Tropical synoptic-scale wave disturbances propagating across the Maritime Continent-Australian monsoon region.

The structure and characteristics of tropical synoptic-scale wave disturbances over the maritime continent (MC) and northern Australia during the Southern Hemisphere summer are analyzed. The tropical synoptic-scale waves are identified by an extended empirical orthogonal function (EEOF) analysis on 2-8-day-filtered daily 850-hPa meridional wind anomalies during December-February for the period 1979-2016. EEOF-based composite analyses reveal a synoptic-scale wave train, with a well-organized structure, which is coupled with deep convection. The waves are considered to be tropical depression type waves that develop along a strong meridional gradient of the mean monsoon westerly flow. These waves propagate westward at approximately 8 m/s, with zonal wavelengths of about 3000-4000 km. Eastward amplification of wave troughs and ridges due to group propagation occurs within the synoptic-scale wave train. Wave activity diagnostics confirm that downstream wave energy propagation facilitates the development of the synoptic-scale wave train along the monsoon westerly flow. Composite rainfall patterns show that rainfall is enhanced over the MC and the ocean in the vicinity of the north coast of Australia, which is associated with the passage of the wave trough. The development of this synoptic-scale wave is found to play an important role in day-to-day weather variation over the islands of the MC. Extratropical forcing of the tropical synoptic-scale waves is also explored. Southerly surges that originated in the midlatitude Indian Ocean could intensify the tropical synoptic-scale wave trough. The southerly surges are induced by the development of midlatitude synoptic-scale baroclinic disturbances over the eastern Indian Ocean.