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

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.

The Origin of Tropical Disturbances in the Equatorial Pacific

“…The change in the positions of tropical cyclone development are thought to be strongly related to the intraseasonal variation of the largescale wind field. Yamazaki and Murakami (1989) analyzed the intraseasonal change of the activities of tropical disturbances by the aid of the idea of amplitude-modulation of the easterly wave. They described from the composite analysis how the lower tropospheric westerly and the upper-level easterly are enhanced at the time of the enhanced activity of the tropical disturbances.…”

Analysis of Tropical Cyclogenesis in the Western North Pacific for 2000 and 2001*

“…It has been suggested that the variability of TC occurrence is modulated by low-frequency atmospheric vari- ability associated with the tropical ISO (Gray 1979). Recent observational studies have confirmed the close relationship between TC genesis and the ISO (Yamazaki and Murakami 1989;Hartmann et al 1992;Liebmann et al 1994;Maloney and Hartmann 2000). Several studies have theoretically addressed how the ISO modulates TC activities (Maloney and Hartmann 2001;Sobel and Maloney 2000;Maloney and Dickinson 2003).…”

Tropical Cyclogenesis in the Western North Pacific as Revealed by the 2008–09 YOTC Data*

“…It has been suggested that the atmospheric intraseasonal oscillation [ISO, 20-90-day mode; Yamazaki and Murakami (1989); Hartmann et al (1992); Liebmann et al (1994); Sobel and Maloney (2000); Maloney and Hartmann (2000, 2002; Maloney and Dickinson (2003); Hogsett and Zhang (2010)] may exert a large-scale control on TC formation through enhancing or suppressing the TD-type disturbances in the lower-level troposphere . The enhancement of the TD-type disturbances is attributed to either Rossby wave accumulation caused by large-scale confluent flows (Kuo et al 2001) or barotropic energy conversion that involves both rotational and divergent winds (Hsu et al 2011).…”