Transports and pathways of the tropical AMOC return flow from Argo data and shipboard velocity measurements

The upper-ocean circulation of the tropical Atlantic is a complex superposition of thermohaline and wind-driven flow components. The resulting zonally- and vertically-integrated upper-ocean meridional flow is referred to as the upper branch of the Atlantic Meridional Overturning Circulation (AMOC) - a major component and potential tipping element of the global climate system. We investigate the tropical part of the northward AMOC branch, i.e. the return flow covering the upper 1,200 m, based on Argo data and repeated shipboard velocity measurements. The western boundary mean circulation at 11°S is realistically reproduced from high-resolution Argo data showing a remarkably good representation of the vertical structure of meridional velocity and the volume transport of water mass layers when compared to results from direct velocity measurements along a repeated ship section. Thus, we extend the analysis to the inner tropical Atlantic. Within the AMOC return flow, a diapycnal upwelling of central water into the thermocline layer of ~2 Sv is derived between 11°S and 10°N which is about half the magnitude of previous estimates, likely due to improved horizontal resolution. The mean strength of the AMOC return flow is ~16 Sv across 11°S and 10°N. At 11°S, northward transport is concentrated at the western boundary where the AMOC return flow enters the tropics at all vertical layers above 1,200 m. At 10°N, northward transport is observed both at the western boundary and in the interior predominantly in the surface and intermediate layer indicating recirculation and transformation of thermocline and central water within the tropics.热带大西洋上层海洋环流是热盐环流与风驱流分量的复杂叠加。经纬向与垂向积分后的上层海洋经向流，被称为大西洋经向翻转环流（Atlantic Meridional Overturning Circulation, AMOC）的上层分支——它是全球气候系统的核心组分，同时也是潜在的气候临界要素。本研究基于阿尔戈（Argo）浮标数据与重复船载流速测量数据，对北向AMOC分支的热带部分展开研究，即覆盖上层1200米的回流体系。南纬11°处的西边界平均环流可通过高分辨率阿尔戈浮标数据得到真实再现，相较于沿重复船测断面的直接流速测量结果，该数据对经向流速的垂直结构以及各水团层的体积输运刻画精度极高。基于此，我们将分析范围拓展至热带大西洋内部海域。在AMOC回流体系中，南纬11°至北纬10°之间存在约2斯维尔德鲁普（Sv，海洋体积输运单位，1 Sv=10^6立方米/秒）的中心水跨等密度面上升进入温跃层的现象，该输运量级仅为既往估算值的一半，这可能得益于更高的水平分辨率。在南纬11°至北纬10°断面上，AMOC回流的平均强度约为16 Sv。南纬11°处，北向输运集中于西边界，AMOC回流在此处通过1200米以浅的所有垂向层进入热带海域；北纬10°处，北向输运同时出现在西边界与海域内部，且主要集中于表层和中层，这表明热带海域内温跃层水与中心水存在再循环与水团变性过程。