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                  森羅万象セミナー                 第 32 回

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             時・場所：3月6日 (月) 13：30〜  理3号館401号室

                            講演： ケント ムーア 
                        (G.W.K.Moore, Tronto 大教授)

           『Reconstruction of the air-sea interaction 
                      associated with the Weddell Polynya』

ABSTRACT
 A recent major climatic event was the occurrence of approximately
350,000 square kilometres of open water in the sea ice near Antarctica 
during the winters of 1974-76. Within this open ocean polynya there was 
vigorous air-sea interaction resulting in the  formation of vast amounts 
of Antarctic Bottom Water, the dominant watermass of the world oceans. 
There is some evidence to suggest that a similar event may have occurred 
in 1960 and indeed that the Eastern Weddell Sea is once again primed for 
polynya  formation due to historically high mid-depth water temperatures. 
The formation of Antarctic Bottom Water in the Weddell Sea, although less 
intensively studied than deep water formation  processes in the North 
Atlantic, is a major potential source of global climatic linkages. In the 
modern ocean, deep water formation in the Antarctic occurs at rates equal 
or greater to that in the North Atlantic, and ventilates a much larger 
portion of the world ocean. Indeed, the possibility of an episodic occurrence
of the Weddell Polynya, and concomitant oscillation in Antarctic Bottom 
Water formation, may be one of the major  expressions of global climate 
variability on the decadal to centennial timescale.

 In order to further our understanding of this important event, we 
make use of the NCAR/NCEP reanalysis dataset to reconstruct the associated 
air-sea interaction. At cold sea surface temperatures such as associated 
with the polynya, non-linearity in the equation of state for sea water 
reduces the efficiency with which cooling results in changes to the density 
of the surface waters. Thus detailed knowledge of both the heat transfer and 
the rate of precipitation are required in order to correctly asses the 
atmospheric forcing of oceanic convection in the polynya. Our work in the 
Labrador Sea suggests that the reanalysis dataset is capable of capturing 
the salient features of the surface meteorological fields but that the 
diagnosed heat flux fields  that are a part of the dataset are overestimates. 
This is due to an inappropriate model bulk flux algorithm. For this reason, 
we used the model surface fields to calculate the heat fluxes with a  more 
conventional algorithm. The precipitation field in the  reanalysis appears 
to be consistent with the sparse data available for the Weddell Sea.
 
 Our results show that previous estimates of the air-sea heat fluxes 
are consistent with the reanalysis data but that the previous estimates of 
the fresh water flux are inconsistent. Indeed the previous estimate is of 
the opposite sign. Our reconstruction shows that the buoyancy flux is 
typically negative, indicating that the surface waters were becoming denser 
thereby driving oceanic convection and Antarctic Bottom Water formation. 
Nevertheless there were periods when the field was positive, indicating the 
fresh water flux due to precipitation was at times larger than the effect 
of local cooling resulting in a reduction in the density of the surface 
waters of the polynya. Furthermore, our work shows that the air-sea 
interaction is highly variable. Indeed, the standard deviation of the fields 
exceeds the mean. Much of this variability can be attributed to the passage 
of transient synoptic scale weather systems.

 世話人 遊馬 芳雄

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次回以降の予定 

 日未定：森田英章 (数学 DX) 『無限可積分系、特にある種のソリトン方程式の話』
 日未定：渡部重十 (地惑 教授) 『近未来の惑星大気探査 (仮題)』   
  
※諸事情により、変更や追加のある場合があります。
※話題希望要望サポーター、大募集中です

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 セミナー幹事：倉本　圭 (Kuramoto Kiyoshi)
　　　　 北海道大学大学院理学研究科地球惑星科学専攻 助手
　　　　 Room:理学部3号館307号室
　　　　 Phone: 011-706-3567, Fax: 011-746-2715 
         E-mail: keikei@ep.sci.hokudai.ac.jp