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第 70 回 大気海洋圏物理系セミナー のおしらせ

日　時：1999年 7月 1日(木) 午前 9:30 〜 12:00
場　所：地球環境科学研究科管理棟 2F 講堂

発表者：木村 詞明 (極域大気海洋学講座 D3 )
題　目：冬季北半球における海氷の動態

発表者：S.V.Gladyshev (極域大気海洋学講座 客員助教授)
題　目：Formation of the Okhotsk Sea Intermediate Water

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冬季北半球における海氷の動態 (木村 詞明) 発表要旨：

  海氷データがある程度蓄積された70年代後半以降の研究により、地域的な 
 海氷域の変動、特に海氷面積の年々変動が大気場の影響を強く受けているこ 
 とが明らかになってきた。しかし、実際にどのようなプロセスによって大気 
 場条件の違いが海氷面積の違いに反映されるのかはよく分かっていない。こ 
 のプロセスを解明することは、大気場が海氷に及ぼす影響の定量化、また、 
 海域による大気からの影響の受け方の違いを知るという意味からも重要な問 
 題である。 
  これまでの解析は、海氷データとして主に海氷密接度だけが用いられてき 
 たために、海氷域内で何がおこっているのかを知ることが困難であった。今 
 回は、マイクロ波放射計DMSP/SSMIの輝度温度データから海氷漂流速度を求 
 め、その結果を用いた議論を行う。具体的には海氷漂流速と地衡風速との比 
 較、さらに、海氷域の拡大速度、漂流速度、地衡風速の三者を比較すること 
 により、海氷が地衡風によってどのような動きをし、それが海氷域の拡大速 
 度にどのような影響を及ぼしているかを考える。これらの解析により、海流 
 の影響を差し引くと海氷は地衡風と平行方向に移動するが、その大きさ(地 
 衡風速に対する比率)は海域によって異なること、海氷域の拡大速度と漂流 
 速度、地衡風の関係も海域によって大きく異なること等が明らかになった。 
  と言っても、劇的に美しい結果をお話し出来る訳ではありません。途中経 
 過の報告という感じです。 

Formation of the Okhotsk Sea Intermediate Water (S.V.Gladyshev ) 発表要旨：

   Okhotsk Sea Intermediate Water (OSIW) originated in the Sea of 
 Okhotsk plays an important role in modification of North Pacific 
 Intermediate Water (NPIW) (Talley 1991; Talley 1993; Talley and Nagata 
 1995; Yasuda 1997; Freeland et al.  1998; Watanabe and Wakatsuchi 
 1998). According to Talley [1991], the former ventilates NPIW in a 
 wide density range between 26.65-27.6 sigma theta through the entire 
 North Pacific. Analyzing Russian historical data, Moroshkin [1966] 
 defined that the largest difference in water properties between the 
 Sea of Okhotsk and NPIW in the vicinity of Kuril Islands is in a layer 
 200-800 m. Watanabe and Wakatsuchi [1998] confirmed this result, based 
 on their Japanese data set. 
   This difference is caused by a few reasons that have been already 
 investigated in more or less degree: strong tidal mixing in the Kuril 
 straits, brine rejection in polynyas over northern Okhotsk shelf and 
 Soya Water outflow through Soya Strait to the Deep Kuril Basin 
 (Moroshkin 1966; Kitani 1973; Takizawa 1982; Talley 1991; Wakatsuchi 
 and Martin 1991; Kowalik and Polyakov 1998; Watanabe and Wakatsuchi 
 1998; Gladyshev et al. 1999). The former mechanism supposed to be 
 predominant in a density range 27.0-27.6 sigma theta (Moroshkin 1966, 
 Talley 1991, Freeland et al. 1998). This process is believed to modify 
 a lower part of OSIW to become colder less saline with high oxygen 
 content in comparison with NPIW.  However, evidences of this are still 
 very limited and definitely require further measurements and 
 considerations. 
   Recently, Gladyshev et. al [1999] estimated that brine rejection in 
 polynyas provides annually average flux 0.4 Sv of less saline water 
 close to the freezing point to OSMW in a density range 26.6-26.9 sigma 
 theta. However, they also pointed out strong interannual variability 
 of dense water production over northern Okhotsk shelf.  Dense shelf 
 water (DSW) mixes with Forerunner Soya Water (FSW) and NPIW and 
 produce together a new water mass which Yasuda [1996] called Okhotsk 
 Sea Mode Water (OSMW). Gladyshev et.al [1999] estimated annually 
 averaged rate of the OSMW formation about 2 Sv that is close to the 
 result of Alfultis and Martin [1987].  As NPIW contribution is the 
 highest to OSMW, one can consider the latter as strongly modified 
 portion of NPIW. Each source-type component of water contributes 
 different properties to OSMW. If NPIW provides background of OSMW as 
 the basic component, then DSW chills freshens and oxygenates it and 
 FSW increases salinity and oxygen content of OSMW. As a result, OSMW 
 is colder, less saline and more oxygenated than NPIW. 
   OSMW has another remarkable properties. Yasuda [1996] reported high 
 isopycnal thickness of OSMW, suggesting strong influence of some 
 density driven mechanism on it formation. Freeland et al. [1998] also 
 defined upper part of OSIW as low potential vorticity region, 
 interpreting it as recently ventilated layer from the 
 surface. However, Gladyshev et al. [1999] showed that DSW is strongly 
 stratified water and can not be a source of a pycnostad within 
 OSMW. Watanabe and Wakatsuchi [1998] also revealed high volume rate of 
 OSMW in the Okhotsk Sea, assuming that this water formed under the 
 influence of regional conditions. They also pointed out that 
 isopycnals remarkably deepen in the Okhotsk Sea. However up to this 
 time, any explanations of this downwelling does not exist in the 
 oceanographical literature. 
   The main purpose of this report is to clarify the role of the Kuril 
 straits and DSW in the OSIW formation, based on the modern CTD data 
 set and some historical Russian data. Some details of the mixing 
 process between different components of OSIW and possible mechanisms 
 are discussed qualitatively. 

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