颱風引起海洋中尺度渦之動力過程

Abstract

過去有部分關於颱風對預先存在的中尺度渦構成影響之相關文獻，卻鮮少探討當海洋為中性環境時，由於颱風通過所引起之海洋中尺度氣漩渦 (Cyclonic Ocean Eddy, COE)。本研究透過比對1995年到2020年之颱風最佳路徑資料 (Best track) 以及中尺度渦漩軌跡圖集 (Mesoscale Eddy Trajectory Atlas)，找出在西北太平洋中，歷年來颱風經過後由中性環境構成COE幅度變化最大的三個範例進行分析，此三例分別為1997年的Rosie、2009年的Nida以及2011年的Ma-on颱風。我們利用區域海洋模擬系統模式 (Regional Ocean Modeling System, ROMS) 對三個例子進行背景環境重建模擬和理想實驗，以還原颱風當下的海洋狀態，並通過設計實驗去確認COE是否為颱風所致。實驗結果顯示在颱風Rosie期間的COE生成係由當時海洋環境與颱風共同作用下的結果，Nida期間的COE則為颱風主導所致，Ma-on期間之COE為海洋環境所主導生成。在此之中特別針對由Nida颱風所生成的COE去進行更進一步的分析，探討其生成過程之動力機制。Nida颱風在生成COE時行進速度緩慢 (1.5216m⁄s) 小於當時海洋的第一斜壓模相位速度 (2.4534m⁄s)，在緩慢移動的情況下使表層海水幅散，導致海表高度下降，同時引起艾克曼抽汲 (Ekman pumping)，而海表高度下降使海水趨向地轉平衡，導致COE的生成。本研究也針對其生成過程進行能量計算，同樣顯示了相同的結果，在颱風駐留時強大的風力擾動上層海洋，使動能快速上升，並帶動艾克曼抽汲，強大的湧升流使等密度線傾斜，因此動能漸漸轉變為勢能，在颱風過後動能與勢能漸漸趨於平衡，也意味著COE的穩定。另外，透過渦度方程進行收支分析，分析顯示除了地轉平衡所帶來的正渦度以外，湧升以及平流造成的傾斜項也是增加相對渦度促使COE形成的主因之一。

There are some studies that mentioned the influences of typhoons (or tropical cyclones) on pre-existing mesoscale eddies. However, less attention has been paid to the new cyclonic ocean eddy (COE) induced by a typhoon. In this study, we compared the data of typhoons and Mesoscale Eddy Trajectory Atlas from 1995 to 2020, to select the possible cases of typhoon-induced COE from the neutral condition during typhoon passages. The three cases with the largest difference in amplitude were used for further analysis. The three typhoon cases are Rosie in 1997, Nida in 2009, and Ma-on in 2011. Numerical modeling based on regional ocean modeling system (ROMS) was executed to reconstruct the background environment during the generations of three possible events. Subsequently, a series of half-idealized experiments, and energetic and vorticity budget analyses were carried out to unveil the key mechanisms therein. The results show that the COE is generated mainly by typhoon wind forcing in Nida’s case. Therefore, we further analyze the dynamic process of COE generation to Nida’s passage. It is noted that the translation speed of typhoon Nida (1.5216m⁄s) is slower than the phase speed of the first baroclinic mode (2.4534m⁄s) of the marine environment by that time. The result shows that the sea surface water will diverge outward directed away from the storm track, resulting in a drop in sea surface height, and causing Ekman pumping at the same time, once the typhoon translation speed is slow enough. The decrease in sea surface height causes the seawater to get geostrophic balance, leading to the generation of COE. We can also see the same result in energetic analysis, the kinetic energy of the sea surface water will increase when the typhoon arrived, which will cause Ekman pumping, and the upwelling flow will lead to the incline of thermocline and increase of baroclinity (potential energy). Kinetic energy and potential energy will eventually reach a balance after typhoon passage, which implies that the COE gets into a stable state. Through the vorticity budget analysis, the results show that the tilting term is also an important factor in the formation of COE under typhoon, in addition to the contribution of horizontal advection resulting from geostrophic balance.