Enhanced water mass mixing in Fram Strait in 2020 and elevated circulation timescales of Atlantic-derived waters in 2021 based on transient tracers I-129 and U-236

The Arctic Ocean is undergoing rapid climate-driven change, making it increasingly important to quantify the circulation pathways and transit times of Atlantic Water entering and exiting the basin. Fram Strait, the primary gateway for Arctic–Atlantic exchange, provides a key location for assessing the evolving connectivity between the Arctic and the subpolar North Atlantic. Here, we combine the anthropogenic radionuclide tracer pair Iodine-129 ( 129 I) and Uranium-236 ( 236 U), with binary mixing and Transit Time Distribution (TTD) modelling to investigate the origin and transit history of surface Polar Water and mid-depth Atlantic Water sampled in Fram Strait between 2016–2021. Our results reveal significant interannual variability. Waters outflowing the Fram Strait in 2020 exhibited a higher degree of mixing and a stronger influence from Amerasian Basin sourced waters compared to 2016 and 2021. We further identify a distinct water parcel on the Greenland Shelf whose tracer signature indicates a long-path circulation originating from the Canada Basin. In contrast, waters sampled in 2021 exhibit generally longer transit times, consistent with either slower circulation or longer transport pathways. These results demonstrate substantial year-to-year variability in Arctic Ocean export through Fram Strait and highlight the need for sustained tracer observations to constrain changes in the circulation of the heat-bearing Atlantic Water layer and its role in Arctic–North Atlantic exchange.