Atmospheric circulation processes and features in the tropical Southwest Pacific
This paper is intended to provide an overview of the atmospheric controls that affect the weather and climate of the island nations of the Southwest Pacific. The geographical focus is on the region to the north of the area dominated by the mid-latitude westerlies, and therefore will largely exclude consideration of Australia and New Zealand and also of Southeast Asia. The review is confined mostly to a discussion of surface features. The large-scale time-averaged circulation features important to the region include the subtropical high pressure belt, the easterly trade winds, the Intertropical Convergence Zone, the South Pacific Convergence Zone, the Monsoon Trough, and the Hadley and Walker circulations. Some island groups will experience different circulation regimes, with their associated weather systems, at different times of the year. Tropical cyclones are also a major feature of the climate of the region south of about 10ºS. Large variability is an intrinsic part of tropical climate. Variations from year to year have been attributed mainly to the El Nino-Southern Oscillation (ENSO) phenomenon, which involves an exchange of air between the tropical eastern Pacific and the Indian Ocean. ENSO changes produce marked rainfall anomalies in many Pacific Island-groups through, among other things, a shift in the position of the South Pacific Convergence Zone. The occurrence of extreme events can also be altered through a shift in the preferred location of tropical cyclone tracks. An important influence on variability within a season is the 30-60 day oscillation. This feature appears as an eastward propagating wave pulse that modulates the intensity of convective systems in the equatorial Pacific, particularly west of the dateline.