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

129 I measurements on samples collected during GEOTRACES oceanographic missions in the Arctic Ocean in 2015 have provided the first synoptic 129 I sections across the Eurasian, Canada, and Makarov Basins. During the 1990s, increased discharges of 129 I from European nuclear fuel reprocessing plants produced a large, tracer spike whose passage through the Arctic Ocean has been followed by 129 I time series measurements over the past 25 years. Elevated 129 I 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 129 I levels were evident in the more poorly ventilated basin interiors. 129 I levels of 200–400 × 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, 129 I input spike. Comparisons of 129 I 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 129 I simulations using the applied circulation model, NAOSIM, agree with both historical 129 I 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 129 I distributions.