EGU21-8499, updated on 04 Mar 2021
EGU General Assembly 2021
© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

A synthesis of observations of aerosol-cloud interactions over the pristine, biologically active Southern Ocean and their implications for global climate model predictions

Isabel L. McCoy1, Daniel T. McCoy2, Robert Wood1, Christopher S. Bretherton1, Leighton Regayre3, Duncan Watson-Parris4, Daniel P. Grosvenor3,5, Andrew Gettelman6, Charles G. Bardeen6, Jane P. Mulcahy7, Yongxiang Hu8, Frida A.-M. Bender9,10, Paul R. Field3,7, Kenneth S. Carslaw3, and Hamish Gordon11
Isabel L. McCoy et al.
  • 1University of Washington, Atmospheric Sciences Department, Seattle, United States of America
  • 2University of Wyoming, Department of Atmospheric Science, Laramie, United States of America
  • 3Institute for Climate and Atmospheric Science, University of Leeds, School of Earth and Environment, Leeds, United Kingdom
  • 4University of Oxford, Department of Physics, Oxford, United Kingdom
  • 5National Center for Atmospheric Science, University of Leeds, Leeds, United Kingdom
  • 6National Center for Atmospheric Research, Boulder, United States of America
  • 7Met Office, Exeter, United Kingdom
  • 8NASA Langley Research Center, Atmospheric Composition Branch, Hampton, United States of America
  • 9Stockholm University, Department of Meteorology, Stockholm, Sweden
  • 10Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden
  • 11Carnegie-Mellon University, College of Engineering, Pittsburgh, United States of America

The change in planetary albedo due to aerosol-cloud interactions (aci) during the industrial era is the leading source of uncertainty in inferring Earth's climate sensitivity to increased greenhouse gases from the historical record. Examining pristine environments such as the Southern Ocean (SO) helps us to understand the pre-industrial state and constrain the change in cloud brightness over the industrial period associated with aci. This study presents two methods of utilizing observations of pristine environments to examine climate models and our understanding of the pre-industrial state.

First, cloud droplet number concentration (Nd) is used as an indicator of aci. Global climate models (GCMs) show that the hemispheric contrast in liquid cloud Nd between the pristine SO and the polluted Northern Hemisphere observed in the present-day can be used as a proxy for the increase in Nd from the pre-industrial. A hemispheric difference constraint developed from MODIS satellite observations indicates that pre-industrial Nd may have been higher than previously thought and provides an estimate of radiative forcing associated with aci between -1.2 and -0.6 Wm-2. Comparisons with MODIS Nd  highlight significant GCM discrepancies in pristine, biologically active regions.

Second, aerosol and cloud microphysical observations from a recent SO aircraft campaign are used to identify two potentially important mechanisms that are incomplete or missing in GCMs: i) production of new aerosol particles through synoptic uplift, and ii) buffering of Nd against precipitation removal by small, Aitken mode aerosols entrained from the free troposphere. The latter may significantly contribute to the high, summertime SO Nd levels which persist despite precipitation depletion associated with mid-latitude storm systems. Observational comparisons with nudged Community Atmosphere Model version 6 (CAM6) hindcasts show low-biased SO Nd  is linked to under-production of free-tropospheric Aitken aerosol which drives low-biases in cloud condensation nuclei number and likely discrepancies in composition. These results have important implications for the ability of current GCMs to capture aci in pristine environments.

How to cite: McCoy, I. L., McCoy, D. T., Wood, R., Bretherton, C. S., Regayre, L., Watson-Parris, D., Grosvenor, D. P., Gettelman, A., Bardeen, C. G., Mulcahy, J. P., Hu, Y., Bender, F. A.-M., Field, P. R., Carslaw, K. S., and Gordon, H.: A synthesis of observations of aerosol-cloud interactions over the pristine, biologically active Southern Ocean and their implications for global climate model predictions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8499,, 2021.

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