Internal lee waves and baroclinic bores over a tropical seamount shark ‘hot-spot’
Internal lee waves and baroclinic bores over a tropical seamount shark ‘hot-spot’
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DOI：
10.1016/j.pocean.2019.01.010
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发表时间：
2019-03
影响因子：
4.1
通讯作者：
P. Hosegood;W. Nimmo-Smith;R. Proud;K. Adams;A. Brierley
中科院分区：
文献类型：
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作者：
P. Hosegood;W. Nimmo-Smith;R. Proud;K. Adams;A. Brierley
在热带印度洋两个相邻海山的顶端和两侧安装了一个地下海洋系泊设备进行了海洋学观测，以查明可能是导致银鳍鲨(Carcharhinus Albimarginatus)在山顶周围深水落点聚集的过程。这些海山位于英国印度洋领土的查戈斯群岛，水平范围很窄(<；10 公里)，两侧陡峭(>；15°)从 >； 600 m深处上升，山顶平坦，深度70 m。它们受到亚惯性、盆地尺度和局部尺度的强迫，包括混合潮汐制度和风暴产生的近惯性波。在落潮深度70-100 m之间，等温线以20-30∼m的昼夜和半日频率振荡。潮汐起源的波伴随着幅度为O(10 m)的短周期(∼5 分钟)内波，频率接近温跃层内的本地浮力频率N，这是自由传播内波的最大频率。潮汐振荡是由超临界海山两侧的优势流产生的垂直波长为30 m的内背风波引起的，其底部斜率大于内潮波斜率。Near-N波是由于从陡峭的边到相对平坦的山顶的突然转变而形成的背风波形成的水力跳跃所引起的剪切增强所致。这些跳跃表现为底部被困的钻孔，沿着斜坡向山顶传播。对山顶的进一步观察表明，钻孔随后以潮汐周期的冷水冲刷山顶。这些钻孔的长波相速度是钻孔颗粒速度的两倍多，具有强烈的垂直速度(>；0.1 m S−1)和推断的局部再悬浮，但基于温度反转的湍流相对较小。我们的结果有力地暗示了背风波作为引导顺序重要性的动力学机制，对先前观测到的在整个群岛中常见的超临界斜坡附近的生物量积累具有重要意义。我们建议进一步的研究应该确定内波活动和顶峰附近鱼类鱼群之间的时空相关性，以及这种鱼群是否吸引捕食者。
Oceanographic observations were made with a subsurface oceanographic mooring over the summit and flanks of two neighbouring seamounts in the tropical Indian Ocean to identify processes that may be responsible for the aggregation of silvertip sharks (Carcharhinus albimarginatus) in the deep water drop-off surrounding the summits. The seamounts, which are in the Chagos Archipelago in the British Indian Ocean Territories, are narrow in horizontal extent (<10 km), have steeply sloping (>15°) sides that rise from depths of > 600 m, and flat summits at a depth of 70 m. They are subjected to forcing at subinertial, basin-scales and local scales that include a mixed tidal regime and storm-generated near inertial waves. At the drop-off, at a depth of between 70 and 100 m, isotherms oscillate at both diurnal and semidiurnal frequencies with amplitudes of∼20–30 m. The waves of tidal origin are accompanied by short period (∼5 min) internal waves with amplitudesO(10 m) and frequencies close to the local buoyancy frequency,N, within the thermocline which is the maximum frequency possible for freely propagating internal waves. The tidal oscillations result from internal lee waves with 30 m vertical wavelength generated by the prevailing currents over the supercritical seamount flanks, whereby the bottom slope is greater than the internal tide wave slope. The ‘near-N’ waves are due to enhanced shear associated with the hydraulic jumps that form from the lee waves due to the abrupt transition from steeply sloping sides to a relatively flat summit. The jumps manifest themselves as bottom-trapped bores that propagate up the slope towards the summit. Further observations over the summit reveal that the bores subsequently flush the summits with cold water with tidal periodicity. The bores, which have long wave phase speeds more than double that of the bore particle velocities, are characterised by intense vertical velocities (>0.1 m s−1) and inferred local resuspension but relatively little turbulence based on temperature overturns. Our results strongly implicate lee waves as the dynamic mechanism of leading order importance to the previously observed accumulation of biomass adjacent to the supercritical slopes that are commonplace throughout the archipelago. We propose that further investigation should identify the spatiotemporal correlation between internal wave activity and fish schooling around the summit, and whether such schooling attracts predators.