Physical mechanisms of the Australian summer monsoon: 1. Seasonal cycle

Abstract

In order to understand the physical mechanisms of the Australian summer monsoon, cyclostationary empirical orthogonal function analysis was conducted on the 23‐year (1979–2002) Xie‐Arkin pentad summer precipitation data and other key physical variables over the Australian monsoon region, including the tropical and southern Indian and Pacific Oceans. The first mode of precipitation represents the El Niño signal, the second mode is the seasonal cycle, the third mode describes the phase transition of El Niño–Southern Oscillation between El Niño and La Niña, and the fourth and fifth modes represent separate components of the Madden‐Julian oscillations. All five modes together explain about one third of the total variance of the pentad precipitation data. This paper investigates the physical mechanisms of the seasonal cycle, which explains the “mean” seasonal evolution of the Australian summer monsoon in the absence of other external physical mechanisms. The onset and termination mechanisms of the Australian summer monsoon are strongly associated with the surface temperature changes over the Australian continent and the resulting sea level pressure (SLP) changes. Almost a month prior to the monsoon onset, the surface temperature over northern Australia increases, and consequently SLP decreases over Australia. This initiates an anomalous cyclonic circulation over Australia. The increased upward motion induced by the surface warming together with the anomalous cyclonic circulation results in the increased lower level moisture convergence over the interior of Australia. This increases the amount of precipitation over northern Australia and eventually starts the monsoon. In conjunction with the anomalous cyclonic circulation, zonal wind anomalies over northern Australia change from easterly to westerly. During the termination stage, physical conditions over Australia are reversed.