The Correlation of the Current Speed and Vorticity by Drifters to the Vertical Motion of Drogue at Thermocline

• Main Authors: Lin, Min-Jyun, 林玟君
• Other Authors: Hu, Jian-Hwa
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/61602837557489171384

碩士 === 國立臺灣海洋大學 === 海洋環境資訊系 === 103 === In order to observe the stratification of currents at thermocline around the south of Taiwan, including the Kuroshio, Luzon Strait and Taiwan Strait, experiments of five spots were set up. Stratified currents were measured by five GPS drifters hung with a plastic can-vas holey-sock drogue of seven meters long and one meter diameter at the conceptual depths of surface, 15m, 30m, 50m and 100m. The weight of the bottom circular pipe of the drogue was little heavier than the buoyance of the top pipe, rendering the drogue sank to its nomina-tive depth. At the top pipe a mini pressure gauge was tied to record the time series of varied depths of the drogue. For each experiment, 8 hours observation was done due to the con-straint of battery, the vertical up-and-down motions of drogues were recorded apparently. The drifters provide Lagrangian data for depicting their trajectories and calculating the velocities. The former can be applied to estimate the instantaneous vorticities according to the angle variations of trajectories. The latter on the other hand is used straightforwardly for the relative vorticity equation. According to the Bernoulli theory, fluid flows faster would be in lower pressure to absorb adjacent fluid nearby of low speed, causing another possible mechanism of drogue’s vertical motions. In concept, at the northern hemisphere, a cyclonic system with a positive vorticity should have a counter-clockwise turning movement, being accompanied with a low pressure to ascend a drogue as a result, and a negative vorticity to descend it. As thought the entire drogue has a slight net weight, it could descend when the cyclonic vorticity is getting weak without a negative vorticity occurrence. In other words, a phase delay could be there between the drogue vertical motion and the vorticity. According to this conjecture, subjectively point out those cases interpretable by the two mechanisms, happened independently or synchronously but needing an overlay cancel-lation, leading that the vertical motions of drogues are caused by vorticities and velocities in durations being about 70% to 80% in statistics.