Abrupt Arctic Sea Ice Decline on Synoptic Timescales during Summer: Physical Processes and Background Climate Impacts

The physical processes of abrupt Arctic sea ice decline events on synoptic timescales in summer, and their dependence on the climatic background, remain insufficiently understood. This study investigates these events in the Laptev Sea and the Eastern Siberian–Chukchi–Beaufort (EsCB) Seas by comparing two periods: 1979–2000 (P1) and 2001–2021 (P2). Using daily sea ice data from the US National Snow and Ice Data Center (NSIDC) and the fifth generation ECMWF atmospheric reanalysis (ERA5), 30 (31) events in the Laptev Sea and 29 (22) in the EsCB Seas during P1 (P2) are selected, and composite analysis is performed to reveal the underlying mechanisms. The results show that: (1) abrupt decline is consistently forced by an atmospheric zonal dipole pattern, comprising an upstream cold cyclonic anomaly and a downstream warm anticyclonic anomaly; (2) this dipole regulates surface energy fluxes—the anticyclone reduces low-level cloud cover and enhances downward shortwave radiation (DSR), initiating sea ice melt, whereas the cyclonic–anticyclonic anchor zone fosters poleward transport of warm, moist air, increasing total column water and downward longwave radiation (DLR); (3) while DSR dominates the entire sea ice loss process in the Laptev Sea during both periods, the dominant energy source in the EsCB Seas shifts from DLR and surface turbulent heat flux (STHF) in P1 to DSR and STHF in P2; (4) this shift is attributed to a more compact and persistent dipole structure under the negative phase of the Pacific Decadal Oscillation (PDO) in P2, which exacerbates regional sea ice loss. These findings underscore the synergistic role of synoptic atmospheric dynamics and background climatic variability in driving rapid Arctic sea ice decline.