Impact of Arctic sea‐ice retreat on the recent change in cloud‐base height during autumn

Abstract

Cloud‐base observations over the ice‐free Chukchi and Beaufort Seas in autumn were conducted using a shipboard ceilometer and radiosondes during the 1999–2010 cruises of the Japanese R/V Mirai. In comparison with cloud‐base heights in an ice‐covered case (the Surface Heat Budget of the Arctic Ocean project in 1998), our ice‐free results showed a 30% decrease (increase) in the frequency of low clouds with a ceiling below (above) 500 m. Temperature profiles revealed that the boundary layer was well developed over the ice‐free ocean in the 2000s, whereas a stable layer dominated during the ice‐covered period in 1998. The change in surface boundary conditions likely resulted in the difference in cloud‐base height, although it had little impact on air temperatures in the mid‐ and upper troposphere. Data from the 2010 R/V Mirai cruise were investigated in detail in terms of air‐sea temperature difference. Stratus clouds near the sea surface were predominant under a warm advection situation, whereas stratocumulus clouds with a cloud‐free layer were significant under a cold advection situation. The threshold temperature difference between sea surface and air temperatures for distinguishing the dominant cloud types was 3 K. Anomalous upward turbulent heat fluxes associated with the sea‐ice retreat have likely contributed to warming of the lower troposphere.