The dominant feature of large-scale mass transfer in the modern ocean is the Atlantic meridional overturning circulation (AMOC). The geometry and vigour of this circulation influences global climate on various timescales. Palaeoceanographic evidence suggests that during glacial periods of the past 1.5 million years the AMOC had markedly different features from today¹; in the Atlantic basin, deep waters of Southern Ocean origin increased in volume while above them the core of the North Atlantic Deep Water (NADW) shoaled². An absence of evidence on the origin of this phenomenon means that the sequence of events leading to global glacial conditions remains unclear. Here we present multi-proxy evidence showing that northward shifts in Antarctic iceberg melt in the Indian–Atlantic Southern Ocean (0–50° E) systematically preceded deep-water mass reorganizations by one to two thousand years during Pleistocene-era glaciations. With the aid of iceberg-trajectory model experiments, we demonstrate that such a shift in iceberg trajectories during glacial periods can result in a considerable redistribution of freshwater in the Southern Ocean. We suggest that this, in concert with increased sea-ice cover, enabled positive buoyancy anomalies to ‘escape’ into the upper limb of the AMOC, providing a teleconnection between surface Southern Ocean conditions and the formation of NADW. The magnitude and pacing of this mechanism evolved substantially across the mid-Pleistocene transition, and the coeval increase in magnitude of the ‘southern escape’ and deep circulation perturbations implicate this mechanism as a key feedback in the transition to the ‘100-kyr world’, in which glacial–interglacial cycles occur at roughly 100,000-year periods.

现代海洋大规模传质的主导特征是大西洋经向翻转环流（AMOC）。这种循环的几何形状和活力在不同的时间尺度上影响着全球气候。古海洋学证据表明，在过去 150 万年的冰川时期，AMOC 具有与今天截然不同的特征¹；在大西洋盆地，源自南大洋的深水体积增加，而在其上方，北大西洋深水 (NADW) 的核心变浅². 缺乏关于这种现象起源的证据意味着导致全球冰川条件的事件顺序仍不清楚。在这里，我们提供了多项代理证据，表明在更新世冰期期间，印度-大西洋南大洋（0-50°E）南极冰山融化的北移系统地先于深水质量重组一到两千年。借助冰山轨迹模型实验，我们证明了冰川时期冰山轨迹的这种转变会导致南大洋淡水的大量重新分配。我们认为，这与海冰覆盖的增加相一致，使正浮力异常能够“逃逸”到 AMOC 的上肢，提供南大洋表面条件与 NADW 形成之间的遥相关。这种机制的幅度和步调在更新世中期的过渡期间发生了重大变化，而“南方逃逸”和深层环流扰动的幅度同时增加，表明这种机制是向“100-kyr 世界”过渡的关键反馈, 其中冰期-间冰期循环发生在大约 100,000 年的周期。