南海巨大振幅內波之研究

Abstract

本論文主要在於研究南海巨大振幅內波，包括其行進的特性，能量、能量通量、消散，與非線性內波表面散射強度與其內部性質的關係。 首先使用三組置放於東沙海底高原的長期ADCP錨碇量測、並整合現場船測與遙測資料來研究非線性內波的行進特性，錨碇位於東沙海底高原東緣，並且沿著21o05’排列，由東而西，連續兩站相距分別為~8.5海浬與~17海浬。非線性內波引發的海流與兩站間波到達時間差分別用以計算波的行進方向與行進速度。研究結果顯示，平均行進方向為165 o，也就是維持西稍偏北的行進方向，於連續兩個站間的平均行進速度，由東而西分別為1.83±0.38 m/s 與1.61±0.20 m/s，以上行進速度與方向的估計可由船載海洋雷達與衛星影像所驗證。行進方向長期而言並無明顯之規則變化，但行進速度的長期變化顯示，於8-10月非線性內波的行進速度較快，而1-3月份行進速度較慢，與典型南海季節性分層：8-10月分層強烈，1-3月分層弱相符，使用氣候平均密度場計算第一斜壓模內波之線性相速度，其趨勢與上述結果吻合。行進方向與行進速度具有明顯日內變化，尤其在最大潮的前後數日期間內常可見兩種型態的非線性內波接續交互出現，兩種型態的波分別是行進速度較快、行進方向較偏西與進速度較慢、行進方向比前一型波稍偏北的波，推測應與呂宋海峽的潮流有關。 分析三組置放於北南海的ADCP錨碇量測，量測位置分別位於東沙海底高原、淺水大陸棚區與陡峭大陸斜坡。資料顯示，非線性內波主要為向西行進，不論是沿著或橫越非線性內波行進的路徑，非線性內波皆有強大的能量與能量通量輻散(divergence)，在海底高原，非線性內波的能量通量為8.5 kWm-1，沿著行進路徑往西220公里遠的大陸棚區，則僅有0.25 kWm-1，橫越其行進路徑到其北向約120公里的大陸斜坡區，能量通量為1 kWm-1。沿著東沙海底高原上非線性內波行進的路徑，平均非線性內波能量通量輻散為~0.04 Wm-2，相當於O(10-7-10-6) Wkg-1的消散率(dissipation rate)。整合現有資料與先前的模式結果，可顯示出南海非線性內波能量通量的分佈態勢。非線性內波於東沙海底高原西邊產生，沿著以~21oN為中心寬約100公里的beam以主要向西的行進方向穿越海底高原，在到達大陸棚區域前，非線性內波幾乎消耗掉所有能量。 於弱東北風(風速~2 m/s)時期，本研究亦使用船載海洋雷達(Marine Radar)、都普勒流剖儀、溫鹽深儀、與EK500聲學回跡儀來同時觀測南海巨大振幅非線性內波的表面訊號與內部性質。當非線性內波未出現時，由海洋雷達量測得的海表面散射強度與當地風速有正相關，當非線性內波抵達時，於其海表面輻合區的散射強度被加強，估計非線性內波引發的海表面散射強度相當於6 m/s的風速所產生的海表面散射強度，也就是約真實風速的3倍強度。被增強的海表面散射強度之水平空間結構可用以預測非線性內波的水平空間結構，所觀測得的平均非線性內波半振幅寬度 為1.09±0.2 km，平均增強散射強度半振幅寬度 為~0.57 ，平均增強的非線性內波水平速度輻合半振幅寬度 相當於 ，被增強的海表面散射尖峰值約領先非線性內波中心點~0.46 。非線性內波水平速度輻合與海表面強化散射強度成正相關，非線性內波振幅與輻合區內海表面散射強度增強值的空間積分呈正相關，由上可獲得以海表面散射強度預測非線性內波水平速度輻合與振幅的經驗公式。本研究分析總結，於低風狀態下，遙測量測可提供對非線性內波水平速度輻合、振幅、與空間結構的預測，這些經驗公式可進一步應用，也可於不同風速、表面波、非線性內波情況下，或其他遙測方法下修改。

