使用數值模擬深入了解颱風引起臺灣東北海域降溫

Abstract

夏季颱風通過後，多次在臺灣東北角龍洞外海引起海表溫下降。2001~2020年18個第5類颱風中有3個颱風路徑十分相似，分別是2001年的尤特(Utor)、2008年的如麗(Nuri)和2008年的哈格比(Hagupit)。然而透過龍洞浮標海表溫資料觀察發現這3個颱風對龍洞海域造成的降溫幅度卻差異甚大，尤特颱風期間下降最多達8.8℃，如麗期間降溫為2.7℃，而哈格比期間海表溫下降幅度僅1.4℃。前人文獻指出，臺灣周遭海域颱風引起之海表降溫與颱風行進軌跡間有良好之關係，為了進一步釐清為何相近軌跡之颱風卻引起近岸海表溫降有如此顯著之差異，本研究使用區域海洋模擬系統模式(Regional Ocean Modeling System, ROMS)，重建此三相近軌跡颱風個別引起之上層海洋響應。同時，為了瞭解潮汐可能對颱風引起降溫過程造成之影響，本研究於數值實驗中亦納入了潮汐作用。透過實驗設計以及熱收支守恆方程診斷分析，探討各物理過程對三個颱風期間海表降溫所造成之影響。模擬結果顯示在尤特颱風期間，東海黑潮入侵最為顯著，亦驅動較強的次層冷水抬升，進而導致較大幅度的海表溫降，如麗東海黑潮入侵幅度最小，次層冷水抬升較不明顯，海表溫降幅度亦較弱，表東海黑潮入侵在近岸海表溫降中扮演著重要的角色，而模擬在納入潮汐效應會強化冷卻響應，並使其更接近真實情況。這三個相似路徑和強度的颱風在臺灣東北海域引起不同的區域風，尤特期間龍洞海域出現強東北風，為三者之中最有利於東海黑潮入侵之風力條件，此與颱風半徑有關。另外，從理想實驗可以得知區域風為主要驅動東海黑潮入侵的因素，海洋的部分初始場對降溫影響較小，潮時則影響較為明顯。熱收支分析結果顯示，三個颱風降溫過程溫度變化主要是受到垂直平流項影響，而潮餘流則會透過垂直平流項強化近表層的冷卻響應。最後，當颱風移動至南海時，流向轉為西北，進而造成臺灣海峽流速增加，流速快且溫暖的臺灣海峽流流經龍洞海域，使海表水溫回復。In summer, after the typhoon pass through the ocean, sea surface temperature drops several times surrounding the Longdong sea area in the northeastern Taiwan. There are 3 out of 18 class 5 Typhoons with similar moving paths from 2001 to 2020, Utor(2001), Nuri(2008) and Hagupit(2008) respectively. However, based on long-term in-situ measurement of sea surface temperature (SST) by temperature meters moored on weather buoys situated at northeast of Taiwan observe that typhoons trigger markedly different magnitude of cooling off northeast Taiwan. Utor (2001) leads to maximum SST cooling of 8.8℃. Nuri (2008) leads to maximum SST cooling of 2.7℃. Hagupit (2008) leads to maximum SST cooling of 1.4℃. Previous studies indicated that moving track plays a dominant role determining the cooling magnitude of this area. For further exploring the key reason(s) contributing to the marked discrepancy coolings, regional oceanic modeling system (ROMS) was used to reconstructive the background environment corresponding to generations of coolings to those three TC cases respectively. In addition, the role of tidal forcing was also examined in our analysis for a more comprehensive understanding. Through the comparison of a series of idealized experiments and heat budget analysis, influences of different physical processes on individual cooling by three TCs were elucidated. The simulation results show that the Kuroshio intrusion(KI) is most significant during typhoon Utor and it also drives a strong uplift of subsurface cold water, which leads to an extreme SST cooling. Typhoon Nuri cause weak KI and it also drives a weak uplift of subsurface cold water, which leads to a weak SST cooling. It indicate that KI plays a key role in those coolings. The simulation inclusion of tidal forcing systematically enhances the coolings and improve the simulations of those coolings, which make it closer to the reality. These 3 typhoons with similar moving paths and intensities caused different local wind in northeastern Taiwan. During typhoon Utor, it provides a most favorable local wind for triggering KI because its wider radius of maximum winds. In addition, from the ideal experiments, the local wind is the key factor driving the occurrence of KI. Initial field is little effect on cooling, while the tidal time is more obvious effect. Heat budget diagnostic shows that vertical advection term generally dominates the cooling process of these 3 typhoons and contributions of vertical mixing for the SST cooling in case Utor is much stronger relative to other two. Tidal residuals essentially enhances those sea surface coolings through the vertical advection. When the typhoon moved to the South China Sea, the flow direction turned to the northwest, which caused the flow velocity in the Taiwan Strait to increase. The fast and warm Taiwan Strait Current flowed through the Longdong sea area, restoring the SST.