Effect of Arctic clouds on the ice-albedo feedback in midsummer

The Arctic clouds should be an important factor that affects the summertime sea ice. By reflecting the incoming solar radiation before it reaches the surface, the Arctic clouds may prevent the surface from absorbing tremendous solar radiation due to the reduced sea ice. This cloud effect will lead to intervene the feedback relation between the solar radiation and the sea ice change. However, few studies have quantitatively investigated the Arctic cloud effect on the ice-albedo feedback. This study found that the Arctic clouds regulate the melting speed of sea ice in midsummer months (June to August) based on the data from multiple sources, that is, satellite, reanalysis, and climate models. During this period, the fraction of Arctic clouds with the net radiative cooling effect is almost invariable with sea ice reduction. However, despite of the steady cloud fraction in the midsummer months, the shortwave cloud radiative effect (total-sky minus clear-sky absorbed shortwave radiation) was found to significantly increase with the reduced sea ice concentration (0.64 W m⁻²%⁻¹ in CERES, 0.73 W m⁻²%⁻¹ in ERA5). This is because the clouds present more contrast of albedo with the sea ice-free ocean than the sea ice-covered ocean. Finally, our analyses show that the Arctic clouds are nearly halving the strength of the ice-albedo feedback in the midsummer months. These results imply that the sea ice reduction could have been much faster in the past decades in the absence of the cloud effect found here.