Persistent bistability of Nordic Seas overflow waters over 800,000 years

The stability of the Atlantic Meridional Overturning Circulation (AMOC) is a major uncertainty in projections of future climate. However, direct evidence for AMOC tipping behavior on orbital timescales remains scarce. Here, we reconstruct the strength of Iceland-Scotland Overflow Water, a key lower branch of the deep Atlantic circulation, over the past 800,000 years by combining 12 current-speed records from North Atlantic sediment cores with a data-driven hybrid model. The resulting reconstruction reveals persistent alternation between weak (glacial) and strong (interglacial) circulation modes, pervasive millennial-scale variability within both regimes, and systematic asymmetry between weak-to-strong recoveries and strong-to-weak collapses. Threshold and phase-space analyses show that regime shifts occur within a narrow window of atmospheric carbon dioxide and sea level and often follow hysteresis-like loops. Although bistability persists throughout the last 800,000 years, its sensitivity and timing evolve through time. These findings provide long-term, high-resolution, data-driven constraints on the bistability of Nordic Seas overflow waters and its implications for AMOC resilience in a warming world.