The lower limb of the Atlantic Meridional Overturning Circulation is an important feature of Earth’s climate system as it returns recently ventilated water to the deep ocean and is a major sink for anthropogenic carbon. The Deep Western Boundary Current—the primary component of the lower limb—flows southwards along the eastern flank of Greenland transporting dense water formed in the Nordic seas. Since 2014, the Deep Western Boundary Current has been continuously monitored at this location from a mooring array to observe the current’s velocity and hydrographic structure close to its source. Here we find that the Deep Western Boundary Current transport has decreased by 26% over the first six years of observations, due to (1) a thinning of the traditionally defined Deep Western Boundary Current layer (σ_θ > 27.8 kg m⁻³) from a known freshening signal propagating through the subpolar region (56%), and (2) weakening velocities (44%). Despite this decrease, the Atlantic Meridional Overturning Circulation has remained relatively steady over the same period. Ultimately, this difference is due to the methods used to define these two circulations. Finding such notably different trends for two seemingly dependent circulations raises the question of how to best define these transports.

大西洋经向翻转环流的下肢是地球气候系统的一个重要特征，因为它将最近通风的水返回到深海，是人为碳的主要汇。西部深部边界流（下肢的主要组成部分）沿着格陵兰岛的东侧向南流动，输送北欧海域形成的稠密水。自 2014 年以来，该位置一直通过系泊阵列持续监测深西部边界流，以观察靠近其源头的洋流速度和水文结构。在这里，我们发现，在观测的前六年中，深西部边界流的传输减少了 26%，原因是 （1） 传统定义的深西部边界流层 （σ_θ > 27.8 kg m⁻³） 从通过亚极地传播的已知清新信号 （56%） 变薄，以及 （2） 速度减弱 （44%）。尽管有所下降，但大西洋经向翻转环流在同一时期保持相对稳定。归根结底，这种差异是由于用于定义这两个环流的方法造成的。在两个看似依赖的环流中发现如此明显不同的趋势，提出了如何最好地定义这些运输的问题。