海洋屏障層對颱風發展的影響之探討

Abstract

海洋結構的特性在颱風的演化中扮演了至關重要的角色。颱風施加的強風應力通過流切逸入和垂直混合作用將冷水從海洋下層拉出，進而降低海表面溫度，並減少颱風的最大潛力和當前強度，形成負反饋現象。垂直混合作用對颱風強度的影響越來越受到關注，尤其是颱風形成前的海洋剖面所扮演的角色。屏障層可以顯著增強海洋的穩定性，進而抑制海表面溫度的降溫效應，導致上層海洋熱含量的消耗。儘管一些研究已著重探討屏障層效應與颱風強度的關係，但深入探究屏障層與不同颱風強度的相關性仍然至關重要。因此本研究旨在進一步探索屏障層對颱風發展的影響。 本研究的目標是探討不同厚度的屏障層對颱風的影響，以及屏障層在不同颱風階段中對海洋結構和颱風強度的影響。我們使用耦合氣象研究與預報模型（WRF）和三維海洋模型 Price-Weller-Pinkel（3DPWP）進行理想化模擬，以簡化環境條件並聚焦於海氣交互作用和海洋變化。我們設置了具有四種不同屏障層厚度的初始海洋剖面，以分析這些模擬中颱風強度和海洋剖面變化的差異。 以上理想化模擬的結果顯示，屏障層對颱風的影響隨颱風強度而異。在環境條件良好且颱風已經較強的情況下，屏障層對颱風的發展影響較小。然而在不利條件下，如移動速度慢、輻射輸入減少以及中度或以上垂直風切的情況下，屏障層可能顯著影響颱風強度。未受擾動的屏障層因吸收短波輻射而顯示出變暖信號，且此效應隨屏障層厚度增加而增強。該過程強化了從海洋到颱風的表面熱通量傳遞，導致颱風增強。另一方面，對於弱颱風，混合層的降溫更加明顯且持久，削弱了颱風對於海表面熱通量的吸收作用，提供不利於颱風增強的條件。

The properties of ocean structure play a crucial role in the evolution of tropical cyclones (TCs). Strong wind stress exerted by TCs pulls cold water from the ocean underneath through entrainment and vertical mixing, cools down the SST and further decreases the maximum potential and current intensity of TCs, resulting in negative feedback. The impact of vertical mixing, which affects TC intensity, has gathered increasing attention, especially regarding the role of pre-TC ocean profile. Barrier layer (BL) can significantly enhance stability in the ocean column and ultimately suppress the SST-cooling effect, leading to the dissipation of upper ocean heat content. Although several studies have focused on the relationship between the BL effect and TC intensity, a comprehensive investigation into the correlation between the BL and varying TC intensities remains crucial. This study seeks to further explore the impact of BLs on the development of TCs. The objectives of this study are to investigate the effect of BLs with different thicknesses on TCs and the influence of the BL on ocean structure and TC intensity during different TC phases. We use the Weather Research and Forecasting (WRF) model coupled with the 3D Price-Weller-Pinkel (3DPWP) ocean model to conduct idealized simulations to simplify environmental conditions and to focus on the examination of the air-sea interaction and ocean variation. Different initial ocean profiles with four BL thicknesses are set to investigate the differences in TC intensity and changes in ocean profiles among these simulations. Results from the above idealized simulations indicate that the influence of BLs on TCs varies with TC intensity. BLs have a minimal impact on TC development when environmental conditions are favorable and TCs are already strong. However, under unfavorable conditions—such as slow movement, reduced radiation input, and moderate or strong vertical wind shear—BLs can significantly influence the TC intensity. An unperturbed BL shows a warming signal due to the absorption of shortwave radiation, with this effect intensifying as BL thickness increases. This process enhances surface heat flux transfer from the ocean to the TCs, leading to accelerated intensification. On the other hand, for weak TCs, the cooling in the mixed layer becomes more pronounced and prolonged, reducing surface heat flux uptake and resulting in more unfavorable conditions for intensification.