East Asia and Western Pacific METEOROLOGY AND CLIMATE

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southeasterly current in the western Pacific.
The typical horizontal structure of the disturbances occurring in the region of maximum
variability in £ has been studied using lag-correlation maps with base points located in the active zone.
An example of such correlation charts is presented in Fig. 1, which shows the distributions of lagcorrelation
coefficients between the bandpass filtered fluctuations in £ at the base point 15°N 132.5°E
and the corresponding fluctuations at all other grid points. The temporal lags used in this example range
from -2 to +2 days. Here negative (positive) lags refer to computations with the time series at
individual grid points leading (lagging) the time series at the base point. A similar technique has been
used by Wallace et. al. 6 ) to illustrate the behavior of midlatitude baroclinic disturbances. These authors
have referred to the patterns in Fig. 1 as 'one-point correlation maps'. Each panel in Fig. 1 is indicative of
a wavelike feature with extrema of alternating polarities. Comparison between the patterns in different
panels reveals a systematic west-northwestward phase propagation.
The average wavelength of the disturbances depicted in Fig. 1, as estimated from the distance
between adjacent correlation centers with the same polarity, is approximately 2500 km. The typical
period, as given by twice the time interval between correlation patterns which are 180° out of phase with
respect to each other (e.g., see panels corresponding to lags of -2 and +1 days), is about 6 days. These
estimates yield a mean phase speed of 4.8 ms"*. A characteristic southwest-northeast tilt of the elongated
extrema is discernible in the patterns of Fig. 1. The placement of such tilted circulation features
to the south of a southeasterly current would imply a downgradient (southward) transport of easterly
momentum, and a barotropic conversion from mean to eddy kinetic energy.
The wavelike nature of the disturbances over the western Pacific may be further illustrated by a
teleconnectivity analysis similar to that performed by Wallace et. al. 6 ) The 'teleconnectivity' for any
point P is defined as the absolute value of the most negative correlation coefficient appearing in a simultaneous
one-point correlation map corresponding to base point P. A large teleconnectivity value for
point P implies that fluctuations at P are accompanied by strong fluctuations of the opposite polarity at
other locations, and is hence indicative of the prevalence of wavelike phenomena in the vicinity of P. Fig.
2 shows the teleconnectivity pattern based on bandpass filtered 850 mb £ data. A region of strong teleconnectivity
(larger than 0.4) is seen to extend west-northwestward from 5°N 16*0°E to South China.
The location of this maximum coincides with the region of enhanced rms of C mentioned earlier, and
with the propagation path of the wavetrain shown in Fig. 1. This suggests that, in addition to monopolar
vortex-like systems such as typhoons and monsoon depressions, a substantial fraction of the synoptic
scale variability in the tropical western Pacific is attributable to propagating, wavy disturbances.
4. PROPAGATION CHARACTERISTICS
By applying the analysis procedures outlined in Wallace et. al. 6 ), the phase velocity vectors and
the temporal coherence of the migratory signal have been computed from one-point correlation maps.
The distributions of these two quantities are displayed in Fig. 3 using arrows and stippling, respectively.
For a series of lag-correlation charts corresponding to the common base point P, the strongest positive
correlation centers appearing in the panels for lags of -2 and +2 days have been identified and labeled as A
and D, respectively (see example in Fig. 1). The line segments AP and PD represent the average spatial
displacement of the disturbance center 2 days before and 2 days after its passage over P, respectively. The
direction of the phase velocity vector at P may hence be inferred from the orientation of the line segment
AD, whereas the magnitude of the phase speed may be approximated by dividing the distance between A
and D by the elapsed time interval (i.e., 4 days). Furthermore, the temporal coherence of the migratory
signal for base point P is defined as the average of the lagged correlation coefficients at points A and D.