HFレーダーを用いた対馬海峡表層海流の観測 : 予備解析

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タイトル別名
  • Meaurement of the Surface Current in the TSUSHIMA Strait by HF Radars: Preliminary Analysis
  • HF レーダー オ モチイタ ツシマ カイキョウ ヒョウソウ カイリュウ ノ カンソク ヨビ カイセキ

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The surface current of the Tsushima Strait was measured by two HF radars, which faced the western channel from August to October and the eastern channel from October to December 1999. Doppler spectra were smooth in the western channel. In the eastern channel, however, they often had a curious pattern of double peaks, which made it difficult to identify a single radial component of current velocity. This fact suggests complicated characteristics of the eastern channel, which is in the wake of the Tsushima Island against the northeastward Tsushima Current. In order to examine the reliability of the data, we compared the surface current measured by the HF radars with nearby available current measurement with a ship-born ADCP by Fukuoka Fisheries and Marine Technology Research Center. A complex regression was made to relate the former current data with the latter: on average, the current velocity measured by the ADCP (nominally at the depth of 5 m) was found to be about 1.1 times larger than and rotated 11 degrees clockwise from that by the HF radars (within the uppermost 0.5 m or so). The tidal ellipses of eight main components were determined from the HF radar measurement and the M2 and 01 components were discussed rather in detail. Their major axes were oriented from southwest to northeast except at distant locations off the western coast of Tsushima. The tidal ellipses were compared favorably with a numerical experiment of fine resolution by Kawano (1999) at least within roughly 40 km from the radars in both the eastern and western channels. At locations watched by a single radar, only amplitudes of tidal oscillation in the radial direction were determined. Those tidal oscillations also agreed well with the radial projection of the tidal ellipses due to the numerical experiment, showing that even the sole radial component obtained by HF radars yields useful information. The current field averaged over each two months of observation was plausible. In the western channel the Tsushima Current flowed northeastward. It was faster and more northward near the western coast of Tsushima. The current in the eastern channel also flowed northeastward, but it was slower than in the western channel including a weak southwestward counter current in the northern part. In contrast to the western channel, the current in the eastern channel was even slower and stagnant nearby the island, indicating again that the eastern channel is in the wake. Although the average field was a likely one, snapshots often showed curious and interesting flow patterns in both channels. An example against our intuition was almost westward flow emanating from Tsushima. Of hydromechanical interest was the generation and movement of an eddy in the eastern channel. Thus HF radars proved to be a useful means for monitoring the surface current in the Tsushima Strait.

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