The Combined Influence of Observed Southern Ocean Clouds and Sea Ice on Top-of-Atmosphere Albedo

摘要

When sea ice concentration decreases, surface albedo decreases. Yet the impact of Southern Ocean sea ice concentration decreases on top-of-atmosphere albedo is uncertain. Why? The cloud cover and opacity response to Southern Ocean sea ice variability has been challenging to quantify. Here we use observations to constrain the cloud response to Southern Ocean sea ice variability and assess the combined influence of sea ice and clouds on top-of-atmosphere albedo. We focus on the spring and summer seasons that dominate the high-latitude shortwave energy budget. To isolate the influence of sea ice concentration on clouds, we analyze spaceborne light detection and ranging (LIDAR) observations in regions where present-day sea ice concentration varies. During spring, low cloud cover is slightly (4%) higher over open water compared to sea ice. During summer, sea ice variability does not affect low cloud cover. During both spring and summer, cloud opacity is larger over open water than over sea ice due to a cloud phase shift from ice toward liquid with warming. Independent ship-based visual and radiosonde observations available during summer corroborate the LIDAR results. Even with the cloud response, satellite-observed top-of-atmosphere albedo is lower over open water than over sea ice. The observations show the cloud response to sea ice retreat with warming will not mask the surface albedo decrease. In other words, more shortwave radiation will be absorbed when Southern Ocean sea ice is lost.