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An effort was made to search for relationships between interannual variations of population, lifetime, genesis locations, and intensity of named typhoons and numbered tropical depressions in the western North Pacific during the 1979-2002 period. To support this research task, climatological relationships of tropical cyclone characteristics were also investigated for these cyclones. Major findings of this study are summarized as follows:1) Climatology: Measured by the intensity scale of the Japan Meteorological Agency, three groups of tropical cyclones were identified in terms of population versus intensity: Group 1 [tropical depression (TD) ϩ typhoon (TY)], Group 2 (strong ϩ very strong TY), and Group 3 (catastrophic TY). This group division coincides with that formed in terms of lifetime of tropical cyclones versus intensity. Weak cyclones (Group 1) have a larger population than strong cyclones (Group 3), while the former group has shorter lifetime than the latter group. For genesis locations, the monsoon trough is established as a favorable region of tropical cyclone genesis because it provides an environment of large vorticity. Therefore, the northward latitudinal displacement of the maximum genesis frequency in the three groups of tropical cyclones follows that of the monsoon trough. 2) Interannual variation: Any mechanism that can modulate the location and intensity of the monsoon trough affects the genesis location and frequency of tropical cyclones. In response to tropical Pacific sea surface temperature anomalies, a short wave train consisting of east-west oriented cells emanates from the Tropics and progresses along the western North Pacific rim. Population of the Group-1 tropical cyclones varies interannually in phase with the oscillation of the anomalous circulation cell northeast of Taiwan and south of Japan in this short wave train, while that of Group 3 fluctuates coherently with the tropical cell of this short wave train. Because these two anomalous circulation cells exhibit opposite polarity, the out-of-phase interannual oscillation between these two cells results in the opposite interannual variation of genesis frequency between tropical cyclones of Groups 1 and 3.

Under a weather hazard prevention program in Taiwan, the Vaisala Meteorological Instrument, Inc., of Finland and the Environmental Science and Engineering Corporation of Taiwan jointly developed and installed more than 300 stations of the Automatic Rainfall and Meteorological Telemetry System (ARMTS) over this subtropical island with an area of approximately 30 000 km 2. The Taiwan ARMTS network can provide the hourly rainfall data of not only a densely instrumented, but also very uniquely observed, geographically complex island embedded in the east Asian monsoon environment. The hourly ARMTS rainfall for the period 1993-96 supplemented with the hourly surface winds and pressure observed by 25 conventional surface stations around this island are analyzed to understand three aspects of the diurnal and seasonal variations of Taiwan rainfall. The preliminary results of the analysis are presented in this paper. 1) The maximum daily rainfall occurs at about 1600 ~ 1700 LST, which is about 2-3 h behind the maximum horizontal convergence of surface airmass flux. This time lag suggests that the interaction between mountains and the land-sea breeze circulation may have a timing regulation effect on the maximum daily rainfall. 2) The daily rainfall variations in Taiwan exhibit a bimodality (an early morning minor maximum and an early evening maximum). Although the timing of these two rainfall maxima coincides with the surface pressure minima of semidiurnal variation, the surface pressure tendency analysis does not support this possible cause of the bimodal rainfall variation. The contrast between daily rainfall variation and divergence of surface airmass flux suggests that the early morning rainfall maximum may be induced by the nocturnal drainage flow along the western slope of the Taiwan central mountain range. 3) The seasonal variation of Taiwan rainfall consists basically of a seasonal seesaw between the summer rainfall over west Taiwan and the fall-winter rainfall over northeast Taiwan. Four rainfall regimes over a year are identified around the island. The seasonal transitions of Taiwan rainfall are essentially formed by a counterclockwise rotation of these four rainfall regimes.

The heavy rainfall/flood (HRF) event in central Vietnam usually occurs in October-November, the maximum rainfall season. This rainfall maximum undergoes a distinct interannual variation, opposite the interannual variation of sea surface temperature (SST) anomalies averaged over the NOAA Niñ o-3.4 area-DSST(Niñ o-3.4)-but coincident with the intensification (weakening) of the low-level easterlies at 158N and westerlies at 58N. The changes of low-level zonal winds reflect the strengthening (weakening) of the tropical cyclonic shear flow in tropical South/Southeast Asia in response to the tropical Pacific SST anomalies. Because the rainfall maximum in central Vietnam is primarily produced by the HRF cyclone, the interannual rainfall variation in this region should be attributed to the HRF cyclone activity-a new perspective of the climate change in precipitation. On average, one HRF cyclone occurs in each cold late fall. The population of the HRF cyclone may not be an important factor causing the interannual rainfall variation in central Vietnam. During the cold late fall, the rain-producing efficiency of the individual HRF cyclone is statistically almost twice those during warm and normal late falls and the most crucial factor leading to the interannual rainfall variation in central Vietnam. It is shown by further hydrological analysis that the increase (decrease) of the HRF cyclone's rain-producing efficiency is determined by the large-scale environmental flow through the enhancement (weakening) of the regional convergence of water vapor flux.