Data from: Fate of internal waves on a shallow shelf

In this paper we present observations of internal waves shoaling on the shallow shelf-slope of Dongsha Atoll, a coral reef ecosystem in the South China Sea (Fig. 1a-c). Dongsha is directly in the path of some of the world’s largest internal solitary waves [Alford, 2015; Guo and Chen, 2014; Hsu, 2000; Lien et al., 2005]. These waves evolve from steepened internal tides in Luzon Strait and propagate westward across the deep basin of the northern South China Sea with wavelengths of O(3-10 km), often developing into wave trains [Alford, 2015]. As the solitary waves shoal up onto the continental slope they steepen even further with amplitudes as large as 150-200 m and wavelengths of only a few hundred meters. At this point they can become susceptible to both convective and shear instabilities which cause them to break and drive huge vertical overturns, energetic mixing, and energy dissipation [Chang et al., 2006; St. Laurent, 2008]. The shallow (< 25 m) slope of Dongsha Atoll is an internal swash zone, where the end-of-life of these large internal waves takes the form of bottom-propagating solibores and boluses. They spend their remaining energy bringing deep water up to the surface where it has significant effects on the reef heat and nutrient budgets [Reid et al., 2019]. From 1-17 June 2014, we deployed Distributed Temperature Sensing (DTS) instrumentation to capture spatially-continuous observations of near-bed temperature on the fore reef slope of Dongsha Atoll to evaluate the path (or “fate”) of internal waves shoaling onto the shelf and moorings to measure the vertical structure of velocity and density stratification. A semi-idealized numerical simulation is used to help interpret the novel perspective provided by the DTS observations. Here a four-channel DTS system (Sensornet Oryx) was used to capture a continuous view of near-bed temperature in a cross-shelf profile on the east fore reef of Dongsha Atoll. Four kilometers of Kaiphone 0.6 cm-diameter fiber optic cable was deployed in a cross-shelf orientation (285°, aligned approximately parallel to the propagation direction of the offshore internal wave field [Ramp, 2010], beginning at the back of the reef flat (~2 m depth) and terminating offshore at 50-m depth on the fore reef (Fig. 1c). In this study we focus on the offshore-most ~1 km of the FO cable - from the reef crest down the fore reef slope – the reader is referred to Reid et al. [2019] for further information about the shallow reef DTS results. The FO cable followed the bottom contours of the bed, except in areas with extreme changes in topography (such as a coral groove) where the cable was raised above the bed by up to 0.5 m. The DTS collected temperature traces along the cable every minute with 2-m spatial resolution from 2-12 June 2014. A tropical storm in the vicinity of Dongsha Atoll reduced solar power and prevented further DTS measurements after 12 June, however, measurements from vertical arrays (Section 2.3) continued until 17 June 2014.

本研究报道了南海珊瑚礁生态系统东沙环礁（Dongsha Atoll）浅陆架坡上内波浅化（shoaling）的观测结果（图1a-c）。东沙环礁正处于全球部分最强内孤立波的传播路径之上[Alford, 2015; Guo and Chen, 2014; Hsu, 2000; Lien等, 2005]。这类内波起源于吕宋海峡（Luzon Strait）经陡化的内潮，向西横穿南海北部深海盆地传播，波长量级为3~10千米，常发展为波列[Alford, 2015]。当这类孤立波爬坡进入大陆坡区域时，其陡化程度进一步加剧，振幅可达150~200米，波长仅数百米。此时，这类内波易受对流不稳定与剪切不稳定影响，进而发生破碎，引发强烈的垂直翻转、剧烈混合与能量耗散[Chang等, 2006; St. Laurent, 2008]。东沙环礁水深小于25米的浅坡属于内波冲带（internal swash zone），这类大型内波在此终结其生命周期，以沿底传播的滑涌波（solibores）与水团（boluses）的形式存在。它们会消耗剩余能量将深层海水抬升至表层，对珊瑚礁的热量与营养盐收支具有显著影响[Reid等, 2019]。2014年6月1日至17日，我们在东沙环礁礁前坡部署了分布式温度传感（Distributed Temperature Sensing, DTS）设备，以获取近底温度的空间连续观测数据，用于分析内波爬坡进入陆架后的路径（或称"归宿"）；同时部署锚系浮标，以测量流速与密度层结的垂直结构。本研究采用半理想化数值模拟，辅助解读DTS观测所提供的全新视角。本研究采用四通道DTS系统（Sensornet Oryx），对东沙环礁东侧礁前的跨陆架剖面开展近底温度连续观测。我们部署了总长4千米、直径0.6厘米的Kaiphone光缆，沿跨陆架方向（方位角285°，大致平行于近海内波场的传播方向[Ramp, 2010]）布设，起点位于礁坪后部（水深约2米），终点延伸至礁前水深50米的近海区域（图1c）。本研究聚焦于该光纤（fiber optic, FO）电缆最靠近海的约1千米区段——从礁冠延伸至礁前坡——关于浅礁区DTS观测结果的详细信息，请参阅Reid等[2019]的研究。该光缆沿海底地形等高线布设，仅在地形突变区域（如珊瑚沟）处，光缆被抬升于海底上方最高达0.5米。2014年6月2日至12日，DTS系统沿光缆以每分钟1次的频率采集温度观测序列，空间分辨率为2米。2014年6月12日后，东沙环礁附近的热带风暴导致太阳能供电不足，致使DTS观测被迫终止；但垂直阵列观测（2.3节）仍持续至6月17日。