Browsing by Author "許瑋真"

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南海與呂宋海峽渦漩動力之研究
(2009) 許瑋真
Satellite observations have shown the abundance of generally westward-propagating eddies in the subtropical regions in the North Pacific Ocean, especially north of 10°N. Eddies transport mass, and can significantly impact the circulation as well as the heat, salt and nutrient balances of the western Pacific marginal seas. This study uses a numerical model to examine the conditions when eddies can or cannot freely propagate westward through the Luzon Strait into the South China Sea (SCS). Composite analyses on the 10-year model data show that the fates of eddies depend on the strength and path of the Kuroshio. In one path which exists mostly during fall and winter, the Kuroshio loops westward into the SCS, the potential vorticity (PV) across the current is weak, and eddies are likely to propagate freely through the Luzon Strait. In another path which exists mostly during spring and summer, the Kuroshio tends to leap directly northward bypassing the SCS, the PV across it strengthens, and eddies are then blocked and are constrained to also follow the northward path. Nonlinear eddy-current interaction and the existence of a cyclone north of the Luzon Island during the looping phase explain why eddies of both signs can pass through the strait. It is shown also that the upstream state of the Kuroshio in the western tropical Pacific plays an important role in dictating the different paths of the Kuroshio. The looping (leaping) path is caused by a weakened (stronger) Kuroshio transport related to the northward (southward) shift of the North Equatorial Current in wintertime (summertime). During El Niño/Southern Oscillation (ENSO) events, the Kuroshio weakens and a large portion of the Philippine Sea water passes through the Luzon Strait. The intensity of the Kuroshio is capable of influencing the seasonal upwelling in the SCS. Seasonal upwelling events along the east coast of Vietnam and west coast of Luzon have been demonstrated by satellite data. In a normal year, a strong eastward jet is associated with the cooling and upwelling off Vietnam. Strong ENSO events have been recorded in the years 1997 and 1998: the satellite data clearly show a vigorous upwelling off Vietnam in August 1997, but a wan one in August 1998. Abnormal warming interrupts the normal upwelling generated off Vietnam and Luzon. Two defined indicators of sea surface temperature anomaly (SSTA), the upwelling off the coast of East Vietnam in summer and off the coast of west Luzon Island, reveal seasonal upwelling activities. Both these upwellings appear in normal years, but they diminish during the active warming period. The seasonal upwelling dynamics is dependent on ENSO and is also coherent with the South Eastern Tropical Indian Ocean (SETIO). The intensity of prevailing monsoon varies the proportion of the response to oceanic circulation and heat content, that is, weak monsoons do not promote water in vertical motion and less heat is released. The ocean heat content anomaly (OHCA) indicates whether the budgets for both the ascending and descending heat content are initiated at the western boundary. Although the OHCA in conjunction with the vortex has not been directly reported, the results of model dynamics studies are favorably comparable with the satellite data. Large-scale Indian-Pacific Ocean meteorological variations have certain strong impacts on the SCS. The teleconnection between ocean and atmosphere shows that the seasonal upwelling is controlled by the complex interplay between the internal and external sea-air interacting processes. The regional monsoon system changes rapidly in response to oceanic variations. The weak wind stress curl during the evolution of the La Niña cycle of 1998 affects the atmosphere-ocean coupling and hampers the generation of the upwelling. Variations in both the large-scale air-sea interaction and the strength of the Kuroshio transport impact (1) the generation of vortices over the SCS and (2) the path of movement vortices in the Luzon Strait.
桑美颱風與台灣東邊海域之黑潮的交互關係
(2006-10-20) 許瑋真; 吳朝榮; 徐月娟; 陳琬婷
臺灣附近海域傳輸量之研究
(2004-05-20) 許瑋真; 辛宜佳; 吳朝榮
臺灣位於西北太平洋(Northwestern Pacific Ocean )的海域上，是東亞島弧的一部份；東連太平洋，北鄰東海(East China Sea，ECS)，西接臺灣海峽(Taiwan Strait)，南靠呂宋海峽(Luzon Strait)，西南方為南中國海(South China Sea，SCS)。在此區域之風系概為：於冬季時低氣壓在北太平洋上形成，盛行東北季風，然於夏季時相反，盛行西南季風，故此季風系統的改變乃是影響臺灣附近海流變化的重要因子之一。臺灣周圍海底地貌有許多重要的通道，這些通道為水團交換的重要場所，黑潮由臺灣東邊的東臺灣通道(East Taiwan Channel， ETC)進入東海，西太平洋水團經由臺灣與呂宋島間的呂宋海峽與南中國海水團會合，東海水團流經位在臺灣與中國大陸(Mainland China)間的臺灣海峽與南中國海水團相會，而在臺灣海峽東南側的澎湖水道(Penghu channel)，是海峽中唯一水深超過200 公尺之處。所以這些通道影響著水團進出的特性，而且從過去到現在已有計畫展開研究。本研究的主要目的是運用臺灣附近海域傳輸量之比較來驗證西北太平洋數值模式的正確性，並探討此海域附近傳輸量的變異。由西北太平洋數值模式之模擬結果，我們可以計算出臺灣附近海域各主要通道之傳輸量的時間序列，其結果與觀測值相仿，如黑潮主軸在PN測線(124.50度E,30.00度 N)~(128.25度E，27.50度N)上往東北方向的流，年平均傳輸量為 26.7.plmin.3.5Sv(1Sv=10m/sup 3/s/sup -1/)，與Ichikawa and Chean(2000)指出年平均傳輸量為27.6Sv，頗為近似，故可說明此模式在臺灣附近海域的模擬具有很高的可信度。