Fig. 5: Schematic diagrams showing the dynamic changes of the upper mixed layer under different sea ice regimes.

a Seasonal mixed layer evolution under the climatology of sea ice and b the new normal of low sea ice. c The battle between wind-induced turbulent kinetic energy (TKE) input and surface buoyancy flux, including air–sea heat (\({Q}_{{\mbox{T}}}\)) and freshwater fluxes (P–E) and sea-ice-derived meltwater in the summertime under the climatology of sea ice and d the new normal of low sea ice. The layer from the sea surface to the white dotted line represented the upper mixed layer. The spiral lines below the mixed layer represented the process of turbulent mixing driven by wind stirring. The density of spirals implied the mean strength of turbulent kinetic energy. The variability of monthly wind stress was indicated by different colors, with red representing regional significantly increased, blue representing regional significantly decreased, and gray representing no significant changes. The traditional viewpoints of enhanced upper ocean stratification are caused by surface warming and ocean freshening with rapid sea ice loss (a and c). Under the new normal of low summer sea ice (b and d), effective wind mixing in a prolonged open water period triumphed the overall buoyancy forcing with reduced meltwater input, which deepened the mixed layer.