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10-,25-, and 60-day Periodic Current Variations on the Western Shelf in Hidaka Bay

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Other Title
  • 日高湾西部陸棚上における10日, 25日, 60日周期流速変動
  • ヒダカワン セイブ リクダナ ジョウ ニ オケル 10カ 25ニチ 60ニチ シュウキ リュウソク ヘンドウ

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Abstract

青森室蘭間を運航する定期旅客船に搭載されたADCPによる2000年4月~2001年4月までの流速データを用いて,日高湾西部陸棚上こおける数十日(10日~60日)周期の流速変動を調べた。主要10分潮の潮流成分をADCP流速データから除去した後,2種類のスペクトル解析法を適用した。その結果,西部陸棚上において10日,25日,60日の三つの周期帯の流速変動が卓越していることが分った。本研究では,不等間隔データに適用可能な新しいバンドバス法(HAB法:Harmonic Analysis Bawl-pass Method)を提案し,各周期変動の水平流速・鉛直流速の構造を抽出した。日高湾西部陸棚上のADCP観測点間において,三つの周期帯の流速変動はいずれも水平的にほぼ同位相であった。また,10日と25日周期変動の鉛直的な位相差はほとんど見られなかったが,60日周期変動には数日の顕著な位相差が見られ,下層流が上層流よりも常に先行していた。流れと海上風とのクロススベクトル解析を行った結果,10日周期帯の流速変動のみが風との高い相関を示し,北西風が北西同流よりも約1.5日先行していた。10日周期帯の流速変動が秋~冬季にかけて卓越していたことと,この見積もられた位相差が,大島(三宅(1990)による日高湾内での風強制順圧モデル計算の見積もりとよく一致していたことから,この周期帯の変動は北西の季節風によって励起された陸棚波の伝播に伴う流速変動と推測される。Variability in intraseasonal current fluctuations (periods between 10 and 60 days) on the western shelf in Hidaka Bay was examined by using an Acoustic Dopplcr Current Profiler (ADCP) installed on a regular ferry shuttling between Aomori and Muroran from April 2000 to April 2001. After ten primary tidal current constituents were subtracted from the ADCP data, two types of spectral analysis were carried out. and three periodic variations, around 10-, 25-, and 60-day periods, respectively, were identified on the shelf. A new band-pass method, the Harmonic Analysis Band-pass (HAB) method, which can be applied to irregularly sampled data, is proposed to extract the horizontal and vertical current structures of each periodic variation. Very little phase lag between ADCP points was found in any of these periodic variations on the western shelf. Slight vertical phase lag was observed in the 10- and 25-day periodic variations, and a significant vertical phase lag of several days (i.e., lower current always leads upper current) was observed for the 60-day periodic variation. Cross-spectral analysis between current and sea surface wind showed that among signals in the periodic variations, only those in the 10-day period wore coherent, and their phase lag indicates that the northwesterly wind leads the northwestward flow by about, 1.5 days. This estimated phase lag agrees well with that of the wind-forced barotropic model of Hidaka Bay by Ohshima and Miyake (1990). Also, the current variation in the 10-day period was dominant from autumn to winter. Therefore, we infer that this 10-day current variation is effectively induced by the northwesterly monsoon wind.

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