East Asia and Western Pacific METEOROLOGY AND CLIMATE

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Indian Ocean and the western Pacific and are known to possess a hierarchy of sub-structures
associated with cloud cluster and superclusters (Nakazawa, 1989, Nakazawa and Lau,
1989 and Lau et al, 1989). During the northern summer, a large number of westward
propagating disturbances associated with easterly waves traverse the western Pacific at
10-15°N. Many of these disturbances reaches East Asia and the South China Sea. It is
therefore necessary to examine the small scales structures embedded in the 30-60 day
oscillation and their possible relationship with the monsoon disturbances. Because
intermediate scales such as 10-20 day fluctuations are often found in groups within the 30-
60 day oscillations, we shall use the term 30-60 days oscillation in its generic sense in the
following to include these intermediate scales.
Figure 6 shows the time-longitude section of 3-hourly CMS convective index
averaged between 1°N and 1°S from March 14 to April 12,1984. During the first half of
the period, the convection over the region is relatively sparse consisting mainly of
westward propagating transients. During the second half, the convection in this region
becomes very active. Two supercluster complexes (labelled A and B) can be identified
during this period. The eastward propagation is seen to be made up of successive formation
of new clusters to the east while individual clusters propagate westward. The detailed 6-
hourly synoptic sequence during the period 12 Z, March 29 to 06 Z, March 31 is also shown.
The spatial scale of the superclusters is of the order of 1000 to 2000 km whereas smaller
clusters of the order of 500-1000 km can be found in the vicinity. The period represents the
beginning of the convective phase of the 30-60 day oscillations in the Western Pacific.
The above is a rather typical situation for the propagation of organized convection
associated with the westward and eastward components of the supercluster. In many noted
synoptic situations, a pair of cyclones on each side of the equator is found associated with
the equatorial supercluster. During northern summer, not infrequently, the northern
cyclones develop into tropical cyclones that are (a) either advected by the mean flow first
northward and then northeastward across Japan or (b) drifted across Indo-China to the Bay
of Bengal giving rise to monsoon depression there. The tropical cyclones in the former
categories can further develop into typhoons especially during the late summer season.
The discussion in this section and in section 2 is also consistent with the classical
zonally symmetric description of the equatorial ITCZ and the monsoon ITCZ. When
averaged over the equatorial belt between the Indian Ocean and the western Pacific, the
arrival of the rising branch of the 30-60 day oscillation feature enhanced convection and