Observations of Island Wakes at High Rossby Numbers: Evolution of Submesoscale Vortices and Free Shear Layers
Observations of Island Wakes at High Rossby Numbers: Evolution of Submesoscale Vortices and Free Shear Layers
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高罗斯贝数岛屿尾流的观测：次中尺度涡旋和自由剪切层的演化
DOI：
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发表时间：
2019
影响因子：
3.5
通讯作者：
Vigan Mensah
中科院分区：
文献类型：
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作者：
Ming‐Huei Chang;S. Jan;Chih;Yu‐Hsin Cheng;Vigan Mensah
本文对台湾绿岛背风面（直径约7公里）黑潮流速为1-1.5 m s-1的海洋涡旋演变进行了观测研究，并与理论和实验室实验结果进行了比较。在近尾流中，再循环发生在相对涡度为100 ~ 20 f（其中f是行星涡度）的情况下，随后在潮汐振荡和涡动演化的内在时间尺度所产生的周期组合中发生。潮汐振荡是主导过程。我们的分析表明，一个岛屿位于黑潮与周期性和横流潮汐漂移是类似的横流振荡柱。因此，旋涡的脱落周期与接近固有周期的潮汐周期同步。黑潮与回流区之间形成自由切变层，其特征是约30 f的相对涡度带（宽约2km）和波状的热锋。在0.5-2小时的时间内发生的锋面波类似于开尔文-亥姆霍兹不稳定性对应于高Re值。对于远尾流，重复的交叉尾流调查表明，气旋和反气旋涡交替存在于接近M2潮汐的周期，与近尾流测量一致。沿尾流方向的重复观测显示，从近尾流开始，气旋涡流以0.35 m s-1的速度向下游脱落，这是上游流速的1/3。将我们的观测结果与之前的水箱实验结果进行比较，预计亚中尺度尾流区的Re值为O（103）。
Oceanic vortex evolution on the lee side of Taiwan’s Green Island (~7 km in diameter), where the Kuroshio flows at a speed of 1–1.5 m s−1, is observationally examined and compared to theories and the preceding results of laboratory experiments. In the near wake, recirculation occurs with a relative vorticity of ζ ~ 20f (where f is the planetary vorticity) and subsequently sheds at a combination of periods resulting from the tidal oscillations and the intrinsic time scale of eddy evolution. The tidal oscillations are the predominant processes. Our analysis suggests that an island positioned in the Kuroshio with periodic and cross-stream tidal excursions is analogous to a cross-stream oscillating cylinder. Consequently, the shedding period of the vortex is synchronized to a tidal period occurring close to the intrinsic period. The free shear layer, which is characterized by an ~30f relative vorticity band (2 km wide) and a wavy thermal front, develops between the Kuroshio and recirculation. The frontal wave occurring over a time period of 0.5–2 h resembles Kelvin–Helmholtz instability corresponding to high Re values. For the far wake, repeated cross-wake surveys suggest that cyclonic and anticyclonic vortices are alternatively present at a period close to the period of M2 tides in agreement with near-wake measurements. Repeated along-wake surveys reveal a cyclonic eddy shedding downstream at a speed of 0.35 m s−1, 1/3 of the upstream current speed, from the near wake. In comparing our observations with the results of previous water tank experiments, an Re value of O(103) for the submesoscale wake regime is expected.