A Changing Arctic Ocean: How Measured and Modeled 129I Distributions Indicate Fundamental Shifts in Circulation Between 1994 and 2015

I-129 measurements on samples collected during GEOTRACES oceanographic missions in the Arctic Ocean in 2015 have provided the first synoptic I-129 sections across the Eurasian, Canada, and Makarov Basins. During the 1990s, increased discharges of I-129 from European nuclear fuel reprocessing plants produced a large, tracer spike whose passage through the Arctic Ocean has been followed by I-129 time series measurements over the past 25 years. Elevated I-129 levels measured over the Lomonosov and Alpha-Mendeleyev Ridges in 2015 were associated with tracer labeled, Atlantic-origin water bathymetrically steered by the ridge systems through the central Arctic while lower I-129 levels were evident in the more poorly ventilated basin interiors. I-129 levels of 200-400 x 10(7) at/l measured in intermediate waters had increased by a factor of 10 in 2015 compared to 1994-1996 owing to the circulation of the 1990s, I-129 input spike. Comparisons of I-129 distributions between the mid-1990s and 2015 delineate large scale circulation changes that occurred during the shift from a positive Arctic Oscillation and a cyclonic circulation regime in the mid-1990s to anticyclonic circulation in 2015. The latter is characterized by a broadened Beaufort Gyre in the upper ocean, a weakened boundary current and partial mid-depth, AW flow reversal in the southern Canada Basin. Tracer I-129 simulations using the applied circulation model, NAOSIM, agree with both historical I-129 results and recent GEOTRACES data sets, thereby supporting the present interpretation of the relationship of changes in arctic circulation to shifts in climate indices revealed by tracer I-129 distributions.