Enhanced 20th-century heat transfer to the Arctic simulated in the context of climate variations over the last millennium

Oceanic heat transport variations, carried by the northward-flowing Atlantic Water, strongly influence Arctic sea-ice distribution, ocean-atmosphere exchanges, and pan-Arctic temperatures. Palaeoceanographic reconstructions from marine sediments near Fram Strait have documented a dramatic increase in Atlantic Water temperatures over the 20th century, unprecedented in the last millennium. Here we present results from Earth system model simulations that reproduce and explain the reconstructed exceptional Atlantic Water warming in Fram Strait in the 20th century in the context of natural variability during the last millennium. The associated increase in ocean heat transfer to the Arctic can be traced back to changes in the ocean circulation in the subpolar North Atlantic. An interplay between a weakening overturning circulation and a strengthening subpolar gyre as a consequence of 20th-century global warming is identified as the driving mechanism for the pronounced warming along the Atlantic Water path toward the Arctic. Simulations covering the late Holocene provide a reference frame that allows us to conclude that the changes during the last century are unprecedented in the last 1150 years and that they cannot be explained by internal variability or natural forcing alone.