The study is focused on the large-amplitude nonlinear internal waves (NLIW) in the South China Sea (SCS): the propagation characteristics, the energy, energy flux and dissipation, and the relationship between the interior properties of NLIW and its surface scattering strength. Three sets of long-term ADCP measurements taken on the Dongsha plateau, integrating with both the shipboard measurement and the remote sensing data, are used to study the propagation characteristics of the NLIWs. The moorings were aligned along 21o05’N near the eastern edge of the Dongsha plateau. From east to west, the distances between the two successive moorings are ~8.5’ and ~17’, respectively. The NLIW propagating directions and speeds were computed by NLIW-induced current velocity and NLIW arrival time between two successive mooring stations, respectively. The averaged propagating direction of NLIW is 165o, which is northwestward. The averaged propagating speeds between two successive mooring stations are 1.83±0.38 m/s and 1.61±0.20 m/s from east to west. The above estimations are further verified by the observations of both shipboard marine radar and satellite images. The propagating directions reveal irregular variation. Nonetheless, the propagating speeds, which are higher in Aug.-Oct. and are slower in Jan.-Mar, reveal apparently seasonal variation. Such seasonal variation could relate with the typically seasonal stratification in the SCS, strong stratification in Aug.-Oct and weak stratification in Jan.-Mar. The linear phase speed, which is calculated using the climatological density profiles of Generalized Digital Environmental Model (GDEM) output, has good correlation with the measured NLIW propagating speed. Both the propagation direction and speed reveal daily inequality. Two types of NLIW appear reciprocally around the spring tide. One of them propagates faster and mainly northwestward and the other propagates slower and more northward than the previous one. It could be associated with the tidal current in the Luzon Strait. Three sets of ADCP measurements taken on the Dongsha plateau, on the shallow continental shelf, and on the steep continental slope in the northern South China Sea are analyzed. The data show strong divergences of energy and energy flux of nonlinear internal waves along and across waves’ prevailing westward propagation path. The NLIW energy flux is 8.5 kW m-1 on the plateau, only 0.25 kW m-1 on the continental shelf 220 km westward along the propagation path, and only 1 kW m-1 on the continental slope 120 km northward across the propagation path. Along the wave path on the plateau, the average energy flux divergence of NLIW is ~0.04 W m-2, which corresponds to a dissipation rate of O(10-7-10-6)Wkg-1. Combining the present with previous observations and model results, a scenario of NLIW energy flux in the SCS emerges. NLIWs are generated east of the plateau, propagate predominantly westward across the plateau along a beam of ~100 km width that is centered at ~210N, and dissipate nearly all their energy before reaching the continental shelf. Surface signatures and interior properties of NLIWs were measured during a period of weak northeast wind (~2 m s-1) using shipboard marine radar, ADCP, CTD, and echo sounder. The surface scattering strength measured by the marine radar is positively correlated with the local wind speed when NLIWs are absent. When NLIWs approach, the surface scattering strength within the convergence zone is enhanced. The sea surface scattering induced by NLIWs is equivalent to that of a ~6 m s-1 surface wind speed, i.e., three times greater than the actual surface wind speed. The horizontal spatial structure of the enhanced sea surface scattering strength predicts the horizontal spatial structure of the NLIW. The observed average half-amplitude full width of NLIWs is 1.09±0.2 km; the average half-amplitude full width of the enhanced scattering strength is ~0.57 . The average half-amplitude full width of the enhanced horizontal velocity convergence of NLIWs is approximately equal to . The peak of the enhanced surface scattering leads the center of NLIWs by ~0.46 . NLIW horizontal velocity convergence is positively correlated with the enhancement of the surface scattering strength. NLIW amplitude is positively correlated with the spatial integration of the enhancement of the surface scattering strength within the convergence zone of NLIWs. The analysis concludes that in low-wind conditions remote sensing measurements may provide useful predictions of horizontal velocity convergences, amplitudes, and spatial structures of NLIWs. Further applications and modification of our empirical formulas in different conditions of wind speed, surface waves, and NLIWs, or with other remote sensing methods are encouraged.