The seasonal cycle of the Arctic Ocean in a summer ice-free climate : changes, driving processes and consequences.

The Arctic Ocean is a hot spot of climate change, with enhanced warming and freshening of near-surface waters and a rapid decline of sea ice in recent decades. Climate model projections suggest that the Arctic Ocean may be ice-free in summer as early as 2030-2050, accompanied by an intensified seasonal cycle of sea ice characterized by earlier melting and later growth seasons. This transition will enhance interactions between the ocean, atmosphere and sea ice, likely altering the stratification of the Arctic Ocean during summer. The projected retreat of summer sea ice in the coming decades raises the question of how the seasonal cycle of the ocean may change, which is critical in regulating chemical, biological and physical processes in the region. Given the non-uniformity of sea ice loss across the Arctic, pan-Arctic averages fail to capture the spatial variability of these changes. In this study, we analyze 36 climate models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) under the SSP5-8.5 scenario to characterize regional changes in the Arctic Ocean seasonal cycle in the near future. Our results reveal an intensified seasonal cycle of sea surface temperature and a weakened seasonal cycle of sea surface salinity with significant regional variability and model dependence. Changes at depth are primarily confined to the mixed layer. By analyzing the mixed layer temperature and salinity budget for each region, we identify the key processes driving these changes. These insights enhance our understanding of the evolving seasonal dynamics of the Arctic Ocean and their broader implications in a rapidly changing climate.