Changes in water mass composition and circulation in the central Arctic Ocean between 2011 and 2021 inferred from tracer observations

Abstract. The Arctic Ocean is changing rapidly and Atlantic Water circulation plays a key role in the warming, sea-ice decline, and ecosystem changes observed in the Arctic. Still, we only have limited understanding of the pathways and circulation times of Atlantic-derived water both at surface and mid-depth layers in the Arctic Ocean, and their evolution over time. Here, we investigate the water mass composition and circulation in the central Arctic Ocean in 2021 and assess temporal changes thereof between 2011 and 2021 by using the long-lived anthropogenic radionuclides 129I and 236U. This study is based on radionuclide data collected in the central Arctic Ocean and for the first time north of Greenland as part of one of the ocean expeditions of the Synoptic Arctic Survey (SAS-Oden 2021), and available historic data across the Arctic Ocean between 2011 and 2021. We obtain tracer ages as well as the mixing of different endmembers in the surface layer using a tracer-based mixing model. Atlantic Water circulation times and mixing in the mid-depth Atlantic layer are obtained from the Transit Time Distribution (TTD) model. For 2021, we find a sharp decrease in surface 129I and 236U concentrations between the Amundsen and Makarov Basins, pointing to substantial fractions of Pacific Water reaching the Lomonosov Ridge from the Amerasian side. In the halocline layer, similar 129I and 236U concentrations on both sides of the Lomonosov Ridge suggest a common 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. Circulation times of Atlantic Water in the mid-depth layer point to a longer transport route on the Makarov Basin side of the Lomonosov Ridge compared to the Amundsen Basin. When looking at the temporal variability between 2011 and 2021, we observe a shift of the Atlantic-Pacific Water front from the Makarov Basin towards the Lomonosov Ridge from 2011/2012 to 2015 and 2021. In the mid-depth Atlantic layer, we find an increase in mean and mode ages from 2015 to 2021, suggesting a slowdown or changes in the pathways of the Arctic Ocean Boundary Current, which is in line with recent studies based on gas tracers.