大西洋经向翻转环流对岁差响应的气候背景依赖性

大西洋经向翻转环流（Atlantic Meridional Overturning Circulation，AMOC）是气候系统重要的组成部分，其强度变化可直接影响南北半球的热量分配，厘清其变化机理对全球变暖背景下的未来预估至关重要。海洋沉积物记录发现，在晚更新世，AMOC的变化与地球岁差周期有紧密联系，但其物理机理尚不清楚。本文利用海洋?大气耦合气候模型—COSMOS（ECHAM5/JSBACH/MPIOM）模型，通过敏感试验，分析在冰盛期冷期和间冰期暖期气候背景下，AMOC对地球岁差变化的响应机理。结果表明:岁差降低引起的北半球夏季太阳辐射增强，会导致间冰期暖期背景下的AMOC显著减弱，但对冰盛期AMOC的影响并不明显。通过进一步分析发现，在间冰期暖期，夏季太阳辐射增强，造成高低纬大西洋海表的升温，同时促进北大西洋高纬度地区的局地降水，两者导致北大西洋表层海水密度降低，共同削弱大西洋深层水生成。而在冰盛期冷期，大西洋高低纬度地区的响应对AMOC的影响反向—副热带升温触发的海盆尺度低压异常，通过其南侧的西风异常削弱大西洋向太平洋的水汽输送，导致净降水增多，海表盐度下降；同时，高纬度升温造成的海冰减少，促进了海洋热丧失，海表失热变重，有利于大西洋深层水的生成，最终两者的共同作用导致AMOC对岁差变化的响应偏弱。本文系统揭示了不同气候背景下，岁差尺度AMOC变化的控制机理，对理解晚更新世AMOC重建记录中持续存在的岁差周期具有重要启示意义。

Background climate dependence of Atlantic meridional overturning circulation responding to precessional change

The Atlantic meridional overturning circulation (AMOC) is an important component of the climate system, of which change in the strength can affect meridional heat distribution between the northern and southern hemispheres. Proxy records show that changes in Atlantic Ocean circulation during the Late Pleistocene is associated with precessional cycle, but its physical mechanism remains unclear. Here we use a fully coupled climate model to investigate dynamics associated with AMOC changes in precessional band under glacial-interglacial climate conditions. Our results show that increase in boreal summer insolation can effectively weaken the AMOC during warm interglacial periods, while this weakening effect is reduced under glacial maximum. We further demonstrate that during the warm interglacial period increase in boreal summer insolation leads to sea surface warming and subpolar rainfall increase in North Atlantic, which jointly reduces sea surface density and hence the strength of deep water formation. During the glacial maximum period, climate responses to precessional change is of anti-phase impacts on the AMOC. At the low latitudes, a low pressure anomaly triggered by subtropical warming weakens atmospheric moisture export from the subtropical Atlantic to Pacific, increasing in net precipitation and hence freshening tropical sea surface in the North Atlantic. At the high latitudes, the warming-induced sea ice retreat promotes ocean heat loss via the enlarged ice-free area, and hence tends to strengthen the vertical mixing. The combined effects of low- and high-latitude responses finally leads to a trivial weakening of the AMOC. Overall, our results provide a systematic understanding of governing mechanism for precessionally-induced AMOC change under glacial-interglacial climatic backgrounds, shedding light on our interpretation of precessional periodicity in reconstructed ocean circulation changes during the Pleistocene.