Symmetry and Asymmetry of the Asian and Australian Summer Monsoons

Abstract

The rainfalls associated with the Asian summer monsoon have significant correlation with succeeding Australian summer monsoon rainfalls. This is partly due to the typical life cycle of the El Niño?Southern Oscillation (ENSO) phase locked with the annual cycle within an Asian monsoon year (May?April). This feature is referred to as the symmetric behavior of the Asian?Australian monsoon system. However, preceding Australian summer monsoon rainfalls have no significant impact on succeeding Asian summer monsoon rainfalls. Thus, there is a one-way (asymmetric) relationship between the two monsoons and a communication gap exists in boreal spring prior to the onset of the Asian summer monsoon. The annual cycles of precipitation associated with the intertropical convergence zone (ITCZ) and the sea surface temperature (SST) anomaly in the Indonesia sector (90°?120°E) that links the two monsoon regions are compared. The continuous propagation of the ITCZ from the Asian to the Australian summer monsoon regions in Northern Hemisphere (NH) fall provides a one-way link between them. However, the SST anomaly shows a sudden change of sign in November, suggesting that the anomaly is mainly a response to the ITCZ movement rather than its cause. From January to May, the ITCZ is confined at the latitudes south of 5°N due to the subsidence in oceanic regions adjacent to the southern coast of Asia. This subsidence prevents the ITCZ from propagating northward until the abrupt onset of the Asian summer monsoon with the reversal of meridional temperature gradient. The discontinuity of the ITCZ movement and the seasonal heating over the Asian continent result in the communication gap in NH spring.