2016-08-012024-05-18https://scholars.lib.ntu.edu.tw/handle/123456789/701615摘要：當河流所帶之淡水進入陸棚，水體的密度差異使淡水上浮並形成向外海擴散的淡水水舌（river plumes）。在地球自轉的影響下，淡水水舌在河口附近產生反氣旋式的環流，此環流最後受海岸邊界的限制，以沿岸流（buoyancy-driven flow）的型式繼續將淡水 向下游輸送（in the direction of coastally trapped waves）。由於淡水水舌與沿岸流系統為連結陸域與海洋生態系的主要管道，瞭解此系統的結構與傳輸特性是近岸海洋學的重要課題。 淡水水舌與沿岸流系統對外力（如潮流、風應力）的響應已受到廣泛的重視與研究。然而，由於這些外力在不同時間尺度下的複雜交互作用，量化、理解、進而估計沿岸流傳輸的時空變化仍然是近岸海洋學最具挑戰性的問題之一。此研究的主要目的為暸解在不同淡水水舌系統中（based on outflow Rossby number），沿岸流傳輸對不同時間尺度外力的反應。我們利用理想化的數值模式來簡化不同系統的複雜度，藉由 momentum budget analysis 及 lagrangian drifters 來探討淡水傳輸的不同路徑與機制。 我們將探討三個子題： ‧ Isobe (2005) 發現，沿岸方向的潮流（no tidally-driven mean flow）會增加沿岸流的傳輸。產生此現象的主要機制為何？ ‧ 當淡水水舌與沿岸流系統受到沿岸方向的風應力（upwelling and downwelling favorable wind stress）的作用之下，我們該如何區分由風及由浮力驅動的傳輸？（Is the coastal current transport of an outflow system dominated by wind or buoyancy-driven flow?） ‧ 當外力減弱或消失後，一淡水水舌與沿岸流系統調整回穩態所需的時間尺度為何？又此調整時間（adjustment time scale）受何物理過程控制？ 我們預期，此研究的結果可增進我們對近岸淡水與物質傳輸之機制與時空變化的基本認識，形成可供未來現場量測檢驗的假設，並對海岸的管理提供相對應物理過程上的指引。 <br> Abstract: River plumes form when a buoyant coastal discharge enters the shelf. Under the influences of Earth’s rotation, they turn anticyclonically and often flow downshelf as a slender, buoyancy-driven coastal current. Since the coastal discharges represent a major link between terrestrial and marine ecosystems, it is critical to characterize the structure and the transport of these buoyant outflows in the coastal ocean. Sensitivities of the buoyant outflows to coastal forcings have been a subject of extensive research. However, due primarily to complex interplay of these forces, quantitative understanding of the processes controlling the plume dispersal has not been fully established. The primary objective of this proposal is to quantify and understand the freshwater transport variability in the presence of transient ambient forcing across different outflow systems. A process-based, idealized modeling approach, which allows exploration of the parameter space, is adopted. Momentum budget analysis and langrangian drifter releases are used to elucidate the transport mechanisms. We focus on coastal current regions and address three main questions. ‧ Isobe (2005) showed that alongshore tidal currents enhances the net coastal current transport. What is the underlying mechanism? ‧ In the presence of upwelling and downwelling favorable wind stress, how do we evaluate the relative importance of wind- and buoyancy-driven transport in different outflow systems? ‧ What is outflow’s adjustment time scale when an applied force (e.g. wind) is removed? What processes control the adjustment time scale? The results from this study will enhance our understanding of fundamental processes involved in time dependent plume dispersal, form testable hypotheses for future field studies, and provide guidance for coastal management.