The Oceanic forecasting system near the Shimokita Peninsula, Japan

下北半島周辺における海洋予報システム

印 貞治*; 中山 智治*; 松浦 康孝*; 島 茂樹*; 石川 洋一*; 淡路 敏之*; 小林 卓也 ; 川村 英之  ; 外川 織彦 ; 豊田 隆寛*

In, Teiji*; Nakayama, Tomoharu*; Matsuura, Yasutaka*; Shima, Shigeki*; Ishikawa, Yoichi*; Awaji, Toshiyuki*; Kobayashi, Takuya; Kawamura, Hideyuki; Togawa, Orihiko; Toyoda, Takahiro*

本州北端に位置する下北半島周辺海域は高温高塩分の海水が日本海から津軽海峡を通過して低温の親潮と混合し、複雑な海洋構造が観測されている。また本海域では日本海を起源とする津軽暖水の季節変動により、津軽暖水が冬から春の季節は沿岸に沿って南下(沿岸モード)し、夏から秋の季節は半島沖で時計回りの循環流を形成する(渦モード)。下北半島周辺海域の海況予報システムを開発するためには、このような水塊構造を数値モデルで詳細に再現することが必要である。そこで1-2km程度の詳細水平格子及び4次元変分法を用いたデータ同化手法を用いて2003年における対象海域の予報計算を実施した(現況再現実験)ところ、良好な再現結果を得た。

The Shimokita Peninsula (hereafter SP)is located at the northeast tip of Honshu (the main island of Japan) and borders on the northwestern part of the North Pacific Ocean (hereafter NWNP) in the east and the Tsugaru Strait (hereafter TS) in the north. It is known that the NWNP off the SP assumes a complex water mass structure. Seawater of Japan Sea origin, called the Tsugaru Warm Water (TWW) component, takes on a characteristic seasonal variation. Conlon (1982) pointed out that during winter and spring, the TWW in the NWNP propagates southward along the coast of the peninsula (the coastal mode), and swirls off the peninsula from summer to autumn (the gyre mode). It is necessary to accurately reproduce the striking circulation patterns and the complicated water mass structures in this area with an ocean general circulation model (OGCM). We need to use a fine-scale grid, probably such as 1-2 km in size to reproduce the water mass structure around the TS and the off the SP in detail. To initialize the forecast system, we use a 4-dimensional variational method of data assimilation. A forecast experiment on the basis of oceanographic observations made in 2003 (a hindcast experiment) illustrates the good performance of our system. We shall focus, in particular, on the shift from the coastal mode to the gyre mode and vice versa.