The hemispheric contrast in cloud microphysical properties constrains aerosol forcing

Abstract

The change in planetary albedo due to aerosol−cloud interactions during the industrial era is the leading source of uncertainty in inferring Earth’s climate sensitivity to increased greenhouse gases from the historical record. The variable that controls aerosol−cloud interactions in warm clouds is droplet number concentration. Global climate models demonstrate that the present-day hemispheric contrast in cloud droplet number concentration between the pristine Southern Hemisphere and the polluted Northern Hemisphere oceans can be used as a proxy for anthropogenically driven change in cloud droplet number concentration. Remotely sensed estimates constrain this change in droplet number concentration to be between 8 cm−3 and 24 cm−3. By extension, the radiative forcing since 1850 from aerosol−cloud interactions is constrained to be −1.2 W⋅m−2 to −0.6 W⋅m−2. The robustness of this constraint depends upon the assumption that pristine Southern Ocean droplet number concentration is a suitable proxy for preindustrial concentrations. Droplet number concentrations calculated from satellite data over the Southern Ocean are high in austral summer. Near Antarctica, they reach values typical of Northern Hemisphere polluted outflows. These concentrations are found to agree with several in situ datasets. In contrast, climate models show systematic underpredictions of cloud droplet number concentration across the Southern Ocean. Near Antarctica, where precipitation sinks of aerosol are small, the underestimation by climate models is particularly large. This motivates the need for detailed process studies of aerosol production and aerosol−cloud interactions in pristine environments. The hemispheric difference in satellite estimated cloud droplet number concentration implies preindustrial aerosol concentrations were higher than estimated by most models.

Evaluation of Cloud and Precipitation Simulations in CAM6 and AM4 Using Observations Over the Southern Ocean

“…The low bias in cloud droplet number concentration in CAM6 is consistent with discrepancies seen between other state of the art models and satellite observations of Southern Ocean cloud droplet number concentrations in summertime low clouds (McCoy et al, 2020; Revell et al, 2019). This low bias is a widespread issue remaining in GCMs that presumably contributes to TOA SW bias for low‐lying liquid clouds over the Southern Ocean.…”

The value of remote marine aerosol measurements for constraining radiative forcing uncertainty

“…The model is structurally consistent in our experiments and so neglects uncertainty caused by choice of microphysical and atmospheric process representations. Our model also neglects some potentially important sources of remote marine aerosol, such as primary marine organic aerosol (Mulcahy et al, 2020) and methane-sulfonic acid (Schmale et al, 2019;Hodshire et al, 2019;Revell et al, 2019). Model inter-comparison projects (such as CMIP6) can be used to quantify the diversity of RF (or ERF) output from models, but they lack information about single model uncertainty.…”

Southern Ocean Cloud Properties Derived From CAPRICORN and MARCUS Data

“…While seasonally varying sea surface temperatures and sea ice contribute to the cloud variability (Huang et al., 2016), the Antarctic Circumpolar Current essentially divides the SO into lower latitude temperate and high latitude oceans. Especially in the high latitude SO, seasonal biological productivity results in significant oscillations in sulfate aerosol sources (Ayers & Gras, 1991; Humphries et al., 2016; O'Dowd et al., 1997; Shaw, 1988) that appear to drive variability in cloud properties across the entire oceanic basin between winter and summer (Mace & Avey, 2017; D. T. McCoy et al., 2015; I. L. McCoy et al., 2020). Liquid clouds within this environment respond to large‐scale meteorological forcing and moisture transports (Field & Wood, 2007; Govekar et al., 2011; Kelleher & Grise, 2019; Klein et al., 2017; D. T. McCoy et al.…”

“…significant oscillations in sulfate aerosol sources (Ayers & Gras, 1991;Humphries et al, 2016;O'Dowd et al, 1997;Shaw, 1988) that appear to drive variability in cloud properties across the entire oceanic basin between winter and summer (Mace & Avey, 2017;D. T. McCoy et al, 2015;I. L. McCoy et al, 2020).…”