Seasonal Variations and Spatial Patterns of Arctic Cloud Changes in Association with Sea-Ice Loss during 1950-2019 in ERA5

Abstract The dynamic and thermodynamic mechanisms that link retreating sea ice to increased Arctic cloud amount and cloud water content are unclear. Using the fifth generation of the ECMWF Reanalysis (ERA5), the long-term changes between years 1950-1979 and 1990-2019 in Arctic clouds are estimated along with their relationship to sea-ice loss. A comparison of ERA5 to CERES satellite cloud fractions reveals that ERA5 simulates the seasonal cycle, variations, and changes of cloud fraction well over water surfaces during 2001-2020. This suggests that ERA5 may reliably represent the cloud response to sea-ice loss because melting sea ice exposes more water surfaces in the Arctic. Increases in ERA5 Arctic cloud fraction and water content are largest during October-March from ~950-700 hPa over areas with significant (≥15%) sea-ice loss. Further, regions with significant sea-ice loss experience higher convective available potential energy (~2-2.75 J kg −1 ), planetary boundary layer height (~120-200 m) and near-surface specific humidity (~0.25-0.40 g kg −1 ) and a greater reduction of the lower tropospheric temperature inversion (~3-4 °C) than regions with small (<15%) sea-ice loss in autumn and winter. Areas with significant sea-ice loss also show strengthened upward motion between 1000-700 hPa, enhanced horizontal convergence (divergence) of air, and decreased (increased) relative humidity from 1000-950 hPa (950-700 hPa) during the cold season. Analyses of moisture divergence, evaporation minus precipitation, and meridional moisture flux fields suggest that increased local surface water fluxes, rather than atmospheric motions, provide a key source of moisture for increased Arctic clouds over newly exposed water surfaces from October-March.