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

Abstract. The world's oceans are responding to anthropogenically induced climate change, with the Arctic Ocean being identified as one of the most rapidly changing regions. Quantifying the circulation and mixing timescales of Atlantic-origin waters in the Fram Strait – the primary gateway for Arctic-Atlantic exchange – is essential for understanding the evolving connectivity between the Arctic and the subpolar North Atlantic. This study utilizes the anthropogenic radionuclide tracer pair, Iodine-129 (¹²⁹I) and Uranium-236 (²³⁶U), to investigate the origin and transit history of water masses sampled between 2016 and 2021. By applying a consistent methodological framework using both binary mixing and Transit Time Distribution (TTD) models to surface Polar Water and mid-depth Arctic Atlantic Water, we assess the temporal stability of the regional circulation regime. Our results reveal significant interannual variability. Waters outflowing the Fram Strait in 2020 exhibited a higher degree of 10 mixing and a stronger influence from Amerasian Basin sourced waters compared to 2016 and 2021. We identify a distinct water parcel on the Greenland Shelf with a tracer signature indicating a long-path circulation from the Canada Basin in 2020. Finally, we find a tendency towards elevated circulation timescales in 2021 that are related to either slower circulation timescales or longer circulation pathways. These findings highlight the importance of further assessing the temporal evolution of Atlantic Waters arriving in Fram Strait as the Atlantic layer brings heat to the Arctic Ocean.