Physical phenology of air-sea heat budget for the Beaufort Sea autumn freeze-up

Recent observations in the Arctic Ocean have confirmed predictions about fifty years ago that the Arctic is the Earth's most sensitive region to climate change. Evident phenomena in the Beaufort Sea include an average of 1-2 days delay per year of the autumn freeze-up. In the same fashion, open-water period during which the Arctic is sea ice-free is lengthening, leading to notable changes in air-sea heat, momentum and chemical fluxes. This study aims to examine the air-sea energy budget by a combination of three reanalyses (ERA5, MERRA-2, and CFSv2) and observations from a recent field campaign in the Beaufort Sea (NASA SASSIE cruise in September 2022). Obvious in all of these four data sources, the accumulative net air-sea heat flux reaches its minimum at the autumn equinox, indicating ocean's transition from heat gain to loss. This onset of ocean cooling is in part owing to the gradual reduction of incoming shortwave radiance, and in part on account of more intense outgoing longwave radiation and turbulent heat flux released from the sea surface. We show that conducting processes measured by air-sea temperature difference play a crucial role in shaping turbulent heat fluxes, while wind speed is not as important. In terms of the entrant buoyancy flux, beta-effect (salinity-driven) dominates over alpha (temperature-determined) in water mass transformation, highlighting the importance of more accurate predictions in sea ice activities and understanding in air-ocean-ice coupled dynamics.