Natural Marine Cloud Brightening in the Southern Ocean

: The number of cloud droplets per unit volume ( N d ) is a fundamentally 23 important property of marine boundary layer (MBL) liquid clouds that, at constant liquid 24 water path, exerts considerable controls on albedo. Past work has shown that regional 25 N d has direct correlation to marine primary productivity (PP) because of the role of 26 seasonally-varying biogenically-derived precursor gasses in modulating secondary 27 aerosol properties. These linkages are thought to be observable over the high latitude 28 oceans where strong seasonal variability in aerosol and meteorology covary in mostly 29 pristine marine environments. Here, we examine N d variability derived from five years of 30 MODIS level 2 derived cloud properties in a broad region of the summertime Eastern 31 Southern Ocean and adjacent marginal seas. We demonstrate both latitudinal, 32 longitudinal, and temporal gradients in N d that are strongly correlated with the passage of 33 air masses over regions of high PP waters that are mostly concentrated along the 34 Antarctic Shelf poleward of 60°S. In particular we find that the albedo of MBL clouds in 35 the latitudes south of 60°S is significantly higher than similar LWP clouds north of this 36 latitude. 37