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

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 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 present 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 I-129 and U-236 in the Transit Time Distribution model. Key findings for 2021 include a decline in surface radionuclide concentrations between the Amundsen and Makarov Basins, pointing to substantial fractions of Pacific Water reaching the Lomonosov Ridge from the Amerasian side. Similar radionuclide concentrations in halocline waters on both sides of the Lomonosov Ridge suggest a common formation region of these waters with a clear Atlantic Water signal. North of Greenland, a mixture of waters from the Canada and Amundsen Basins is observed at both surface and mid-depth. Between 2011 and 2021, we observe a shift of the Atlantic-Pacific Water front from the Makarov Basin towards the Lomonosov Ridge and an increase in circulation times in the mid-depth Atlantic layer. Overall, our findings provide a baseline of the circulation of Atlantic-derived waters in 2021 and provide evidence of circulation changes both in the surface and intermediate waters between 2011 and 2021.