Implications of shortwave cloud forcing and feedbacks in the Southern Ocean

Abstract

Measurements of the incident solar radiation taken during the Antarctic Remote Ice Sensing Experiment (ARISE) aboard the R/V Aurora Australis in the Southern Ocean and springtime Antarctic ice pack are analyzed together with all-sky cloud imagery to determine the incident shortwave cloud radiative forcing at the surface. For most solar zenith angles (Z < 82°) in this dataset, the primary shortwave cloud effect is to induce cooling of the surface; as the sun approaches the horizon, however, the shortwave effects become negligible or even positive. The clear-sky atmospheric transmissivity over the length of the cruise is 0.91, a value comparable to those derived from measurements taken at various locations in the Arctic during daylight periods. Although the presence of clouds has a great effect on the surface heat budget and provides a negative shortwave feedback that may stabilize the polar atmosphere, the effect on the photosynthetically active radiation available to ice algae is relatively small in comparison to the effects of even small amounts of snow on sea ice.