2018年馬登-朱利安振盪(Madden-Julian Oscillation)活躍期下風所引發之混合層加深

Abstract

在北半球冬季，馬登-朱利安振盪 (MJO) 為季節內天氣系統具有顯著的深對流，從印度洋西部開始沿著赤道向東移動。2018年11月在澳洲西北所佈放的兩個EM-APEX floats、兩個ALAMO floats和一個 FIO buoy，測量2018年12月中旬MJO通過期間的海洋溫度、鹽度、水平流速與基本大氣參數。自12月14日以來，浮標量測到混合層在五天內從25m快速加深到50m，並且該段時間內MJO所帶來的西風維持9 ms-1以上，引起高達0.4 ms-1的海流，破壞上層海洋的穩定。透過計算梯度理查森數(Ri)以發現不穩定性，由於經常觀測到小於0.25的Ri，因此推測在強風作用下，上層海洋可能會出現不穩定和強烈的紊流混合。本研究使用Thorpe scale方法估算紊流耗散率，結果顯示混合層的紊流耗散率約為10-7 Wkg-1至10-6 Wkg-1，大於典型溫躍層內的紊流耗散率。在MJO連續幾天的風力作用下，剪切不穩定可能會發生強烈的紊流混合，從而使混合層加深。混合層加深導致海表溫度(SST)冷卻約1.1°C，SST的變化改變了潛熱加顯熱量由100 Wm-2增至400Wm-2，並有可能影響MJO的發展。由於混合層加深可能有助於海表冷卻，因此MLD變化在模式模擬中至關重要，研究中模式結果顯示，在MJO下使用COARE 3.0算法計算的風應力可能低估。因此通過觀測資料測量與估算正確風應力，可以在模式中更好地模擬MJO觀測的特徵，並進一步改進MJO的預報。

During the boreal winter, Madden–Julian Oscillations (MJOs) as organized deep convections and intra-seasonal weather systems propagate eastward along the equator, starting from the west of the Indian Ocean. Two EM-APEX, two ALAMO floats, and an FIO buoy were deployed in the northwest coast of Australia, which captured the ocean responses of temperature, salinity, and horizontal current velocity during the passage of one MJO in the middle of December 2018. The four floats captured a rapid deepening of mixed layer depth (MLD) from 25 m to 50 m since 14th Dec in five days. At the same time, strong westerly wind associated with MJO was mostly > 9 m s-1. The wind-induced a strong current up to 0.4 m s-1 for destabilizing the upper ocean. The gradient Richardson number (Ri) was computed for identifying the instability. Because the low Ri < 0.25 was frequently observed, instability and strong turbulence might occur in the upper ocean under the strong wind forcing. Using the Thorpe-scale method, the turbulent dissipation rate was approximately 10-7 to 10-6 W kg-1 in the MLD, which was larger than those within the typical thermocline. Strong turbulent mixing might occur via shear instability under the consecutive days of wind forcing, thereby MLD deepening. MLD deepening contributed to cooling sea surface temperature (SST) by about 1.1 °C. The heat fluxes were modulated by SST variation from 100 to 400 W m-2. The heat flux variation might affect the development of MJOs. Because MLD deepening may contribute to the cooling of SST, the simulation of MLD variation is critical in models. In the study, model results demonstrate that the computation of the wind stress using the COARE 3.0 algorithm may be underestimated under MJO. Therefore, with correct wind stress based on the float measurements, several features of the observations can be better captured in models and further improve MJOs’ forecasts.