Causal drivers of North Atlantic glacial climate variability

Terrestrial and marine proxy records suggest that climate variability was higher during the last glacial cycle than today. This is particularly true in the North Atlantic region where this enhanced variability is associated with changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) on different time scales. Despite prolonged efforts to explain an oscillatory behaviour of glacial AMOC strength, it is still not fully understood; moreover, studies often describe its mechanism qualitatively or attempt to identify its drivers using methods that do not imply causation. In this study we use the isotope-enabled Earth system model iCESM1.2 to simulate the glacial climate under conditions representing the Marine Isotope Stage 3 (about 38 thousand years before present). Our results show the AMOC oscillating with a period of roughly 500 years and an amplitude of 4 Sv (1 Sv = 106 m3s-1). Surface air temperature varies by about 1-2C in Northern Europe, 4C in Greenland, and up to 15C over the North Atlantic Ocean where sea ice cover varies the most. Based on a supervised machine learning method (causal discovery), we find causal links between AMOC and North Atlantic Ocean salinity and meridional salt transport. We show how changes in the salinity of water advected into key deep-water forming areas feed back to the AMOC, thus driving the oscillation. Our findings from applying causal discovery outline the mechanism of a salt-oscillator in a fully coupled model, and indicate the potential of this method to identify causal drivers that trigger variations in AMOC strength on different time scales too.