The Atlantic Meridional Overturning Circulation is the main driver of northward heat transport in the Atlantic Ocean today, setting global climate patterns. Whether global warming has affected the strength of this overturning circulation over the past century is still debated: observational studies suggest that there has been persistent weakening since the mid-twentieth century, whereas climate models systematically simulate a stable circulation. Here, using Earth system and eddy-permitting coupled ocean–sea-ice models, we show that a freshening of the subarctic Atlantic Ocean and weakening of the overturning circulation increase the temperature and salinity of the South Atlantic on a decadal timescale through the propagation of Kelvin and Rossby waves. We also show that accounting for upper-end meltwater input in historical simulations significantly improves the data–model agreement on past changes in the Atlantic Meridional Overturning Circulation, yielding a slowdown of 0.46 sverdrups per decade since 1950. Including estimates of subarctic meltwater input for the coming century suggests that this circulation could be 33% weaker than its anthropogenically unperturbed state under 2 °C of global warming, which could be reached over the coming decade. Such a weakening of the overturning circulation would substantially affect the climate and ecosystems.

大西洋经向翻转环流是当今大西洋热量北移的主要驱动力，决定了全球气候模式。全球变暖是否影响了过去一个世纪这种翻转环流的强度仍存在争议：观测研究表明，自 20 世纪中叶以来，环流一直在持续减弱，而气候模型系统地模拟了稳定的环流。在这里，使用地球系统和允许涡流耦合的海洋-海冰模型，我们表明，亚北极大西洋的更新和翻转环流的减弱通过开尔文波和罗斯比波的传播在年代际时间尺度上增加了南大西洋的温度和盐度。我们还表明，在历史模拟中考虑高端融水输入显着提高了大西洋经向翻转环流过去变化的数据模型一致性，自 1950 年以来每十年放缓 0.46 斯沃德隆。包括对未来世纪亚北极融水输入的估计表明，在全球变暖 2°C 的情况下，这种环流可能比其人为未受干扰的状态弱 33%，而全球变暖可能在未来十年内达到。这种翻转环流的减弱将对气候和生态系统产生重大影响。