Weiss et al, 2012;Kim et al, 2015); however, they are less important to the column-integrated energy budget, which is the focus of this study. For each plume size and location, two different cases of SSA (representing the radiative properties of the aerosols) are simulated-0.8 (representing relatively absorbing plumes) and 0.95 (representing less absorbing plumes).…”

Detecting cross‐equatorial wind change as a fingerprint of climate response to anthropogenic aerosol forcing

“…Distinct from well-mixed greenhouse gases (GHGs), the spatial distributions of anthropogenic aerosols are strong due to the inhomogeneous emission sources and short lifetimes. Compared to greenhouse gas forcing, aerosol forcing is effective in inducing atmospheric circulation and hence regional variations in climate response [Shindell and Faluvegi, 2009;Shindell et al, 2012;Ming and Ramaswamy, 2011;Li et al, 2015;Kim et al, 2015].…”

Climate Impacts of CALIPSO‐Guided Corrections to Black Carbon Aerosol Vertical Distributions in a Global Climate Model

“…Theoretically, the poleward extent of the HC is also related to the location of the ITCZ based on baroclinic instability arguments (Kang & Lu, ) and the metrics for the two have been found to be correlated in GCM simulations (Allen & Sherwood, ; Kang & Lu, ; Kim et al, ; Kovilakam & Mahajan, ). Figure e also shows a near linear relationship between tropical expansion metric and the shift in the ITCZ center for all BC experiments, consistent with UC only experiments (Kovilakam & Mahajan, ).…”

“…Since the NH is colder in the CC runs, with little difference in the SH -as suggested by the surface temperature difference (Figure 2b) -the northward ITCZ shift and cross-equatorial southward heat transport are also weaker. Although not directly comparable, this result is broadly consistent with the result from Kim et al (2015) that as NH BC are raised from below the low-level cloud layer to above the cloud layer, the ITCZ northward shift weakens.…”

“…The impact of vertical location of BC in altering cloud structures via the indirect effects is less studied (Bond et al, 2013). 10.1002/2017GL074652 Ming et al (2010) in a suite of idealized experiments found that uniformly raising the vertical location of BC globally increases atmospheric absorption and decreases global precipitation. Recently, Kim et al (2015) found that the ITCZ response to prescribed hemispherically asymmetric BC forcing is sensitive to the altitude of the prescribed BC concentrations in the NH subtropics in an aquaplanet GCM. They found that when BC are placed near (above, below, or within) the low-level cloud layers, BC-induced warmer subtropical free troposphere results in a northward shift of the ITCZ (favoring the warmer hemisphere) and a poleward expansion of the NH Hadley cell.…”