| Summary: | Analysis of modern and historical observations demonstrates that the temperature of the intermediate-depth (150–900 m) Atlantic water (AW) of the Arctic Ocean has increased in recent decades. The AW warming has been uneven in time; a local;18C maximum was observed in the mid-1990s, followed by an interveningminimumand an additional warming that culminated in 2007with temperatures higher than in the 1990s by 0.248C. Relative to climatology from all data prior to 1999, the most extreme 2007 temperature anomalies of up to 18C and higher were observed in the Eurasian andMakarov Basins. The AWwarming was associated with a substantial (up to 75–90 m) shoaling of the upper AW boundary in the central Arctic Ocean and weakening of the Eurasian Basin upper-ocean stratification. Taken together, these observations suggest that the changes in the Eurasian Basin facilitated greater upward transfer of AW heat to the ocean surface layer. Available limited observations and results from a 1D ocean column model support this surmised up-ward spread of AW heat through the Eurasian Basin halocline. Experiments with a 3D coupled ice–ocean
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