A synthesis of observations of aerosol-cloud interactions over the pristine, biologically active Southern Ocean and their implications for global climate model predictions
Authors: McCoy Isabel L. , McCoy Daniel T. , Wood Robert , Bretherton Christopher S. , Regayre Leighton , Watson-Parris Duncan , Grosvenor Daniel P. , Gettelman Andrew , Bardeen Charles G. , Mulcahy Jane P. , Hu Yongxiang , Bender Frida A.-M. , Field Paul R. , Carslaw Kenneth S. , Gordon Hamish 
Published: March 2021

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&lt;p&gt;The change in planetary albedo due to aerosol-cloud interactions (aci) during the industrial era is the leading source of uncertainty in inferring&amp;#160;Earth's&amp;#160;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.&lt;/p&gt;
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&lt;p&gt;First, cloud droplet number concentration (&lt;em&gt;N&lt;sub&gt;d&lt;/sub&gt;&lt;/em&gt;) is used as an indicator of aci. Global climate models (GCMs) show that the&amp;#160;hemispheric contrast&amp;#160;in liquid cloud &lt;em&gt;N&lt;sub&gt;d&lt;/sub&gt;&lt;/em&gt; between the&amp;#160;pristine SO&amp;#160;and the polluted&amp;#160;Northern Hemisphere observed in the present-day can be used&lt;strong&gt; &lt;/strong&gt;as&amp;#160;a proxy for the increase&amp;#160;in &lt;em&gt;N&lt;sub&gt;d&lt;/sub&gt;&lt;/em&gt; from&amp;#160;the pre-industrial. A hemispheric difference constraint&amp;#160;developed from MODIS satellite observations indicates that pre-industrial&amp;#160;&lt;em&gt;N&lt;sub&gt;d&lt;/sub&gt;&lt;/em&gt; may have been higher than previously thought and provides an estimate of radiative forcing associated with aci between -1.2 and -0.6 Wm&lt;sup&gt;-2&lt;/sup&gt;. Comparisons with MODIS &lt;em&gt;N&lt;sub&gt;d &amp;#160;&lt;/sub&gt;&lt;/em&gt;highlight significant GCM discrepancies in pristine, biologically active regions.&lt;/p&gt;
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&lt;p&gt;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 &lt;em&gt;N&lt;sub&gt;d&lt;/sub&gt;&lt;/em&gt; against precipitation removal by small, Aitken mode aerosols entrained from the free troposphere. The latter may significantly contribute to the high, summertime SO &lt;em&gt;N&lt;sub&gt;d&lt;/sub&gt;&lt;/em&gt; 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 &lt;em&gt;N&lt;sub&gt;d &amp;#160;&lt;/sub&gt;&lt;/em&gt;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.&lt;/p&gt;
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