ESSAY: Model interpretation of climate signals: an application to the Asian monsoon climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 305

Figure 9.8 Anomalous
velocity potential (JJA 1998
minus JJA 1997) for (a)
observed, (b) all-model
mean, (c) model minus
observed and (d) standard
deviation from all
model-mean.
of the Walker circulation associated with the anomalous warm SST of the
central and eastern Pacific (Ju and Slingo 1995 ; Lau and Nath 2000 ).
Figure 9.8 shows a comparison of the observed and the simulated model
mean velocity potential difference (JJA 1997 minus JJA 1998) at 200mb.
Here, the positive contours indicate anomalous large-scale ascent over the
central and eastern Pacific, and the negative contours anomalous descent over
the Indian Ocean. However, the difference map between the model mean and
observation (Figure 9.8c ) indicates that large errors are found in the Asian
monsoon region, with the model overestimating the anomalous sinking
motion over the maritime continent and the rising motion over the South
China and East China Seas. This may be interpreted as the AGCM's inability
to model the regional anomaly induced by the Walker circulation (Soman
and Slingo 1997 ; Lau and Wu 2001 ). The models tend to disagree most
among themselves over the Indian Ocean and the tropical western Pacific
(Figure 9.8d ).
Many modeling studies have been conducted to unravel the causes of the
record summer monsoon flooding over central East Asia (Shen et al. 2001 ;
Wang et al. 2000 ). Figure 9.9 (Plate 7 ) shows the ensemble mean of the
rainfall and 850mb wind anomalies (1998 minus 1997) for JJA and for each
model participating in the CLIVAR MMIP. For comparison, the observation
is shown in the bottom right panel labeled ''obs''. The observation shows a
zonally oriented rainfall anomaly pattern with reduced rainfall over the
tropical western Pacific, and Indo-China along 108 N, increased rainfall
over the maritime continent/equatorial eastern Indian Ocean, and the sub-
tropics between 30 and 408 N. The regions with rainfall increases are located
on the northern and southern flanks of a subtropical anticyclone, which is