Abstract: Interactions between the open ocean and marginal seas have been suggested to be critical to the redistribution and dissipation of global energy. Here, we propose a mechanism for the upper open ocean influencing the deep flow in marginal seas that hinges on the formation and propagation of topographic Rossby waves (TRWs). Observations and high-resolution simulations suggest substantial intraseasonal variability with periods of 5–60 days associated with the deep flow over continental slopes of the northern South China Sea (NSCS). These fluctuations generally account for over 40% of the total deep kinetic energy variability and the number can reach 70% over the slopes to the west of the Luzon Strait, southwest of the Dongsha Islands, and northeast of the Xisha Islands. By examining the spatiotemporal features of the fluctuations, we demonstrate that the intraseasonal variability of the deep flow in the NSCS is closely associated with TRWs. Using a recently developed multiscale energetics analysis in combination with a ray tracing model, we show that the energy sources of TRWs can be traced back to the east of the Dongsha Islands, where the Kuroshio intrusion and related eddies energize the TRWs primarily through pressure work. These waves propagate westward across the NSCS and drive the intraseasonal variability of the deep flow over continental slopes. Our findings highlight an energy pathway from the open ocean western boundary current to the abyssal marginal sea that could modulate regional circulation as well as exchanges between major ocean basins.

中国南海北部黑潮入侵对深层流动季节内变化的影响

中文摘要：开阔大洋和边缘海的相互作用对于全球能量的再分配和损耗至关重要。本文提出了一种上层开阔大洋影响边缘海的深层流动的机制，该机制取决于地形罗斯贝波（TRWs）的形成和传播。观测和高分辨率模拟结果表明周期为5－60天的显著季节内变化与中国南海北部（NSCS）深层跨陆坡流动有关。在吕宋海峡西部、东沙群岛西南部和西沙群岛东北部陆坡上，这些波动一般超过总深层动能变化率的40%，且其数量超过70%。通过分析波动的时空特征，我们证明了NSCS深层流动的季节内变化与TRWs密切相关。使用最近开发的多尺度能量分析和射线跟踪模型，我们发现TRWs的能量源可追溯到东沙群岛东侧，该处黑潮入侵和相关的涡旋主要通过压力功给TRWs提供能量。这些波动越过NSCS向西传播并驱动陆架坡上深层流动的季节内变化。我们的结果强调了从开阔大洋西边界流到深层边缘海的能量通路，它可能调节区域环流和主要海洋盆地间的交换。