Changes in Atlantic Water circulation in the central Arctic Ocean between 2011 and 2021 inferred from tracer observations

Atlantic Water circulation and associated changes play a crucial role in the future of the Arctic Ocean, especially in light of ongoing "Atlantification" trends. Still, the pathways, mixing properties, and circulation times of Atlantic Water in the Arctic Ocean remain to be better understood in view of recent variability and trends. Here, we use the long-lived anthropogenic radionuclides I129 and U236 to investigate mixing between Atlantic and Pacific Waters in the surface layer and to determine Atlantic Water circulation times and mixing in the mid-depth Atlantic layer of the Arctic Ocean using the Transit Time Distribution (TTD) model. The study is mainly based on radionuclide data that has been collected in the central Arctic Ocean during the "SAS-Oden 2021" expedition aboard the Swedish research icebreaker Oden, which was part of the Synoptic Arctic Survey (SAS) programme. Furthermore, to assess temporal changes in the circulation pattern and circulation times of Atlantic Water between 2011 and 2021, we use available historic data on I129 and U236 between 2011 and 2021. For 2021, we find a sharp decrease in surface I129 and U236 concentrations between the Amundsen and Makarov Basins, pointing to significant fractions of Pacific Water reaching the Lomonosov Ridge from the Amerasian side. In the halocline layer below, similar and comparably high I129 and U236 concentrations suggest a similar formation region of halocline waters with a clear Atlantic Water signal. North of Greenland, we find a mixture of waters that originate from the Canada and Amundsen basins, both in the surface and the mid-depth layer. The TTD model shows higher circulation times pointing to a longer transport route on the Makarov Basin side of the Lomonosov Ridge. Regarding temporal changes, we find a shift in the location of the Atlantic-Pacific Water front from 2011/12, when it was located further into the Makarov Basin, to 2015 and 2021. In the mid-depth Atlantic layer, mean and mode ages show an increase from 2015 to 2021, which is in line with recent studies based on gas tracers and suggests a slowdown or changes in the pathways of the Arctic Ocean Boundary Current.