Natural marine cloud brightening in the Southern Ocean

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 N(d )has a direct correlation to marine primary productivity (PP) because of the role of seasonally varying, biogenically derived precursor gases 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 environments. Here, we examine N-d variability derived from 5 years of MODIS Level 2-derived cloud properties in a broad region of the summer eastern Southern Ocean and adjacent marginal seas. We demonstrate latitudinal, longitudinal and temporal gradients in N-d that are strongly correlated with the passage of air masses over high-PP waters that are mostly concentrated along the Antarctic Shelf poleward of 60 degrees S. We find that the albedo of MBL clouds in the latitudes south of 60 degrees S is significantly higher than similar liquid water path (LWP) clouds north of this latitude.

Automated summary

The number of cloud droplets per unit volume (Nd) is a crucial property of marine boundary layer (MBL) liquid clouds and has a significant impact on albedo at a constant liquid water path. Previous studies have shown a direct correlation between regional Nd and marine primary productivity (PP) due to the influence of seasonally varying, biogenically derived precursor gases on secondary aerosol properties. These connections are believed to be observable in high-latitude oceans, where aerosol and meteorology exhibit strong seasonal variability in mostly pristine environments. This study examines the variability of Nd derived from 5 years of MODIS Level 2-derived cloud properties in a broad region of the summer eastern Southern Ocean and adjacent marginal seas. The results demonstrate latitudinal, longitudinal, and temporal gradients in Nd that are strongly associated with the passage of air masses over high-PP waters, particularly along the Antarctic Shelf poleward of 60 degrees S. Furthermore, the albedo of MBL clouds in latitudes south of 60 degrees S is significantly higher than that of similar liquid water path (LWP) clouds north of this latitude.