Atlantic Water (AW) is the largest reservoir of heat in the Arctic Ocean, isolated from the surface and sea-ice by a strong halocline. In recent years AW shoaling and warming are thought to have had an increased influence on sea-ice in the Eurasian Basin. In this study we analyse 59000 profiles from across the Arctic from the 1970s to 2018 to obtain an observationally-based pan-Arctic picture of the AW layer, and to quantify temporal and spatial changes. The potential temperature maximum of the AW (the AW core) is found to be an easily detectable, and generally effective metric for assessments of AW properties, although temporal trends in AW core properties do not always reflect those of the entire AW layer. The AW core cools and freshens along the AW advection pathway as the AW loses heat and salt through vertical mixing at its upper bound, as well as via likely interaction with cascading shelf flows. In contrast to the Eurasian Basin, where the AW warms (by approximately 0.7 °C between 2002 and 2018) in a pulse-like fashion and has an increased influence on upper ocean heat content, AW in the Canadian Basin cools (by approximately 0.1 °C between 2008 and 2018) and becomes more isolated from the surface due to the intensification of the Beaufort Gyre. These opposing AW trends in the Eurasian and Canadian Basins of the Arctic over the last 40 years suggest that AW in these two regions may evolve differently over the coming decades.

Key Points

Atlantic Water is evolving in opposing ways in eastern and western sectors

Data suggest Atlantic Water cools during transit via vertical mixing at its upper bound and through interaction with cool dense shelf waters

Atlantic Water core temperature is generally effective in assessing Atlantic Water heat content but does not always capture temporal trends

Plain Language Summary

A few hundred meters beneath the surface of the Arctic Ocean lies a warm, salty layer of Atlantic origin, called Atlantic Water (AW), which is isolated from sea-ice and the ocean surface by a vertical salinity gradient that acts as a barrier between the AW and the surface. In recent years, weakening of this barrier and warming of AW in the eastern Arctic have contributed to unprecedented sea-ice loss. This study analyses 59000 vertical temperature and salinity profiles from the Arctic Ocean from the 1970s to 2018 to obtain a broad picture of the AW and its variability. The AW temperature maximum is found to be an easily observable, generally effective way to assess how much heat is stored in the AW layer. Over the period studied, the AW in the eastern Arctic warmed and had an increasing influence on the amount of heat in the surface layer, whereas AW heat became increasingly isolated from the surface in the west due to changes in local winds. The emergence of a characteristically different eastern and western Arctic Ocean in the future could have important consequences, both in terms of Arctic sea-ice loss and global ocean circulation.