上層海洋對颱風通過反應之數值模擬研究

Abstract

颱風是台灣常見的劇烈天氣現象，不僅對陸地造成威脅，同樣也能引起上層海洋強烈的變化。本研究利用三維原始方程數值模式，模擬颱風通過時上層海洋所產生的反應。為了解析出海洋中相關的動力過程，模式提高了混合層的解析度；此外，模式採用二階的紊流閉合方法來估算垂直的紊流黏滯係數，以便合理的模擬強風下動量在水中向下傳遞的過程。 本研究首先討論深海區不同移動速度的颱風在上層海洋所引發的運動。模式結果顯示，改變颱風移動速度使風場頻率接近慣性頻率時，風與流會發生共振，能量得以進入並驅動溫躍層海水運動，進而與混合層的流產生第一斜壓模的耦合，使慣性震盪得以持續，其伴隨的混合效應使海水在風力消失後能夠持續冷卻。而無共振發生時，海水冷卻主要藉由風力作用期間Ekman Pumping造成的等溫線抬升與紊流混合等機制達成，動量垂直輸送不易到達溫躍層，慣性運動隨時間與深度衰退，颱風過後溫度便逐漸回昇。當颱風進入地形複雜以及有強流通過的區域時，水平方向的輸送過程也可能成為冷卻上層海水的首要機制。例如本文第二部分探討的台灣東北海域，此處鄰接東西走向的東海陸棚，沿台灣東岸往北的黑潮在此區轉向，隨著地形往東北而去。綜合模式與觀測資料的分析可得知，在通過台灣附近的颱風事件中，部分路徑颱風能迅速改變此區流場的平衡，由夏季阻斷黑潮入侵的型態，變成冬季有利於黑潮入侵的型態，黑潮因而得以進到東海陸棚，而海水的冷卻主要由伴隨入侵而湧升的黑潮次表層水所造成，垂直紊流混合等機制則屬次要。當颱風行經陸棚而無背景強流的影響時，其引發的環流則受到海岸線與地形的影響，海水溫度變化分布型態不一。以本文第三部分所探討的通過南海北部的丹瑞颱風為例，它在抵達海南島前，已經在其東邊形成一股向南的強流，有助於颱風登陸前引起的下沉增溫結構的維持，並可能延緩登陸時其強度的衰減。此外，颱風在250公尺深處亦引發了靠地形支撐的波動，沿著陸棚向南傳遞，使未受風場直接作用區域的海水亦逐漸冷卻，對南海的水文分布確有影響。

Upper ocean response to tropical cyclones is studied, using a three-dimensional primitive equation model that has higher vertical resolution in the mixed layer and uses level-2 turbulence closure scheme to estimate vertical mixing. In this study, the model is applied to investigate ocean responses to typhoons passing over open oceans and near the continental shelf region with or without strong background currents. The model results showed that in the open ocean, without the interference of topography and background flows, the ocean cooling is primarily influenced by the wind frequency of the passing storm. When the wind frequency is lower than the local inertial frequency, upwelling is the dominant cooling process. When the wind resonates with the current at the inertial frequency, the mixing process becomes important. In resonant cases, momentum penetrates to the thermocline and forces the water to move. The flow is then coupled with that in the mixed layer, so the inertial oscillation of the upper ocean is persistent, which results in long-lasting cooling by mixing. For non-resonant cases, the vertical transfer scale is limited and cooling decays in 5 inertial periods. However, the vertical processes may become secondary when the typhoon moves to a region with both complicated topography and strong background flows, such as the area on the shelf of the East China Sea, near northeastern Taiwan. Both observations and model results indicate that the strong northeast wind in this area accompanied by a typhoon of certain path could significantly alter the circulations on the shelf. An intrusion event may be triggered through a similar mechanism as induced by the winter monsoon but at faster pace. Therefore, the cooling in this area is due to the onshore transport of the Kuroshio’s subsurface water onto the continental shelf, rather than entrainment mixing. If the background current is insignificant, the storm-induced circulation is then influenced by the regional geometry and topography and some warming areas may be produced. As shown in the event of a typhoon moving across the continental shelf of the northern South China Sea, the storm had built up a southward current east of Hainan Island before its center arrived. This flow feature is supportive of the downwelling structure later induced by the passage of the storm, and the warming effect that it creates remains after the storm impacts. Such warming will lead to an increase of upper ocean heat content, which may assist to uphold the storm’s intensity upon landing. In addition, there were topographically trapped waves, which were generated at 250 m depth, thus the cold anomaly quickly propagated southward along the shelf. Through this mechanism, the southern South China Sea, though remote from the storm’s wind field, was cooled in the storm event.