Quantifying the Role of Sea Ice in Seasonal and Interannual Variability in Surface Stress and Mixed-Layer Entrainment in the Central Canada Basin

Abstract. Over the last few decades, the Beaufort Gyre region (BGR) has experienced significant ice loss, leading to changes in momentum transfer within the atmosphere-ice-ocean system. We evaluate monthly and interannual variability of surface stress in the BGR, and a sub-region (the Central Canada Basin, CCB) within it, from 2003–2023. We find an increase in surface-ocean stress magnitude in the CCB of 0.007 ± 0.001 N m⁻² per decade, with the largest linear increases in November–December. We demonstrate that the shifting sea-ice regime, characterized by greater mobility and increased responsiveness to wind forcing, is the primary driver of increased surface-ocean stress, as opposed to changes in the wind field itself. Anomalously High-Stress Events (HSEs, characterized by stress magnitudes an order of magnitude larger than background levels) in the CCB occur most frequently in fall and have increased in frequency over 2003–2023, contributing to an increase in annual time-integrated surface stress of 0.54 ± 0.45 N m⁻² per decade during HSEs. We examine the implications of increased sea-ice mobility and changing surface-ocean stresses on the ocean mixed layer using an energy-balance entrainment parameterization. We find that stress-driven entrainment can account for ∼65 % of October–March deepening, while buoyancy-driven processes contribute ∼7 % of deepening during freeze-up. Mechanical mixing during HSEs contributes to ∼28 % of all stress-induced mixing, even though HSEs occur less than 7 % of the time during the 2003–2023 period. In November and December, the only months with statistically significant increases in surface stress, we estimate that increased surface stress can account for ∼0.15 and 0.1 m year⁻¹ more deepening in each month over 2003–2019. This is ∼20 % of the observed ∼0.7 and 0.6 m year⁻¹ increased deepening in November and December over the CCB for 2003–2019, and we infer that other physical processes dominate the observed mixed-layer deepening trend. These results highlight how sea-ice decline and mobility enhance momentum transfer, suggesting a potential mechanism for a positive feedback via increased ocean-to-ice heat fluxes.