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Please use this identifier to cite or link to this item: http://ntour.ntou.edu.tw:8080/ir/handle/987654321/52877

Title: 海洋與大氣多尺度變動對台灣附近黑潮的影響
Effects of Oceanic and Atmospheric Multi-scale Variations on the Kuroshio Adjacent to Taiwan
Authors: Hsu, Po-Chun
許伯駿
Contributors: NTOU:Department of Marine Environmental Informatics
國立臺灣海洋大學:海洋環境資訊系
Keywords: 黑潮;中尺度漩渦;島嶼尾流;衛星影像;水下滑翔機;颱風
Kuroshio;mesoscale eddy;island wake;satellite imagery;glider;typhoon
Date: 2018
Issue Date: 2020-01-20T06:20:35Z
Abstract: 本研究利用現場觀測、高解析度衛星影像、衛星遙測數據和再分析資料探討海洋和大氣多尺度變化對台灣附近黑潮的影響。首先,利用衛星遙測數據分析台灣東部的黑潮路徑和海水表面溫度及葉綠素濃度的變化。台灣東部的黑潮會受到從北太平洋向西傳播的冷渦影響,在1993年至2013年間向東蜿蜒13次。黑潮蜿蜒的最長持續時間可達80天。受冷渦大小的影響,黑潮主軸最遠可東移距離其原始位置約270 km,且造成黑潮主軸的流速降至其季節平均值的84%,約為0.75 m/s。 當黑潮流經台灣東北部時,會與宜蘭灣附近的沿岸流作用。本研究使用歷史水文測量數據和衛星遙測數據分析,發現宜蘭灣沿岸有逆流(與北向的黑潮流向相反)情況發生。根據聲波都卜勒流剖儀的速度剖面歷史數據顯示,逆流以約0.1 - 0.2 m/s的速度沿台灣東北海岸向南流向宜蘭灣。另外根據水質指標結果,宜蘭灣水體主要受台灣北部的陸棚水域影響,並與黑潮近岸水體混合,因而形成了兩海域之間明顯的海洋鋒面。從動力學過程分析來看,宜蘭灣沿岸逆流主要受台灣東北部的冷丘所影響,以及台灣海峽東北向流動引起的反氣旋流場或中國沿海的東南流所造成的。 在台灣東南部,當黑潮沿著台灣東部經過綠島時,黑潮與島嶼之間的相互作用形成了島嶼尾流。應用衛星中解析度成像分光輻射計(MODIS)的海水表面溫度數據,將島嶼尾流海面溫度的空間分佈劃分為四種不同的類型。最常發生的類型是尾流單獨形成,佔島嶼尾流型態的86.7%。其他三種類型是尾流隨下游延伸(4.0%);一個小冷渦旋形成但尾流沒有伸展至下游(6.8%)和尾流形成S形彎曲的尾跡(2.5%)。另外使用衛星測高儀和聲波都卜勒流剖儀(ADCP)數據計算黑潮的速度,發現在綠島下游面形成漩渦列時,島嶼西側的流場速度增加。而風力作用也會對漩渦列的特徵有明顯的影響,根據高空間解析度衛星影像分析,在偏南風下,漩渦列的平均縱橫比為2.09,無因次寬度比為2.02,而在北風下則分別為1.91和2.76。在綠島海域,由尾流引起的垂直混合,導致海表面有相對溫度較冷和鹽度較高的水體以及島後海面的葉綠素甲濃度提高。島嶼下游海水之等溫線、等鹽線和等密線等被抬升,則提供了海水湧升的證據。在垂直混合作用下,密度翻轉的Thorpe尺度在2.9〜20.5 m之間,湍動能耗散率為0.2×10^-6〜8.5×10^-5 W kg^-1,相當於渦流擴散係數範圍為0.01-0.23 m^2/s。 颱風的強風及降雨對於黑潮水文特性也會影響。本研究使用2010年至2013年期間在台灣附近黑潮地區的水下滑翔機數據,探討在14次颱風案例發生的狀態下,海洋表面和次表層的溫度及鹽度反饋。結果顯示在垂直混合過程的熱泵效應下,颱風會引起次表層增溫,最大溫度變化發生在混合層的底部或更深,而颱風前的海洋斜溫層梯度是決定次表層增溫幅度的重要因素。而伴隨著颱風的大量降雨,將大量淡水引入上層海洋,淡化了表層鹽度。稀釋的表層鹽度伴隨著混合層的加深,透過垂直混合向下移動將淡水輸送至次表層。本文的研究結果有助於理解台灣附近海洋環境變化的影響。
Effects of oceanic and atmospheric multi-scale variations on the Kuroshio adjacent to Taiwan are analyzed using in-situ observations, satellite imagery, satellite data, and reanalysis numerical data. First, satellite and glider data are used to study the Kuroshio meander and surface properties east of Taiwan. The Kuroshio meandered eastward 13 times between 1993 and 2013 because of cold eddies westward propagating from the Norht Pacific. The maximum duration of the meanders was 80 days. The farthest eastward shift of the Kuroshio maximum velocity axis was 270 km from its original position, depending on the size of the cold eddy. Cold eddies reduce the current speed at the Kuroshio maximum velocity axis to 84% of its seasonal average, which is 0.75 m/s. When the Kuroshio flows through northeastern Taiwan, the interaction of coastal water in I-Lan Bay, a bay near northeast Taiwan, and the Kuroshio Current is studied using the data from hydrologic survey and satellite remote sensing. An index for water mass distinguished is used for clarifying the original of water mass in I-Lan Bay. The velocity profile from acoustic Doppler current profile data indicates a countercurrent that flows southward along the northeast coast of Taiwan toward I-Lan Bay with a speed around 0.1-0.2 m/s. The index of water mass indicates that the water of I-Lan Bay is mainly affected by the northern shelf waters of Taiwan and mixed with Kuroshio nearshore water, thus forming a clear ocean front between these two areas. From the analysis of dynamic process, the coastal countercurrent in I-Lan Bay is primarily affected by 1) the occurrence of cold dome in northeast of Taiwan, and 2) anti-cyclone flow fields due to the northeastward movement of the Taiwan Strait Current and/or the southeastward flow of the China Coastal Current. When the