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subtropical jet stream (Lau and Weng 2002 ). During JJA 1997, the Walker
circulation shifts eastward in response to the El Ni˜o SST forcing, inducing
strong downward motion over the maritime continent and suppressing the
ITCZ. The anomalous anticyclone is the cause of the major flooding over the
Yangtze River Valley during JJA 1998 (Lau and Wu 2001 ; Lau and Weng
2001 ;Shenet al. 2001 ). The establishment of the anticyclone is related to
descending motion associated with the eastward shift of the Walker circula-
tion, and also to amplification by local air-sea interactions.
The models' ability to simulate the aforementioned features is generally
not very impressive. While most models show the correct sign of the large-
scale response in the AM region, most models, except perhaps SNU, fail to
simulate the observed zonally oriented rainfall structure. It is clear from the
results shown that the simulation of the East Asian monsoon rainfall anoma-
lies is critically dependent on the anomalous anticyclone, which governs the
moisture available for precipitation. Because the anticyclone is generated by
large-scale dynamics, its broad feature is represented in most models.
However, it is the simulation of the exact location and magnitude of the
anticyclone that is required in order for climate models to simulate the
regional AM rainfall anomalies, and hence the severe floods and droughts
in the AM region. The use of higher resolution climate models, or the use of
downscaling methodologies is required.
The performance of AGCMs to simulate the 1997-8 rainfall anomalies can
also be evaluated from examination of the distribution of model climate states
represented as two-dimensional scatter plots along axes representing key
climate variables. Shown in Figure 9.10 (Plate 8 ) are model rainfall ano-
malies averaged over selected domains (labeled by latitude-longitude bound-
aries in Figure 9.10 ), plotted against the SOI. The domains are for the AM
region as a whole, and for its component parts over the MC, the South Asian
Monsoon (SAM) and the South East Asian Monsoon (SEAM). Each data
point represents one ensemble member of each model. The heavy shaded
symbols represent the observed states. It can be seen that the models are quite
responsive to the El Ni˜o signal, in that there is a clear separation of model
states between 1997 and 1998 along the SOI axis in all panels. Indeed, the
models tend to overestimate the east-west see-saw, as evident in the larger
spread along the SOI axis compared to the observations. For the AM region as
a whole (Figure 9.10a (Plate 8a ) ), a reduction in rainfall during 1997 com-
pared to 1998 can be discerned. This reduction is mainly due to the rainfall
anomalies over the MC, which is situated at the descending branch of the
Walker circulation. For the SAM and SEAM region, the large clusters of
model states for 1997 and 1998, and the lack of obvious shift of the center of
gravity of the clouds on the y-axis during these two years suggest that there is
large rainfall variability, but there is no significant climate impact from the
 
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