We use an ocean general circulation model and its adjoint to analyze the causal chain linking sea surface buoyancy anomalies in the Labrador Sea to variability in the deep branch of the Atlantic meridional overturning circulation (AMOC) on inter-annual timescales. Our study highlights the importance of the North Atlantic Current (NAC) for the north-to-south connectivity in the AMOC and for the meridional transport of Lower North Atlantic Deep Water (LNADW). We identify two mechanisms that allow the Labrador Sea to impact velocities in the LNADW layer. The first mechanism involves a passive advection of surface buoyancy anomalies from the Labrador Sea towards the eastern subpolar gyre by the background NAC. The second mechanism plays a dominant role and involves a dynamical response of the NAC to surface density anomalies originating in the Labrador Sea; the NAC adjustment modifies the northward transport of salt and heat and exerts a strong positive feedback, amplifying the upper ocean buoyancy anomalies. The two mechanisms spin up/down the subpolar gyre on a timescale of years, while boundary trapped waves rapidly communicate this signal to the subtropics and trigger an adjustment of LNADW transport on a timescale of months. The NAC and the eastern subpolar gyre play an essential role in both mechanisms linking the Labrador Sea with LNADW transport variability and the subtropical AMOC. We thus reconcile two apparently contradictory paradigms about AMOC connectivity: (1) Labrador Sea buoyancy anomalies drive AMOC variability; (2) water mass transformation is largest in the eastern subpolar gyre.

我们使用海洋大气环流模型及其伴随分析在年际时间尺度上将拉布拉多海的海面浮力异常与大西洋经向翻转环流 (AMOC) 深分支的变化联系起来的因果链。我们的研究强调了北大西洋洋流 (NAC) 对于 AMOC 南北连通性和北大西洋下游深水区 (LNADW) 经向输送的重要性。我们确定了两种允许拉布拉多海影响 LNADW 层速度的机制。第一种机制涉及由背景 NAC 从拉布拉多海向东部副极环流的表面浮力异常的被动平流。第二种机制起主导作用，涉及 NAC 对源自拉布拉多海的表面密度异常的动态响应；NAC 调整改变了盐和热量向北的传输，并施加了强烈的正反馈，放大了上层海洋浮力异常。这两种机制在数年的时间尺度上使副极地环流向上/向下旋转，而边界困波迅速将该信号传递到亚热带，并在数月的时间尺度上触发 LNADW 传输的调整。NAC 和东部副极地环流在将拉布拉多海与 LNADW 传输变率和亚热带 AMOC 联系起来的两种机制中都发挥着重要作用。因此，我们调和了关于 AMOC 连通性的两个明显矛盾的范式：（1）拉布拉多海浮力异常驱动 AMOC 可变性；