East Asia and Western Pacific METEOROLOGY AND CLIMATE

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taken during the Global Atmospheric Research Program (GARP) Atlantic Tropical Experiment
(GATE) in 1974, Reed et. al. 3 ), and others have substantially improved our knowledge of the easterly
waves originating from central and western Africa. Much less is known about tropical transient phenomena
occurring elsewhere, especially in maritime regions where observations are scarce. In view of the
large differences in the ambient atmospheric flow pattern in various longitudinal sectors, the structure
of the African easterly waves is not expected to prevail throughout the tropical zone. Many studies [e.g.
Nitta et. al. 4 ) ] have already noted that atmospheric fluctuations at various tropical sites exhibit
different spectral characteristics. For instance, the dominant period of the African easterly waves (3-5
days) is known to be shorter than that found over the oceanic areas (5-7 days).
The availability of routinely produced global analyses after the First GARP Global Experiment
(FGGE) has revived the interest of the meteorological community in the behavior of tropical disturbances
at different locations. For example, Nitta et. al. 4 ) and Nitta and Takayabu 5 ) have conducted a
global survey of such phenomena using the FGGE dataset. These authors have identified several regions
of strong transient activities, and have described the characteristic spectra and vertical structure of fluctuations
in these regions. The present study has been launched with a similar goal in mind. By
diagnosing the global gridded analyses for an 8-year period, we attempt to document the three-dimensional
structure and propagation characteristics of summertime transient disturbances in different
tropical regions. In this particular report, the attention is focused on results pertaining to the tropical
western Pacific.
2. DATASETS
The primary dataset for this study consists of twice daily operational analyses archived at the
European Centre for Medium-Range Weather Forecasts (ECMWF) for the period 1980-1987. The data
grid has a uniform horizontal resolution of 2.5° by 2.5°. The variables examined include zonal and
meridional wind, pressure velocity, temperature, geopotential height and specific humidity at 7 standard
pressure levels (100,200,300,500,700,850 and 1000 mb).
Twice daily gridded data for the outgoing longwave radiation produced by the U.S. National
Oceanic and Atmospheric Administration for the period 1974-1987 have also been used to investigate the
relationships between tropical disturbances and convective activities. As an independent check of the
results derived from the ECMWF dataset, some of the diagnostics have been applied to a set of gridded
wind analyses compiled by the Royal Observatory of Hong Kong (ROHK) for the period 1981-1984.
3. TIME SCALE, HORIZONTAL STRUCTURE AND PATH OF MIGRATION
Following the earlier studies of Nitta and Takayabu 5 ) and others, we have chosen the relative
vorticity £ at 850 mb as the key meteorological parameter for identifying tropical disturbances. Power
spectra of £ at 850 mb have been computed at a large number of grid points scattered throughout the
tropical zone. These results (not shown) indicate considerable variations in the dominant time scales of
the fluctuations at different locations. Such time scales range from 7-8 days in northeastern India to 5-8
days in the western Pacific, and 3-4 days in western Africa. In view of the diverse temporal
characteristics in various regions, a 61-point digital filter has been designed to retain fluctuations
residing within a broad spectral band corresponding to the 3-10 day period range.
The summertime distribution of the root-mean-squares (rms) of 850 mb £ over the western
Pacific (not shown) is characterized by maximum values along a zone extending west-northwestward
from about 5°N 165°E towards the East and South China Seas. This region of enhanced transient activity
lies in the vicinity of the monsoon trough, and is displaced to the south/southwest of the mean