The hemispheric contrast in cloud microphysical properties constrains aerosol forcing
Isabel L. McCoy, Daniel T. McCoy, Robert Wood, Leighton A. Regayre, Duncan Watson‐Parris, Daniel P. Grosvenor, Jane P. Mulcahy, Yongxiang Hu, Frida A.‐M. Bender, Paul R. Field, K. S. Carslaw, Hamish Gordon
Abstract
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.