盧碧颱風造成上層海洋之溫度變化

Abstract

2009年10月的盧碧颱風(Lupit)中心近乎通過了佈放於西北太平洋上觀測站(A1)之上方，而後再通過另一站(A3)右側，儀器紀錄了時間解析度良好之海面大氣(含氣壓、風速與風向、太陽短波輻射、濕度、與氣溫)，與上層1至500公尺海洋的溫度變化情形，資料頗具有獨特性與珍貴性，藉此資料陳述討論海面大氣及上層海洋溫度的變化，並進而估算討論海氣的熱交換、海溫回復至一平衡溫度所需時間、及海溫變化的可能動力機制。 盧碧颱風接近測站時，A1所量測之最低氣壓為941.9 hPa，最高風速達59.2 m/s，A3因與颱風距離較A1遠，所記錄的最低氣壓為954.6 hPa，最高風速為46 .1 m/s。於颱風通過期間，兩測站相對溼度升至~95%，太陽短波輻射量與氣溫下降。海表溫與混合層溫度隨著盧碧颱風接近，溫度僅先略微下降0.4~1℃，而不同於混合層的溫度下降現象，混合層以下於颱風通過時，可見溫度有增加之現象。於颱風通過A1與A3後，上層海溫快速的下降，其中於混合層的降幅最為明顯，約於29~38小時後，海溫近乎同時下降至最低值，造成海表溫約為4.3℃的降幅；但因盧碧颱風路徑轉彎，造成A3海溫再次下降，使海表溫最大溫降可達5.2℃。隨著盧碧颱風逐漸遠離，混合層海溫逐漸回暖至一平衡溫度，但受季節變化之影響，海溫仍較颱風通過前的平均海溫低，而混合層以下呈現近慣性震盪(near inertial oscillation)。 由A1混合層(1~40 m)與A3混合層(1~30 m)之熱含量變化，顯示於颱風通過前2~3日，開始有些微下降之趨勢，而此期間海氣淨熱量通量顯示海洋為散失狀態，混合層熱含量初期略微降低可能源於此。颱風通過時，海流輻合造成上層較溫暖海水下沉，使分層明顯減弱，混合層厚度增加，因而可見A1斜溫層上端(40~100 m)與A3斜溫層上端部分(30~100 m)之熱含量皆有增加之現象。於颱風通過後29~38小時，混合層與斜溫層上端之熱含量快速下降至最低值，且浮力頻率最大值的深度位置變淺，混合層厚度減少，此應為下層冷海水湧升(upwelling)之故。 使用指數曲線迴歸(Exponential regression)對混合層各層海溫的時序變化進行擬合，估算回暖所需時間(the warming time, Γ)。結果顯示A1上層1~20公尺近乎同步增溫，Γ為16.7天，A3上層1~8公尺處，Γ為20.3天，此為因海溫回暖過程中，兩站的混合層厚度分別變淺至20與8公尺之故；以下深度至進入斜溫層之間，Γ則隨深度增加而增加。另外，由混合層熱平衡的各項收支結果顯示，於回暖過程中，雖淨熱通量為正值，大氣供給海洋能量，但海氣的熱交換僅提供部分熱量，由水平平流項(horizontal advection)提供海溫回暖的能量是不可忽略的。

ATLAS buoys wewe deployed in the Northwestern Pacific, the eye of typhoon Lupit passed over buoy A1 and to the right of buoy A3 last October 2009. These buoys measured surface metrological parameters including air pressure, wind speed and wind direction, solar radiation, humidity, and air temperature, and the water temperature in the upper 500 m. In this study, we use this unique and valuable data to discuss the upper ocean thermal variations and air-sea interaction, to estimate the e-folding time of the cold anomaly, and to understand the dynamic mechanism caused by Typhoon Lupit. From observations, the lowest air pressure is 941.9 hPa and 954.6 hPa with wind speed up to 59.2 m/s and 46.1 m/s at A1 and A3 respectively. An increase of humidity, decrease of solar radiation and air temperatures was also observed. Meanwhile, when Lupit approached, the temperature in the sea surface and mixed layer(ML) decreased by 0.4℃ at A1 and 1℃ at A3 while the temperature beneath ML was increased. After 29 ~38 hours, the temperature dropped significantly to the minimum in ML, causing SST to drop by 4.3℃at A1 and A3. Later, the cooling at A3 was enhanced because Lupit reapproached again, and reached the maximum cooling 5.2℃. After the passage, the cool water in ML warmed toward equilibrium with temperature ~1.4℃ lower than before due to seasonal cooling, moreover, near-inertial oscillations of temperature below the ML were also noticed. Integrating the heat content(HC) of the upper 40 m at A1 and upper 30 m at A3 showed a slight decrease. This can be attributed from the loss of the net heat flux. During the passage of Lupit, downwelling caused weakening of the stratification and thickening of the mixed layer depth(MLD), allowing HC at depth 40~100 m at A1 and 30~100 m at A3 to increase. After its passage, an abrupt upwelling caused the MLD to become shallower, making the HC in the ML and at the top of thermocline to decrease rapidly almost at the same time. Meanwhile, the warming time(Γ) was derived from using exponential regression of the water temperature data. The MLD decreased to 12m at A1 and only 8m at A3 during the warming process, allowing the temperature which within this interval to recover after 16.7 days and 20.3 days at A1 and A3, respectively. However, Γ increased with depths 20~50m at A1 and 8~50m at A3, which indicates a longer time is essential to reach the equilibrium temperature. To understand all of this, we estimated the heat balance in the ML during the warming process. The results showed even through the net heat flux was positive(which means the atmosphere transmit heat to the ocean), the surface heat flux was only a part of the heat sources, in other words, the horizontal advection is non-neglectable in the warming process.