Multiscale Phenomena in the Tropical Atmosphere over the Western Pacific

Multiscale variabilities in the atmosphere over the tropical western Pacific during the 1979 Northern Hemisphere winter are studied with an aim at identifying possible interactions between phenomena of different spatial and temporal scales. Based on the convection-index information derived from satellite measurements, two intra-seasonal oscillations (ISOs) are identified within the equatorial belt between 0° and 10°S in the analyzed period. The two ISOs, accompanied by both rotational and divergent circulations, propagate eastward from the Indian Ocean to the western Pacific. Over the warm pool in the western Pacific, the ISOs develop into quasi-stationary systems with an enhanced rotational circulation characterized by a strong westerly jet in the lower troposphere. The ISOs appear to interact with a number of regional- and synoptic-scale phenomena in the maritime continent and western Pacific region. For example, the onset of the monsoon coincides with the arrival of the first ISO at northern Australian region (140°E) in late December. The passage of ISOs in the monsoon flow are also associated with surface westerly wind outbreaks. On shorter lime scales (<10 days), the ISOs appear to provide a favorable condition over the warm ocean for the development of 2–4-day disturbances that further organize mesoscale cloud clusters. In addition, the diurnal cycle provides another important forcing mechanism modulating cloud clusters, particularly over the maritime continents. There appears to be an inverse relationship between diurnal cycle and intraseasonal disturbances (⩾10 days) over the maritime continent; that is, periods of active intraseasonal variabilities are characterized by diminished diurnal cycles and vice versa. The possible role of these multiscale processes in the coupled ocean-atmosphere system is discussed.