Persistent monsoonal forcing of Mediterranean Outflow dynamics during the late Pleistocene

The mode and vigor of the global oceanic circulation critically depend on the salinity of (sub)surface water masses advected to the loci of deep-water formation. Within the Atlantic meridional overturning circulation (AMOC), an important supplier of high-salinity waters is the Mediterranean Outflow Water (MOW), discharging into the North Atlantic via the Strait of Gibraltar. Despite its importance for the North Atlantic salinity budget, the long-term dynamics of MOW production have remained poorly understood. Here we present high-resolution records of bottom-current velocity from three drill sites within the Gulf of Cádiz that document a persistent low-latitude forcing of MOW flow speed over the past ~150 k.y. We demonstrate that the African monsoon is the predominant driver of orbital-scale MOW variability via its influence on the freshwater budget of the eastern Mediterranean Sea. Consequently, MOW formation fluctuates in concert with orbital precession overprinted by centennial-scale oscillations of high-latitude origin. We further document that Northern Hemisphere summer insolation minima stimulate maximal injection of MOW-derived salt into the North Atlantic, likely strengthening the intermediate AMOC branch. The direct coupling of MOW dynamics to low-latitude climate forcing represents a hitherto neglected process for propagating (sub)tropical climate signals into the high northern latitudes.全球大洋环流的运行模式与活跃程度，关键取决于被平流输送至深水形成区域的（次）表层水团的盐度。在大西洋经向翻转环流（Atlantic meridional overturning circulation, AMOC）中，高盐度水的重要供应源之一是地中海流出水（Mediterranean Outflow Water, MOW），该水团经直布罗陀海峡注入北大西洋。尽管MOW对北大西洋盐度收支至关重要，但学界对其形成的长期动力学机制仍知之甚少。本研究基于加的斯湾内3个钻探站位的底流流速高分辨率记录，揭示了过去约15万年来MOW流速持续受到低纬度强迫作用的现象。研究表明，非洲季风通过影响东地中海的淡水收支，成为轨道尺度上MOW变化的主导驱动因子。因此，MOW的形成随轨道进动发生波动，同时叠加了源自高纬度地区的百年尺度振荡信号。本研究还发现，北半球夏季日照极小值会促使MOW携带的盐分最大程度地输入北大西洋，这可能会增强大西洋经向翻转环流的中层分支。MOW动力学与低纬度气候强迫的直接耦合，代表了迄今尚未被关注的、将（亚）热带气候信号传递至北高纬度地区的过程。