East Asia and Western Pacific METEOROLOGY AND CLIMATE

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The pattern of arrows in Fig. 3 clearly portray the general west-northwestward propagation of
transient disturbances over the western Pacific. The average phase speed of about 5 m s" 1 is consistent
with the estimate based on the characteristic wavelength and period deduced from Fig. 1. The temporal
coherence of the migratory signal (see stippling in Fig. 3) generally exceeds 0.3 over the maritime areas,
and attains maximum values along a belt extending from the equatorial Pacific to the South China
coasts. The latter region is characterized by strong teleconnectivity (see Fig. 2) and enhanced variability
in £ (see Section 3). Very weak temporal coherence is seen over the Asian land mass.
The magnitude and spatial distribution of typical growth/decay rates of the disturbances have
been estimated using the algorithm developed by Wallace et al. 6 > The results, shown in Fig. 4, are again
based on one-point correlation maps. For a series of such maps corresponding to base point P, the
strongest positive correlation centers at lags of -1 and +1 day have been noted and labeled as B and C, respectively
(see example in Fig. 1). The regression coefficients between the normalized time series of
filtered C at P and the unnormalized time series of the same quantity at B and C were then computed at
time lags of -1 and +1 day, respectively. These regression values provide for a measure of the amplitudes
of the vorticity fluctuations at B and C one day before and one day after, respectively, the passage over P
of a disturbance with an amplitude of one standard deviation. The growth/decay rate at P was then computed
by subtracting the regression coefficient for point B at -1 day lag from the corresponding value for
point C at +1 day lag, and then dividing the result by the appropriate time interval (i.e., 2 days).
The general pattern in Fig. 4 is characterized by growth over the Philippine Sea, the Philippine
Islands and the South China Sea. The steepest decay rates are found along the South China coast. This
result is consistent with the land-sea contrast in the temporal coherence of the migratory signal shown
in Fig. 3. The large growth rates over the Bay of Bengal are associated with the developing monsoon
disturbances in that region.
5. VERTICAL STRUCTURE
The time-space behavior of the 850 rnb £ field has also been diagnosed using rotated extended
empirical orthogonal functions (REEOF). The horizontal structure of the leading pair of REEOFs (not
shown) is very similar to that displayed in Fig. 1. The temporal coefficients of these REEOFs provide
for a logical basis for constructing composites of the three-dimensional structure of the tropical disturbances
of interest. In Fig. 5 are shown the vertical cross-sections of perturbations in (a) relative vorticity,
(b) vertical pressure velocity, (c) temperature, and (d) specific humidity, as obtained from such a
composite procedure. The abscissas of these plots extend southeastward from South China (27.5°N
110°E) in the extreme left to the western equatorial Pacific (2.5°N 142.5°E) in the extreme right.
The pattern for £ [panel (a)] indicates a strong northwestward tilt with increasing height over
the equatorial ocean, where initial wave growth occurs (see Fig. 4). The vertical tilt is seen to decrease
gradually as we scan the panel from right to left, so that there is almost no vertical tilt as the disturbances
approach the South China coast. Perturbations in pressure velocity [panel (b)3 attain extrema at 300
and 850 mb. Comparison between panels (a) and (b) indicates the occurrence of maximum upward
motion at 850 mb (and hence near-surface convergence) at the trough. At the 200-300 mb level, the
upward motion tends to lag the positive £ fluctuations by a quarter wavelength. This phase relationship
may be attributed to the confinement of £ fluctuations in the middle and lower troposphere, and differential
relative vorticity advection by the prevalent southeasterly current in this region. The most notable
features in the cross-section for temperature [panel (c)] are the extrema at 300 mb. These upper tropospheric
warm and cold centers are located directly above the 850 mb trough and ridge, respectively.
The warm center may be associated with condensational heating accompanying the low level conver-