The Arctic is warming at an alarming rate, more than four times the global average, resulting in significant sea ice loss. This reduction in sea ice extent dramatically alters the mechanical and thermodynamic coupling between the atmosphere and the ocean. Wind-driven surface currents and vertical mixing are key ocean processes, and in the Arctic, sea ice acts as a mediator, either amplifying or dampening momentum transfer depending on ice pack rigidity, concentration, and surface/bottom roughness. The continued retreat of sea ice is expected to make the Arctic Ocean more energetic, increasing turbulent mixing. The 'ice-ocean stress governor', a negative feedback mechanism limiting wind stress impact, will likely weaken with thinner, less compact ice cover. At low ice concentrations, sea ice behaves like freely flowing particles (free drift), while higher concentrations create a more compact pack, restricting flow and converting wind energy into internal stress. Ice deformation, through ridging, dissipates kinetic energy, making internal stress a key factor in determining wind-to-ocean momentum transfer. Climate models project continued sea ice decline and increased wind speeds, but the combined impact on Arctic Ocean surface stress and circulation remains unclear. This study analyzes the atmosphere-ice-ocean momentum budget using data from 16 state-of-the-art climate models from CMIP6 to assess future Arctic Ocean surface momentum balance and seasonality, using NorESM2-MM for detailed analysis due to its comprehensive output.

Previous research has highlighted the rapid warming and sea ice loss in the Arctic, emphasizing the impact on atmosphere-ocean coupling. Studies have shown increased turbulent mixing and suggested that the continued sea ice retreat could lead to a more energetic Arctic Ocean. The concept of an 'ice-ocean stress governor' has been proposed, describing a negative feedback mechanism where sea ice limits the effect of wind stress on the ocean. However, this mechanism is expected to become less effective with decreasing sea ice extent. Existing literature also describes the differing behavior of sea ice at various concentrations, from free drift at low concentrations to a more rigid, stress-absorbing pack at higher concentrations. While projections of increasing wind speeds exist, the combined effect of these changes on Arctic Ocean surface stress and circulation has remained under-investigated. This study builds upon these existing works by providing a comprehensive analysis of future projections using a large ensemble of climate models.

This study utilizes monthly mean model output from 16 CMIP6 climate models, chosen for their diverse range of characteristics and good bathymetry representation in the Arctic region. The models provide data on surface wind speed, sea ice concentration and thickness, ocean and atmospheric wind stress, ice and ocean velocity, and, in some cases, stress components at the ice top and bottom, and internal ice stress. NorESM2-MM is used as a case study model due to its comprehensive output, including detailed stress components and vertical mixing data. The high-emission scenario (ssp585) is used to clearly identify climate change signals. The Arctic Ocean region is defined to include the deep central Arctic and shelf seas. Seasonal averages are calculated for winter (January-March), summer (June-August), and fall (September-November). Trends in ocean stress and wind speed are calculated from 2015-2060. Statistical significance is assessed at the 95% confidence level. Ocean surface stress is decomposed into contributions from atmosphere-ocean stress and ice-ocean stress, the latter further divided into atmosphere-ice stress and internal ice stress in NorESM2-MM. The ‘dampening effect’ metric, inspired by a previous study’s ‘amplification index’, quantifies energy loss due to sea ice dynamics, comparing integrated atmospheric wind stress to integrated ocean surface stress. Ekman pumping velocity is calculated using the curl of surface stress, and liquid freshwater content is calculated relative to a reference salinity. Ocean kinetic energy is calculated as the integrated kinetic energy down to 100 m. All computations are performed on the models' native grids.

Multi-model ensemble mean results show consistent increases in Arctic Ocean surface stress across all seasons, with the largest increase in winter (+5.1% per decade). While wind speeds increase most during fall (+2.2% per decade), the surface stress increase is amplified in winter due to reduced internal ice stress. Analysis using NorESM2-MM reveals that the positive trends in total ocean stress are driven by increased total atmosphere-ocean stress (due to both reduced ice area and increased atmosphere-ocean stress from higher wind speed) and, surprisingly, an increase in total ice-ocean stress in late winter and spring. This latter increase is attributed to reduced dissipation of atmospheric stress into internal ice stress due to the thinning ice pack. The model ensemble shows large differences in mean ocean surface stress and wind speed, attributable to differences in sea ice area among the models. However, the climate response (trends) of both wind speed and surface stress were consistent across models, with no correlation between the mean state and the trend strength. Seasonal changes are also significant: the annual cycle amplitude of sea ice concentration decreases, whereas that of wind speed increases. The amplitude of ocean stress also increases, with a shift in the peak stress from December to February. The 'dampening effect' of sea ice (energy loss) is consistently positive across models, indicating that sea ice dampens momentum transfer. However, this dampening effect is consistently reduced in all models as sea ice declines in the future. The reduction in sea ice thickness (85% in fall and 78% in winter) supports the findings regarding reduced internal ice stress. Sea ice speed increases in the annual mean, with an increase of 30% projected by the end of the century. Increased ocean surface stress results in stronger upper ocean circulation and significantly increased surface velocity (29% in winter and 13% in fall by 2100). NorESM2-MM shows a substantial increase in vertical mixing (74% in fall and 48% in winter by 2100), impacting nutrient distribution and ecosystem dynamics. Finally, enhanced surface stress increases Beaufort Gyre Ekman pumping (33% in winter) and freshwater content in all seasons, influencing Arctic Ocean circulation and potentially the North Atlantic.

This study's findings confirm the significant impact of projected sea ice decline and increased wind speeds on Arctic Ocean dynamics. The amplified winter increase in surface stress is a key finding, explained by the reduced dissipation of wind energy into internal ice stress due to a weaker ice pack. This interaction of wind and ice underscores the complex interplay of processes within the Arctic system. The results have implications for Arctic ecosystems, as increased vertical mixing affects nutrient distribution and primary productivity. The altered Beaufort Gyre dynamics also have broader consequences, potentially impacting global ocean circulation. The consistent trends across models, despite the differences in mean state, highlight the robustness of the overall climate response. However, uncertainties exist due to variations in model formulations and the simplified representation of ice-ocean drag. Future studies should focus on improving model coupling of these processes and include more realistic representations of sea ice internal stress and form drag, as well as the incorporation of spatial and temporal variability in sea ice surface and bottom roughness.

This research demonstrates the substantial impacts of projected changes in the Arctic Ocean momentum budget, highlighting the future increase in Arctic Ocean surface stress due to the combined effects of increasing wind speeds and declining sea ice. The amplified winter response, explained by reduced internal ice stress, is a crucial finding. The changes in ocean circulation, vertical mixing, and Beaufort Gyre dynamics have profound consequences for Arctic ecosystems and global climate. Further research is needed to reduce uncertainties associated with model parameterizations and improve the representation of complex interactions at the atmosphere-ice-ocean interface. Continued monitoring and improved modeling are crucial to understand the full extent of these changes and their implications.

The study relies on climate model outputs, which have inherent limitations and uncertainties related to parameterizations of complex processes like sea ice dynamics and momentum transfer. Variations in model formulations and the simplified representation of ice-ocean drag introduce uncertainties, especially in quantifying the contributions of individual components to the overall stress response. The availability of detailed model output varied among models, limiting some analyses, such as decomposition of surface stress and detailed mixing information. Finally, the reliance on a high-emission scenario limits the generalizability to other emission scenarios.