Numerical Model Study of the Mesoscale Dipolar Eddy

中尺度海洋渦旋是大洋上常見的海洋現象，而此類海洋渦旋能夠夾帶水團與大量能量的傳遞。近年來研究發現大洋中同時出現由兩個旋轉方向相反的渦旋組成的中尺度雙極渦旋。本研究目的主要是探討雙極渦旋在f平面上的特性與其動力特徵。使用方法為減重力原始方程的渦旋解析模式探討中尺度雙極渦旋運動行為與動力過程。模擬六個實驗（A-F），結果如下： 在相同半徑及相同強度下，設定高壓渦旋在北，低壓渦旋在南，兩個渦旋中心相距720公里（實驗A，control case），此距離兩渦旋皆是靜止不動。當兩個渦旋中心距640公里（實驗B）時，發現兩渦旋開始緩緩移動，移動軌跡為向西平移；當兩個渦旋中心距離減少至560公里（實驗 C），發現兩渦旋不只有向西平移現象，在南北向還會互相接近。兩渦旋移動的原因是北方的高壓受到南方低壓的速度平流推著走，反之亦然，也就是雙極本身就有自行移動的機制。兩渦旋在實驗C中會靠近的原因則是由 vortex force所引起，使高壓產生一向南的加速度，低壓產生一向北的加速度；此加速度使中間交互作用區向西的動量通量產生輻合作用，造成其向西的流速變快。實驗E和實驗F是在相同半徑，不同強度的情況，結果顯示強度較弱的渦旋會移動的比強度強的快，且移動軌跡並非為直線平移而是繞著他們共同的質心在轉。 位於兩渦旋中央交互作用區的地方其能量都是增加，而在交互作用區外的能量則是減少，這之間的轉換是因為vortex force 產生的南北向加速度影響，使交互作用區的能量增加而產生輻合，交互作用區外則會因能量減少造成輻散。Mesoscale eddies are common features of the open ocean and they can provide an efficient mechanism for the water masses and transport of energy. Recent studies found mesoscale dipolar eddy that consists of two counter-rotating eddies in the ocean. This study aims to investigate characteristics and dynamics of mesoscale dipolar eddy in f-plane. A reduced-gravity primitive equation eddy-resolving model was used to study them. Five sets of experiments (A-F) are simulated. The results are described as follow. We add a pair of anticyclonic eddy and cyclonic eddies with same radius and intensity to the model. The anticyclonic eddy sits on the north, whereas the cyclonic eddy locates 720 km apart on the south (experiment A, control case). These two eddies are both still on experiment A. When the distance between these two eddies is 640 km (experiment B), the eddies begin to slowly propagate westward and the trajectory is translation. If the distance between eddies decreases to 560 km (experiment C), they not only propagate westward but also become closer meridionally. The eddies have tendency to move because the northern anticyclonic eddy is advected by velocity of the southern cyclonic eddy, and vice versa. In other words, the dipole has mechanism of self-propelling in itself. The vortex force makes the anticyclonic eddy generate southward acceleration and makes the cyclonic eddy generate northward acceleration, so the eddies meridionally approach to each other while propagating zonally. This acceleration results in a convergence of the westward momentum flux and a strengthening of the westward velocity between the two eddies. Experiments E and F are tested using same radius but different intensity. The results show that the weaker eddy would move faster than the stronger. The dipole would move around their common center of mass instead of along a straight trajectory zonally. The energy increases between the two eddies (the interaction zone), but decreases elsewhere. The conversion between them is due to the effect of the vortex force, which generates the northward and southward acceleration. The energy then increases (decreases) and results in convergence (divergence) in (outside) the interaction zone