Exceptional 20th Century Ocean Circulation in the Northeast Atlantic

The North Atlantic subpolar gyre (SPG) connects tropical and high‐latitude waters, playing a leading role in deep‐water formation, propagation of Atlantic water into the Arctic, and as habitat for many ecosystems. Instrumental records spanning recent decades document significant decadal variability in SPG circulation, with associated hydrographic and ecological changes. Emerging longer‐term records provide circumstantial evidence that the North Atlantic also experienced centennial trends during the 20th century. Here, we use marine sediment records to show that there has been a long‐term change in SPG circulation during the industrial era, largely during the 20th century. Moreover, we show that the shift and late 20th century SPG configuration were unprecedented in the last 10,000 years. Recent SPG dynamics resulted in an expansion of subtropical ecosystems into new habitats and likely also altered the transport of heat to high latitudes. Plain Language Summary The Northeast Atlantic is of crucial importance for the global climate system and marine ecosystems. We can use sediment from the bottom of the ocean to reconstruct how the Northeast Atlantic has changed over thousands of years. In this study, we present the first evidence that 20th century Northeast Atlantic surface ocean circulation was unusual compared to the last 10,000 years. This change caused a replacement of cool, subpolar waters with warmer subtropical waters near Iceland and has impacted the distribution of marine organisms. The most striking aspect of our work is the exceptional nature of the shift in the 20th century (in contrast to thousands of years of relative stability), with implications for understanding future change. Key Points Ocean sediments suggest that the Iceland Basin warmed during the industrial era Basin‐wide subpolar gyre circulation change contributed to warming during this period Late 20th century subpolar gyre state unprecedented in the last 10,000 years