黑潮上游海區海洋動力學實驗─子計畫一：黑潮上游海域潮汐及底層環流的研究(Ⅲ)

行政院國家科學委員會補助專題研究計畫成果報告

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黑潮上游海區海洋動力學實驗之子計畫一

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黑潮上游海域潮汐及底層環流的研究（三）

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計畫類別：□個別型計畫

□整合型計畫

計畫編號：NSC90－2611－M－002－021－OP2

執行期間：

90 年 08 月 01 日至 91 年 07 月 31 日

計畫主持人：陳慶生

共同主持人：

本成果報告包括以下應繳交之附件：

□赴國外出差或研習心得報告一份

□赴大陸地區出差或研習心得報告一份

□出席國際學術會議心得報告及發表之論文各一份

□國際合作研究計畫國外研究報告書一份

執行單位：台灣大學海洋研究所

中

華

民

國

91 年

10 月 31

日

行政院國家科學委員會補助專題研究計畫成果報告

計畫編號：NSC90－2611－M－002－021－OP2

執行期間：90 年 8 月 1 日至 91 年 7 月 31 日

主持人：陳慶生 台灣大學海洋研究所物理組

計畫參與人員：陳慶生、李逸環、潘怡蘋

From a 3-dimensional eddy-resolving numerical model of the South China Sea (SCS), we found that there is a cyclonic circulation in the deep basin (below 1000m) throughout the year. The strong upwelling of the deep SCS water is associated with this circulation pattern. Besides the mean cyclonic circulation, there is also a westward propagating Rossby wave of an annual period in the depth region below the main thermocline (below 250 m).

Keywords: South China Sea, cyclonic circulation, Rossby wave

二、緣由與目的 水深為 2750 公尺、儀器深度分別是在水下 1500、2000、2400 公尺處。這組錨碇資料 可以提供南海深層水的入流量及溫度通量 的估算。數值模擬則是建立一個可以解析 渦漩運動的南海環流模式，探討季風改變 對於南海環流及呂宋海峽內黑潮支流入侵 南海北部的影響。 三、結果與討論 A three-dimensional eddy-resolving model of Semtner and Mintz (1977) is adopted to study the circulation in the SCS. Our model domain covers a region between 99-122 E and 3-23 N. The horizontal resolution is 1/5.5 degree (about 20 km). There are 18 layers in the vertical, with 25 m in the top four levels, 50 m in the next three levels, 150 m in the following five levels and 500 m in the lower six levels, yielding a maximum model basin depth of 4000 m. Chern and Wang (2003) reported the seasonal variation of the upper ocean circulation of the model results. We will focus on the intermediate and deep water flow in this report. Figure 1 depicts the horizontal distribution of the annual mean temperature and velocity field at 10th layer (550-700 m). The intruded Kuroshio originated warm water forms an anticyclonic loop current in the northern SCS basin and

annual mean circulation of the deep water depicts a cyclonic pattern with relatively strong current occurring along the western slope region of the basin. The upwelling of the deep SCS water is associated with this cyclonic circulation.

We also calculate the difference between the computed flow and its annual mean field as shown in Figs. 1 and 2. The seasonal variations of these anomaly fields are shown in figures 3 and 4. From Fig. 3, we infer that a cyclonic anomaly eddy occurs to the west of Luzon in autumn, probably due to the onset of northeast monsoon winds. An anticyclonic anomaly eddy presents in the western half of the SCS basin at this season.

The cyclonic anomaly grows and moves toward west in winter. A cyclonic gyre presents almost in the whole SCS basin at this depth during the winter. In spring, the circulation to the west of Luzon changes into an anticyclonic pattern and the relatively cold anomaly water is confined along the northwestern slope region. The anticyclonic flow also grows and moves westward and occupied the whole basin in the summer season. This seasonal cycle of the intermediate circulation in the SCS may have strong effects on the exchange of water mass between SCS and Kuroshio. The

westward propagation of the anomaly fields also occurs at depth below 1000 m, as shown in Fig. 4, but its strength becomes much weaker. 四、計畫成果自評 本計畫原訂的工作無論是現場實測或是數 值模式的建立都能依照預期目標執行。現 場實測的結果証實了深層外洋水全年穩定 流入南海下層，最終會以垂直方向的湧升 而流出南海。這個過程應是南海水團有較 強分層的一個主因。數值模式結果顯示出 南海中、下層環流的型態，尤其是南海深 層的氣旋型渦流及中層年週期的渦度波傳 遞行為，都是本計畫的主要貢獻。 五、參考文獻

[1] Chern, C.S. and J. Wang (2003) Numerical study of the Upper-layer circulation in the South China Sea. Journal of Oceanography, 59, 1 (in press).

[2] Semtner, A. J. and Y. Mintz (1977)

Numerical simulation of the Gulf Stream and mid-ocean eddies. Journal of Physical Oceanography, 7, 208-230.

Fig.3 Seasonal Variation of the Temperature and Velocity anomaly fields at 10th level (550-700 m)

Fig. 4 Seasonal Variation of the Temperature and Velocity anomaly fields at 13th level (1000-1500 m)