2007 Annual Report - jamstec japan agency for marine-earth ...

along the Gulf Stream, and in July and August along the
Kuroshio/Oyashio. An important point to note is that major
variations in the Gulf Stream or Kuroshio/Oyashio do not necessarily
generate significant atmospheric anomalies, since their
impact on the near-surface baroclinicity may be canceled by the
land surface temperature anomalies or dwarfed by baroclinic
anomalies associated with the land surface temperature anomalies.
Their impact on the near-surface baroclinicity may be
ignored by the large-scale atmospheric motions when the jet
flows away from the area of baroclinic anomalies also. These
factors make it difficult to detect the impact of extratropical
SST anomalies on the large-scale atmospheric state when the
data are analyzed with the SST in focus. We will be extending
this research by examining the atmospheric response to SST
anomalies in the vicinity of the Kuroshio/Oyashio in the summer,
using a regional atmospheric simulation model at a very
high spatial resolution required to resolve narrow bands of SST
anomalies along the oceanic fronts.
Interannual variability of the East Asian winter monsoon
exerts significant influence on weather conditions over Japan,
including severe cold waves in the 2005/6 winter and anomalous
mildness in the 2004/5 winter. We have been investigating
the causes and predictability of the monsoon variability,
focusing on the remote influence of the tropical variability and
anomalous sea ice cover in the Arctic. It is suggested though
our experiments with an atmospheric general circulation model
anomalous Aleutian Low requires our deeper understanding of
the ocean-to-atmosphere feedback in the subpolar oceanic
frontal zone. We have found through our analysis of an eddyresolving
OFES hindcast integration that cool SST anomalies
that form off the Hokkaido Island along the strengthened
Oyashio tend to extend eastward along the subpolar front in
cooling the overlying atmosphere. This tendency indicates that
SST anomalies generated through such oceanic processes as
advective effect of the Oyashio and equatorward displacement
of the subpolar front can exert thermal forcing on the atmosphere.
In order to further explore the characteristic nature of airsea
interaction in a midlatitude oceanic frontal zone, we analyze
an output of a high-resolution CGCM CFES on the Earth
Simulator. With its eddy-permitting ocean component, CFES
can represent a well-defined oceanic front on the warmer flank
of the strong Antarctic Circumpolar Current. Tight surface air
temperature SAT gradient that is maintained across the
prominent SST front energizes individual cyclones and anticyclones
to anchor the core of the Southern Hemisphere storm
track where the surface westerlies are particularly strong due to
poleward eddy heat transport. Our analysis reveals Fig.5
that on the warmer side of the SST front a huge amount of heat
is released from the ocean into cold air behind a cold front
while on the cooler side of the SST front the ocean cools off
warm air behind a warm front. After relaxed by poleward heat
transport by atmospheric disturbances, SAT gradient across the
AFES on the Earth Simulator that below-normal sea ice
cover within the Barents/Kara Seas in late autumn may possibly
lead to above-normal intensity of the wintertime Siberian High.
Furthermore, we have identified a stationary tropospheric wave
train in late winter over the Eurasian continent as a precursory
signal of a midwinter surface pressure anomaly similar to the
Arctic Oscillation AO. The wave train modifies the upward
propagation of the climatological planetary waves, leading to
the formation of an AO-like anomaly in the stratosphere and its
subsequent extension down to the surface. These are significant
findings that can be key factors for seasonal forecast of wintertime
weather over the Far East.
Deeper understanding of the decadal climate variability
inherent to the midlatitude North Pacific that accompanies the
Fig.5 Air-sea interaction characteristic of the oceanic frontal zone
in the South Indian Ocean reproduced in a high-resolution coupled
ocean-atmosphere GCM CFES. Latitude-time section for 55˚E from
July to September of the first year. upper Surface air temperature
SAT; contoured for every 2 degs. and SST-SAT colored as indicated
on the right; bright colors for positive values. lower SST contoured
for every 2 degs. and sensible heat flux from the ocean surface
colored as indicated on the right; bright colors for upward flux