An Intraseasonal Amplitude Modulation of the Short-term Tropical Disturbances over the Western Pacific

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Abstract
The intraseasonal change of the activities of the short-term tropical disturbances are investigated in terms of their amplitude modulation. It has been revealed that the amplitude modulation in the tropics bears a periodicity around 30 days. It was also found that the enhanced phase of the amplitude modulation is often accompanied by the clustered genesis and/or development of tropical cyclones. The space-time analysis of the amplitude modulation has shown that it consists of two characteristic modes. One is the eastward-propagating mode with tonal wavenumber 1. Another mode shows a westward propagation and zonal wavenumber 4 to 6 as a characteristic longitudinal scale. Both modes appear equally significant in leading to enhanced disturbance activity.The large-scale circulation changes associated with the intraseasonal amplitude modulation are also investigated referring to the modulation over the tropical western Pacific. The composite analysis has revealed that the lower tropospheric westerly and the upper-level easterly are enhanced at the time of the enhanced activity of the tropical disturbances. The horizontal distribution of the circulation anomalies indicates that an intrusion of the low-level westerlies into the western Pacific takes place at this stage from the equatorial Indian Ocean through the South China Sea. This intrusion is also accompanied by enhanced cross-equatorial southerlies over the Indonesian region. In the upper levels, the intensification of the mid-Pacific trough is observed at the same stage.