致沉流風場下湧升環流的生成機制

2012 年,Moffat 和 Lentz 在二維的海洋模式中發現,在有沿岸淡水 (buoyant coastal current) 的情況下,致沉流風場 (downwelling-favorable wind) 能夠於近岸側產生湧升環流 (upwelling circulation)。本研究利用 ROMS (Regional Ocean Modeling System) 建立一三維模式,針對 Moffat 和 Lentz 提出之此一現象進行敏感度測試和動力分析。本模式沿用 Moffat 和 Lent 的初始設定,設置固定的淡水流量持續注入簡單斜坡地形上。當淡水沿著海岸形成細長的沿岸流後, 加入穩定向著沿岸流流向吹送的風場 (致沉流風場)。與 Moffat 和 Lentz 的模式結果相同,一開始,致沉流風場造成下沉環流。表面速度向岸,底部速度離岸的下沉環流使沿岸流之等密度線變陡。當等密度線近乎垂直時,沿岸流的前緣卻產生湧升環流。而此湧升環流的強度會隨著淡水流量的增加,坡度變陡和初始的淡海水密度差增加而增強。由拆解沿岸速度,分成地轉流速和非地轉流速發現,當密度梯度造成的沿岸地轉流切 (geostrophic shear) 大過風所能支持的速度切 (wind-supported shear) 時,即有表面速度離岸,底部速度向岸的湧升環流於密度梯度大處 (density front) 產生。此現象的發生是由於跨岸方向的壓力梯度力和柯氏力不平衡所導致。湧升環流之表面離岸速度試圖減緩側向密度梯度和表面水位梯度,以減少表面大過於科氏力的壓力梯度力。我們也利用了考慮側向密度梯度的 Ekman model 對應到沿岸速度分析的結果:地轉流切和風應力所提供的速度切,兩者之相對大小會影響跨向環流的方向和強度。

關鍵字

湧升環流；致沉流風場；沿岸流；地轉流切；風持切變

並列摘要

A two-dimensional modeling study by Moffat and Lentz (2012) showed that,in the presence of buoyant coastal currents, upwelling circulation could be generated under downwelling-favorable wind. The sensitivities and the dynamics of this upwelling circulation are studied using a three-dimensional ocean model (ROMS). The model is configured with a river source discharging onto a constant sloping shelf. After a slender of buoyant coastal currents develops, stable downwelling wind stress is applied. Consistent with Moffat and Lentz (2012), downwelling wind stress initially produces downwelling circulation (i.e. onshore surface flow) that steepens the isopycanl and enhances the cross-shelf density gradient. When the isopycnals become nearly vertical, upwelling circulation is formed within the coastal currents. The circulation is strengthened by increasing discharge and increasing slope. Analyses of decomposing along-shelf velocity into geostrophic and ageostrophic parts show that, the upwelling circulation forms when the geostrophic shear of alongshelf velocity due to cross-shelf density gradients is greater than the shear supported by wind. This indicates that cross-shore pressure gradient force and Coriolis force are imbalanced, and the surface offshore flow is induced to weaken the density gradient. A simple analytical Ekman model accounting for the cross-shelf density gradients is used to demonstrate the control of cross-shelf circulation by the relative strength of geostrophic shear and wind-supported shear.