Eddy-induced transport of the Kuroshio warm water around the Ryukyu Islands in the East China Sea

琉球諸島周辺海域における乱流混合による黒潮系暖水塊の波及促進効果の解析

上平 雄基   ; 内山 雄介*; 御手洗 哲司*

Kamidaira, Yuki; Uchiyama, Yusuke*; Mitarai, Satoshi*

沖縄本島を含む琉球諸島周辺海域を対象とした2段ネストJCOPE2-ROMSダウンスケーリング海洋モデルを用いて、本海域特有のサブメソスケール(SMS)乱流の発達とそれに伴う3次元的な物質混合過程を評価するべく、季節変動成分を除去した渦成分に対してeddy heat flux (EHF)解析を行った。EHFに対してHelmholtz分解を適用することで、EHFを黒潮流軸成分と直交成分とに分解し、その3次元構造について検討した、その結果、本海域では、傾圧不安定と地形性シアの相乗的な効果により、黒潮流軸に対して東シナ海側では低気圧性のSMS渦が表層および水深500m程度の中層に発達すること、反対に沖縄本島側では混合層周辺に高気圧性のSMS渦が発生すること、およびその両者によって黒潮横断方向の渦熱輸送が3次元的に促進されていることを見出した。

In this study, an oceanic downscaling model in a double-nested configuration was used to investigate the role played by the Kuroshio warm current in preserving and maintaining biological diversity in the coral coasts around the Ryukyu Islands (Japan). A comparison of the modeled data demonstrated that the innermost submesoscale eddy-resolving model successfully reproduced the synoptic and mesoscale oceanic structures even without data assimilation. The Kuroshio flows on the shelf break of the East China Sea approximately 150-200 km from the islands; therefore, eddy-induced transient processes are essential to the lateral transport of material within the strip between the Kuroshio and the islands. The model indicated an evident predominance of submesoscale anticyclonic eddies over cyclonic eddies near the surface of this strip. An energy conversion analysis relevant to the eddy-generation mechanisms revealed that a combination of both the shear instability due to the Kuroshio and the topography and baroclinic instability around the Kuroshio front jointly provoke these near-surface anticyclonic eddies, as well as the subsurface cyclonic eddies that are shed around the shelf break. Both surface and subsurface eddies fit within the submesoscale, and they are energized more as the grid resolution of the model is increased. An eddy heat flux (EHF) analysis was performed with decomposition into the divergent (dEHF) and rotational (rEHF) components. The rEHF vectors appeared along the temperature variance contours by following the Kuroshio, whereas the dEHF properly measured the transverse transport normal to the Kuroshio's path. The diagnostic EHF analysis demonstrated that an asymmetric dEHF occurs within the surface mixed layer, which promotes eastward transport toward the islands. Conversely, below the mixed layer, a negative dEHF tongue is formed that promotes the subsurface westward warm water transport.