Internal gravity waves, the subsurface analogue of the familiar surface gravity waves that break on beaches, are ubiquitous in the ocean. Because of their strong vertical and horizontal currents, and the turbulent mixing caused by their breaking, they affect a panoply of ocean processes, such as the supply of nutrients for photosynthesis, sediment and pollutant transport and acoustic transmission; they also pose hazards for man-made structures in the ocean. Generated primarily by the wind and the tides, internal waves can travel thousands of kilometres from their sources before breaking, making it challenging to observe them and to include them in numerical climate models, which are sensitive to their effects. For over a decade, studies have targeted the South China Sea, where the oceans’ most powerful known internal waves are generated in the Luzon Strait and steepen dramatically as they propagate west. Confusion has persisted regarding their mechanism of generation, variability and energy budget, however, owing to the lack of in situ data from the Luzon Strait, where extreme flow conditions make measurements difficult. Here we use new observations and numerical models to (1) show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena, (2) reveal the existence of >200-metre-high breaking internal waves in the region of generation that give rise to turbulence levels >10,000 times that in the open ocean, (3) determine that the Kuroshio western boundary current noticeably refracts the internal wave field emanating from the Luzon Strait, and (4) demonstrate a factor-of-two agreement between modelled and observed energy fluxes, which allows us to produce an observationally supported energy budget of the region. Together, these findings give a cradle-to-grave picture of internal waves on a basin scale, which will support further improvements of their representation in numerical climate predictions.

南海内波的形成和宿命

中文摘要：内重力波是一种与在沙滩破碎的重力波相似的次表层波动现象，在海洋中无处不在。由于其强大的垂直和水平流，以及由其破碎引起的湍流混合，它们影响了一系列的海洋过程，如光合作用的营养物质的供应、沉积物和污染物的运输以及声学传输；也对海洋中的人造结构构成了危险。内波主要由风和潮汐产生，在破碎之前可以从其源头传播几千公里，这使得观察它们和将它们纳入对其影响敏感的数字气候模型具有挑战性。由于南海有已知的海洋最强大的内波，这些内波在吕宋海峡产生，并在向西传播时急剧变陡，过去十几年来的内波研究也集中于此。然而，由于缺乏来自吕宋海峡的现场数据，在极端的流动条件下很难进行测量，因此对其产生机制、变异性和能量收支一直存在困惑。在这里，我们利用新的观测数据和数值模型，(1)表明内重力波是以正弦波干扰开始的，而不是由强烈的海洋动力现象引起的；(2)揭示了在产生波浪的区域存在>200米高的破碎内波，引起的湍流水平是开放海洋的>10000倍；(3)确定黑潮西部边界海流明显地折射了来自吕宋海峡的内波场，以及(4)证明模式模拟和观测的能量通量之间有2倍的一致性，这使我们能够产生一个有观测支持的该地区的能量收支。这些发现结合在一起，描绘了南海内波的从生成-发展-消亡的过程，这将支持进一步改进它们在数值气候预测中的表现。