Kuroshio passes Green Island of Southeastern Taiwan, well-organized wakes are formed by interaction of the Kuroshio with the island. High-resolution satellite imagery to conduct a statistical study of the ocean vortex train induced by the Kuroshio Current, including optical imagery from the Satellite Pour l'Observation de la Terre and the Formosat-2 satellites and synthetic aperture radar imagery from the European Remote Sensing Satellite, Advanced Land Observing Satellite, and Sentinel-1. Satellite altimetry data and a moored acoustic Doppler current profiler (ADCP) were used to calculate the velocity of the Kuroshio Current. The ADCP data suggest that the velocity increases on the western side of the vortex train when it is formed on the leeward side of Green Island. Wind forcing had a pronounced effect on the characteristics of the vortex train. High-resolution satellite images indicate that the averaged aspect ratio of the vortex train is 2.09 and the dimensionless width is 2.02 under southerly winds, compared to 1.91 and 2.76, respectively, under northerly winds. Wake-induced vertical mixing gives rise to a mixed layer in the lee of the island, which results in relatively colder and saltier waters and higher chlorophyll-a concentration on the sea surface behind the island. Uplifting of the isotherms and isohalines, and isopycnals shoaling on the leeward side of the island provide evidence of upwelling. Under vertical mixing, the density overturns with a Thorpe scale of between 2.9 and 20.5 m, and the turbulent kinetic energy dissipation rate is 0.2×10^-6〜8.5×10^-5 W kg^-1, which corresponds to eddy diffusivity in the range of 0.01-0.23 m^2/s. The spatial distribution patterns of the sea surface temperature (SST) in the island wake could be separated into four distinct types from moderate-resolution imaging spectroradiometer (MODIS) SST images. The most frequently occurred type is the wake alone, which occupies 86.7% of the island wake patterns. The other three types are the wake with a tail stretching downstream (4.0%); the wake with a small cyclonic cold core but no tail stretching downstream (6.8%); and S-shaped meandering wakes (2.5%). This study also explored the impact of typhoon on the hydrological characteristics of Kuroshio. Underwater gliders are used to investigate the variations on the ocean surface and subsurface during the 14 typhoons that passed over the Kuroshio region near Taiwan in 2010–2013. Typhoon-induced subsurface layer warming, which was formed based on the heat pump effect of vertical mixing process, is observed in this study. Besides, the gliders observe the variations in salinity during the passage of the typhoons. Typhoons, accompanied by heavy rainfall, introduce considerable amount of fresh water into the upper ocean, diluting the surface salinity. The diluted salinity accompanied deepening of the mixed layer which moved downward to the subsurface by vertical mixing, supplying fresh water to both the surface and subsurface layers. Additionally, because of vertical mixing, maximum temperature variations occur at the bottom of the mixed layer or at a level deeper than the mixed layer. The upper ocean thermocline gradient before a typhoon plays an important factor to determine the magnitude of subsurface warming. The results of this study are helpful for understanding the impact of different scale changes in the marine environment adjacent to Taiwan.
URI: http://ethesys.lib.ntou.edu.tw/cgi-bin/gs32/gsweb.cgi?o=dstdcdr&s=G0020281001.id
http://ntour.ntou.edu.tw:8080/ir/handle/987654321/52877
Appears in Collections:[海洋環境資訊系] 博碩士論文

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