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第 235回 大気海洋物理学・気候力学セミナー のおしらせ

日　時： 11月 08日(木) 午前 09:30
場　所： 低温科学研究所 3階 講堂

発表者： 青木 邦弘（PD）
\\Kunihiro Aoki (PD)
題名： 黒潮続流域および湾流域に生じる渦熱輸送の興味深い特徴

Speaker: C.C.Bajish（D3）
Title： Quasi-decadal variability of Antarctic sea ice

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黒潮続流域および湾流域に生じる渦熱輸送の興味深い特徴
\\(青木 邦弘 \\Kunihiro Aoki）発表要旨 :

The present study investigates meridional heat transport induced by oceanic
mesoscale variability in the World Ocean using a global eddy-resolving Ocean 
General Circulation Model (OGCM) running on the Earth Simulator. The results 
indicate prominent poleward eddy heat transport around the Western Boundary 
Currents and the Antarctic Circumpolar Current, and equatorward eddy heat 
transport in the equatorial region, consistent with the results of previous 
studies using coarser resolution OGCMs. Such poleward eddy heat transport in 
mid-latitude oceans suggest that the eddies act to reduce meridional background 
temperature gradients across the currents, as would be expected based on 
baroclinic instability. Interestingly, however, along the southern flanks of 
the eastward jets of the Kuroshio Extension and the Gulf Stream, southward eddy 
heat transport occurs in subsurface layers. This is likely to be the result of 
southward migration of warm water cores originating from southern areas adjacent 
to these currents. Southward movement of these cores is caused by interactions 
with unsteady meanders and cold eddies detaching from the meanders. The potential 
impact on biological production in the subtropical surface layers of these 
southward travelling warm water cores is also discussed.


Quasi-decadal variability of Antarctic sea ice
\\(C.C.Bajish）発表要旨 :


In the last 30 years, contrary to the Arctic sea ice rapid decrease, 
the Antarctic sea ice extent has been increasing, with significant 
regional scale increase at the Ross Sea (RS) sector and decrease at 
Bellingshausen Sea (BS). However latest IPCC models (Coupled Model 
Intercomparison Project 5) predicted a decrease in Antarctic sea ice 
extent in this time period, thereby unable to reproduce this observed 
increasing trend. The long-term Antarctic sea ice variability (~10-30 
years) is difficult to examine due to the limitation in the satellite
 observation record. Hence a high resolution (0.5º x 0.5º in ocean 
and 1º x 1º in atmosphere) coupled ocean-atmosphere-ice model (CFES
 Mini), is used to study the sea ice long-term variability in relationship
 with physical variables such as sea surface temperature (SST) and 
geo-potential height (Z) along with the observed data. The model does 
not include the anthropogenic and external (volcanic eruption) effects, 
which is suitable to examine the natural quasi-decadal variability.

The observed long term circumpolar SIE and SST shows a decadal variability 
with increasing trend in the sea ice and corresponding decreasing trend 
in the SST, although they are not statistically significant. As for the 
model, some modes of sea ice concentration (SIC) variability reveal 
realistic patterns. The second mode of the annual mean SIC exhibits a 
spatial dipole structure with opposite signed anomalies in the Atlantic 
and Pacific oceans which is similar to the leading mode of the observed 
satellite SIC (~4-5 years). Similarly the SST modes also shows the same 
spatial and temporal relationship. However, the leading mode of SIC in 
the model shows an quasi-circumpolar pattern with a dominant time scale 
of 15-20 years, which is absent in the observations due to its shorter 
time scale (32 years). The leading SST mode also reveals a similar spatial 
pattern with the time score showing a high correlation (~0.75) with the 
SIC further confirming the SST and sea ice relationship . The model 
southern annular mode (SAM) projects on well with these leading modes of
 both SIC and SST with significant correlation. However the SST and SIC 
in the model are more closely related to each other than with SAM.

Intensification of westerlies due to positive SAM causes advection of 
colder water equatorward between 50ºS and 60ºS reducing the SST, 
accompanied by upwelling. This reduced SST contributes to the increase 
in sea ice and also the SST feedback may further intensify SAM. This 
indicates that the oceanic natural variability is the key to understand 
the variability in sea ice.

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