Southern Ocean phytoplankton increases cloud albedo and reduces precipitation

Abstract

[1] Effects of natural and anthropogenic aerosol particles on the radiation budget in cloudy atmospheres are still a major research topic. For example, can an increase or decrease in aerosol particle number, originating from changed dimethylsulfide (DMS) and isoprene emissions by marine phytoplankton, impact the earth radiation budget via increasing or decreasing planetary albedo and lifetime of clouds? And if so, is a shifted cloud droplet spectrum accompanied by a regional change in precipitation? Here, we show by a synergistic analysis of satellite observations (MODIS, SeaWiFS, AIRS, SSM/I and CERES) that the phytoplankton related emission of the mentioned gases into the atmosphere strongly influences cloud properties within a broad latitude belt in the Southern Hemisphere during the austral summer. For this season we detected indirect aerosol effects over the Southern Ocean from 45°S to 65°S, especially in regions with plankton blooms, indicated by high chlorophyll-a concentration in seawater. The strong increase in cloud condensation nuclei column content from 2.0 × 108 to more than 5.0 × 108 CCN/cm2 for a chlorophyll increase from 0.3 to about 0.5 mg/m3 in these regions decreases cloud droplet effective radius and increases cloud optical thickness for water clouds. Consequently, the upward short-wave radiative flux at the top of the atmosphere increases. Our analysis also reveals reduced precipitation over the Antarctic Polar Frontal Zone during strong plankton blooms. We suggest that due to fine particles formed in the atmosphere originating from gaseous DMS and possibly isoprene emissions the reduction of precipitation is caused by delayed homogeneous freezing in water clouds.