遠域颱風對臺灣東北龍洞海域降溫之影響

Abstract

颱風生成於廣大的熱帶海洋上，當其經過海洋時，沿著颱風附近的海表面溫度，會因颱風的經過而有明顯的海表溫下降，並且前人文獻指出颱風造成海表降溫通常發生在颱風行進路徑右側附近( Price, 1981 )。然而，我們通過龍洞氣象浮標過去20年 ( 1998至2017年)的連續觀測資料，發現行經呂宋海峽的遠域颱風多次在遠在颱風軌跡420 km外的龍洞海域造成6 ℃以上的降溫，為了釐清其中的機制，本研究使用區域海洋模擬系統模式 ( Regional Ocean Modeling System, ROMS )，模擬2001年至2018年具備類似路徑之6個颱風案例，分別為2001年的尤特( Utor )、2003年的杜鵑( Dujuan )、2005年的珊瑚( Sanvu )、2011年的南瑪都( Nanmadol )、2013年的天兔( Usagi )及2016年的莫蘭蒂( Meranti )；並搭配龍洞潮位站之潮位資料與浮標之海溫資料對模擬進行驗證，系統性分析遠域颱風在龍洞地區造成上層海洋冷卻響應之過程。此外，由於龍洞位於近岸區域，潮汐作用對颱風引起冷卻響應過程可能造成之影響，亦為此研究欲釐清之重點。研究結果顯示，遠域颱風在龍洞引發較為強烈之降溫，主要透過颱風引起臺灣東北海域黑潮入侵之機制，並且，敏感度實驗顯示，臺灣東北海域當地風力為決定黑潮入侵是否形成之關鍵因素。同時，結果亦顯示，在未加入潮汐作用之實驗當中，颱風引起降溫在六個研究範例當中平均較觀測資料弱6.8 °C。在納入潮汐作用後，颱風引起近岸區域降溫趨於強烈，與觀測資料對比，溫度平均差異大幅度縮小至1.0 °C，整體而言，納入潮汐作用後，颱風引起近岸區域降溫之過程模擬獲得系統性改善。最後，透過系統性分析發現，加入潮汐作用主要透過底下機制增強颱風引起上層海洋降溫之過程，包含: (1) 在半個潮汐週期性振盪內會有上升流，因此潮汐作用可以加強上升流的強度，而且潮汐混合會破壞水體分層，提供有利環境供次層冷水上升，加上颱風通過時造成此區的強烈的上層混合和上升流，更容易使冷水抬升至較淺的水層、(2) 潮流與海底地形的交互作用，導致底部水層動能增強且底部應力增大。此外，底部應力也可以驅動額外的底部艾克曼傳輸( Bottom Ekman transport )，弱化底部水體分層，容易產生較強的上升流，進而使颱風通過後造成的海表降溫更強、(3) 龍洞次表層的南向潮餘流引發向下的底部艾克曼流( Bottom Ekman flow )，造成底部向東的艾克曼傳輸( Bottom Ekman transport )，而後因海底地形的關係，在離岸約20公里處產生向上的流，整體構成一逆時針環流，由於該區域上下翻轉流作用，使潮餘溫呈現次層水冷卻而下層水增溫之情形。綜合上述效應，我們建議後續在模擬颱風對近岸上層海洋冷卻響應時，納入潮汐作用可相當程度改善近岸降溫之情形並使之更符合真實之海洋狀態。Typhoons are generated on the vast tropical ocean. When they pass through the ocean, the sea surface temperature (SST) might decrease significantly due to their passages. In addition, previous literatures point out that the SST drop usually occurs at the right side of the typhoon trajectory. However, in this study, through the continuous measurement of SST by Longdong weather buoy in the past 20 years (1998-2017), we found that the far field typhoons passing through the Luzon Strait repeatedly caused SST drop more than 6℃surrounding the Longdong sea area. To clarify the possible mechanism(s), this study uses the Regional Ocean Modeling System (ROMS) to simulate the oceanic environment corresponding to 6 far field typhoon cases with similar tracks occurred from 2001 to 2016. They are typhoons Utor ( 2001 ), Dujuan ( 2003 ), Sanvu ( 2005 ), Nanmadol ( 2011 ), Usagi ( 2013 ), and Meranti ( 2016 ). The simulations are validated by the sea levels data from in-situ tidal gauges of Longdong tide station and continuous temperature measured by moored buoy, and the processes of the upper ocean cooling response caused by far field typhoons in Longdong sea area are systematically analyzed.The results show that the strong cooling caused by the far field typhoon in Longdong is mainly through the mechanism of the Kuroshio intrusion. In addition, the local wind surroudning Northeast of Taiwan is the key factor determining the occurrence of Kuroshio intrusion or not. Moreover, in the experiments without tidal effect, the average temperature difference is 6.8℃, which is much weaker relative to in-situ measurements. After including tidal effect, the temperature drops in the nearshore resion tend to be strong. Compared with the in-situ data, the average temperature difference significantly reduces to 1℃. In general, after the tidal effect is included, the simulations of temperature drops in the nearshore region caused by typhoon have been systematically improved. Finally, through systematic analysis, it’s shown that, the addition of tidal effect enhances the cooling process through mainly the underlying mechanism. (1) Tidal mixing can de-stratify the water column and provide a favorable environment for cold water to rise. Consequentially, upper ocean mixing and stronger upwelling allow enhanced SST drop. (2) The interaction between tidal currents and bottom topography results in enhanced bottom kinetic energy and bottom stress. (3) The southward tidal residual current in the subsurface of Longdong caused a downward bottom Ekman flow. Then, due to the bottom topography, an upward flow was generated about 20 kilometers offshore, forming a counterclockwise circulation. This process leads to tidal residual temperature getting cold at the top and warm at the bottom. Finally, it is suggested that, for simulating the response of upper ocean cooling to typhoon passages at nearshore regions, including tidal effect can provide a more comprehensive understanding about the process of occurrence of SST cooling to typhoon passage.