熱帶西太平洋上近一年之錨碇量測上層海洋溫度與颱風對其影響

Abstract

為研究颱風與海洋間交互作用，2009年由台灣與美國合作之西北太平洋及其邊緣海之颱風-海洋交互作用與內波之研究，英文簡稱ITOP，實驗規劃在西北太平洋中施放三組海氣象觀測即時傳輸平台，觀測項目包含氣象參數與水下溫度，在2009年3月至2010年3月，共記錄了一年水下資料與片段的氣象資料。水下溫度記錄顯示海洋溫度變化以季節性變動為主，另外亦夾帶有較小時間尺度的降溫或升溫；後者中有些降溫與冷鋒通過有關，其特徵是溫降由海表開始向下略有延遲，時間尺度約為一週；另外也有海洋中尺度渦旋（eddy）通過所引起之升溫或降溫，其時間尺度則為數週。 觀測期間，盧碧颱風（Lupit, 2009）通過其中兩個測站，一個記錄到颱風眼通過，而另一個則是記錄到颱風眼牆區通過時之氣象與水下溫度反應。颱風本身的行進速度與颱風在路徑變化時風應力變化趨勢，皆是造成颱風冷尾跡（cold wake）呈現對稱分佈或是偏向路徑右側的原因。測站所記錄到之最高風速為60公尺/秒，最低氣壓為942毫巴；颱風造成在颱風眼下方測站海表面溫度降低4.5 °C，在眼牆區亦降低4.5 °C。在颱風眼下方的海洋，受到直接輻散作用影響，溫度約在0~0.5個慣性週期時降低，而在颱風路徑左側，因有水平平流（advection）效果而使得降溫時間延後約0.2個慣性週期。 積分上層100公尺溫度可得上層海洋熱含量，其顯示慣性震盪在颱風通過後約持續5天，受到季節變化影響，上層海洋熱含量並未恢復到颱風前的狀態。海洋中的冷渦在颱風通過前已經存在，其受到颱風影響而增強；有冷渦存在的區域，因溫躍層受冷渦影響抬升，颱風在此處引起之降溫幅度會更大，其亦為颱風降溫幅度之主要因素。另外在其中一個測站量測到次中尺度流絲（filament），其在颱風引起之冷尾跡回溫時造成再降溫，然而此一再降溫僅持續三天，其回溫速度很快，並未影響到原來冷尾跡之回溫速度。 比較衛星資料之海表面異常值與積分至360公尺處之海洋熱含量顯示高相關性，顯示此處之溫度變化主要受到中尺度海洋運動影響。使用經驗正交函數（EOF）分析所量測之海洋溫度，在三個測站皆顯示第一模態佔80%以上之變異量，而第一模態之時間序列與衛星資料之海表面異常值亦有高相關性，利用量測資料與量測期間內之海表面異常值為資料庫，反推二者關係，進而推估垂直分布之海溫結構，可提供予未來預測研究區域之垂直海溫結構。

Three surface buoys at stations A1, A2 and A3, equipped with surface meteorological and subsurface temperature sensors, were deployed in the tropical western Pacific. Together, they recorded a year of upper ocean thermal structure and some meteorological data from March 2009 to March 2010. Seasonal cooling and warming trend was often interrupted by intermittent cooling and sudden warming. Some cooling episodes were induced by cold air passages in spring. Others, cooling or warming, were often related to the migration of cold and warm eddies. Lupit (2009), once intensified to a category-5 typhoon, ran over one buoy with its eye and skirted by another with its eye wall when it was weakening from category 3 to 2. All buoys worked well except for a few instrument failures after Lupit’s passage. Changes in the translation speed and wind stress field during typhoon re-curvature or left-turning influenced the appearance and location of the cold wake. The highest wind speed of 60 m/s and lowest air pressure of 941 mb were recorded under the eye. The net sea surface temperature drop was 4.5 °C under the eye and 4.5 °C under the eye wall. During Lupit’s passage, temperature in the upper thermocline increased first before decreasing. The upper-ocean current divergence under the typhoon’s eye caused the temperature drop in half an inertial period or less. On the left-side of Lupit’s track, horizontal advection delayed the cooling by about 0.2 inertial period. Upper ocean heat content in the top 100 m showed post-typhoon near-inertial oscillations for 5 days and did not bounce back to the pre-Lupit value thereafter. Preconditioning by a cold eddy enhanced the post-typhoon cooling at one station. A sub-mesoscale filament was observed during the warming or relaxation of the cold wake at station A3. It induced a three-day cooling pulse superimposed on the otherwise warming trend. Applied Empirical Orthogonal Functions (EOFs) analysis to the observed temperature, Mode 1 explained over 80% variance at all three stations. Mode 1 amplitude has a high correlation with satellite derived sea level anomaly (SLA). The relationship between SLA and Mode 1 amplitude, found by linear regression, may be useful for prediction of vertical temperature structure in the Phillipine Sea.