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

Iceland-Scotland Overflow Water shows persistent bistability over the past 800,000 years, alternating between weak glacial and strong interglacial circulation modes.

• Why it matters: AMOC stability is a major uncertainty for future climate projections, but direct evidence for tipping-like behavior on orbital timescales is limited.
• What they did: Strength was reconstructed for Iceland-Scotland Overflow Water using 12 current-speed sediment-core records from the North Atlantic plus a data-driven hybrid model, then analyzed with threshold and phase-space methods.
• The result: Regime shifts occur within a narrow window of atmospheric carbon dioxide and sea level, often following hysteresis-like loops, implying evolving sensitivity and timing while bistability persists.

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