Anomalous North Atlantic oceanography and overturning during the last 250 years

ATLAS work package 1 presentation at ATLAS 3rd General Assembly

The circulation of the Atlantic Ocean plays an essential role in climate and ecosystems through its redistribution of heat, nutrients and mixing, and its influence on the carbon cycle. However, short observational datasets preclude a longer-term perspective on the modern state and variability of key features, such as Labrador Sea convection, the Atlantic Meridional Overturning Circulation (AMOC) and gyre variability. Here, we provide several lines of paleoceanographic evidence that circulation in the North Atlantic has been anomalous since 1750-1850, including weakened Labrador Sea deep convection, weakened Iceland-Scotland overflows and weakened AMOC, a northerly shift in the position of the Gulf Stream and a warming and reduction of frontal activity in the Iceland Basin. The reconstructions suggest that transitions began around the end of the LIA, with some differences in timing between regions, and have continued over the past 150-250 years. We suggest that enhanced freshwater fluxes from the Arctic and Nordic Seas towards the end of the LIA, sourced from melting glaciers and sea-ice, weakened Labrador Sea convection and the AMOC. The lack of a subsequent recovery may result from hysteresis or anthropogenic warming and freshening of the North Atlantic. Our results highlight that recent decadal variability in the North Atlantic has occurred during an atypical background state. Future work should aim to constrain the role of internal climate variability versus early anthropogenic forcing in the changes described here.