Preprints
https://doi.org/10.5194/acp-2022-571
https://doi.org/10.5194/acp-2022-571
18 Aug 2022
18 Aug 2022
Status: a revised version of this preprint was accepted for the journal ACP and is expected to appear here in due course.

Natural Marine Cloud Brightening in the Southern Ocean

Gerald G. Mace1, Sally Benson1, Ruhi Humphries2,3, Mathew Peter Gombert1, and Elizabeth Sterner1 Gerald G. Mace et al.
  • 1Department of Atmospheric Sciences, University of Utah, Salt Lake City, Utah
  • 2Climate Science Centre, CSIRO Oceans and Atmosphere, Melbourne, Australia
  • 3Australian Antarctic Program Partnership, Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, Australia

Abstract. The number of cloud droplets per unit volume (Nd) is a fundamentally important property of marine boundary layer (MBL) liquid clouds that, at constant liquid water path, exerts considerable controls on albedo. Past work has shown that regional Nd has direct correlation to marine primary productivity (PP) because of the role of seasonally-varying biogenically-derived precursor gasses in modulating secondary aerosol properties. These linkages are thought to be observable over the high latitude oceans where strong seasonal variability in aerosol and meteorology covary in mostly pristine marine environments. Here, we examine Nd variability derived from five years of MODIS level 2 derived cloud properties in a broad region of the summertime Eastern Southern Ocean and adjacent marginal seas. We demonstrate both latitudinal, longitudinal, and temporal gradients in Nd that are strongly correlated with the passage of air masses over regions of high PP waters that are mostly concentrated along the Antarctic Shelf poleward of 60° S. In particular we find that the albedo of MBL clouds in the latitudes south of 60° S is significantly higher than similar LWP clouds north of this latitude.

Gerald G. Mace et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-571', Anonymous Referee #1, 07 Sep 2022
    • AC1: 'Reply on RC1', Gerald Mace, 03 Nov 2022
  • RC2: 'Comment on acp-2022-571', Anonymous Referee #2, 09 Sep 2022
    • AC2: 'Reply on RC2', Gerald Mace, 03 Nov 2022

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-571', Anonymous Referee #1, 07 Sep 2022
    • AC1: 'Reply on RC1', Gerald Mace, 03 Nov 2022
  • RC2: 'Comment on acp-2022-571', Anonymous Referee #2, 09 Sep 2022
    • AC2: 'Reply on RC2', Gerald Mace, 03 Nov 2022

Gerald G. Mace et al.

Gerald G. Mace et al.

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Executive editor
The Southern Ocean can be considered a region exhibiting pristine conditions as during the pre-industrial time. Thus, any changes in radiative forcing in this region can be attributed to natural factors. Feedbacks of ocean biological activity on Earth’s radiation budget have been put forward as the CLAW hypothesis (Charlson et al., 1987, https://www.nature.com/articles/326655a0). It implies that emissions of biogenic sulfur-containing compounds result in the formation of cloud condensation nuclei, which lead to higher cloud droplet number concentrations. Such clouds are more reflective and thus lead to a cooling effect. The current study provides satellite-based evidence of the increase droplet number concentrations and cloud reflectivity (‘albedo’) triggered by chlorophyll emissions, as a proxy for biological activity. Specifically, it demonstrates for the first time the extent to which the cloud albedo is modulated by biological factors as a function of latitude along the Antarctic shelf. While the study does not extend to discussing the subsequent feedbacks of cloud reflectivity to biological activity, it clearly demonstrates how biological ocean activity affects cloudiness above the Southern Ocean and thus may regulate temperature.
Short summary
The number cloud droplets per unit volume is a significantly important property of clouds that controls their reflective properties. Computer models of the Earth's atmosphere and climate have low skill at predicting the reflective properties of Southern Ocean clouds. Here we investigate the properties of those clouds using satellite data and find that the cloud droplet number in the Southern Ocean is related to the oceanic phytoplankton abundance near Antarctica.
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