內孤立波受淺化地形影響之觀測：自下沉型轉化為上舉型

Abstract

本論文使用現場錨碇觀測及衛星資料研究內孤立波自下沉型轉化為上舉型內孤立波之現象。於2007年兩組位於東沙台地附近海域且沿著21° 05’ N之ADCP錨碇資料(為期4個月)顯示，約2月上旬，內孤立波由下沉型(深水站LR1，位於水深605公尺)轉變為上舉型(淺水站LR2，位於水深427公尺)之事件頻繁發生，事件發生時下沉型內孤立波變寬並呈現結構不對稱，前緣接近與地形斜坡平行，其後並跟隨一上舉型內孤立波。於此期間有一個半徑約50公里的反氣旋式渦漩從錨碇站東北方進入錨碇觀測海域，造成顯著的背景流場，U_max~0.4 m/s，V_max~0.4 m/s。在KdV內孤立波理論中指出非線性係數α大於0及小於0時分別有利於上舉型及下沉型內孤立波的存在，我們的分析發現，反氣旋式渦漩存在期間確實可滿足上舉型內孤立波發生的條件(α>0)，然而此期間的內潮亦扮演重要的角色，當內潮位於上舉(elevated)相位時，可抵銷反氣旋式渦漩的效應，此時仍為下沉型內孤立波(α<0)，觀測資料與KdV理論相符。此外，能量收支分析顯示淺水站的上舉型內孤立波係由深水站的下沉型內孤立波的能量轉化而來，深水站的下沉型內孤立波之動能相當於淺水站的下沉型及上舉型內孤立波之動能的總和，而淺水站的下沉型內孤立波之動能則少於深水站約10%。

Using observation and satellite data, the conversion from depression to elevation internal solitary waves (ISWs) was investigated. Observations were taken by one set of two ADCP moorings aligned along 21° 05’ N near the eastern flank of the Dongsha Plateau for 4 months. In early February 2007, the conversion from depression ISWs in deep water site (LR1, at a depth of 605 m) to elevation ISWs in shallow water site (LR2, at a depth of 427 m) was frequently observed. The conversion process showed the depression ISWs became broad and asymmetric with the front edge nearly parallel to the bottom slope, followed by an elevation ISW. In the period, an anticyclonic eddy with a scale of 100 km propagated into our mooring sites from northeast of Dongsha Plateau, producing a background flow of U_max~0.4 m/s and V_max~0.4 m/s. In KdV theory, the phase of depression and elevation ISWs are determined by the nonlinear coefficient α, i.e. the conditions of α>0 and α<0 favor the presence of elevation and depression ISWs, respectively. We found that the hydrographic condition in the presence of anticyclonic eddy indeed support the occurrence of elevation ISWs (α>0). Furthermore, the internal tides play a role as well, i.e. the effect of anticyclonic eddy can be cancelled by the elevated phase of internal tides while ISWs are depression waves (α<0). Analysis of energy budget supports that the energy of elevation ISWs at LR2 is converted from the depression ISWs at LR1. The kinetic energy of depression ISWs at LR1 is approximately equal to the summation of kinetic energy of depression and elevation ISWs at LR2. The kinetic energy of depression ISWs at LR2 is ~10% less than that at LR1.