Variabilities of the Asian-Australian monsoon system from annual to interdecadal timescales

Among the monsoons of the world, the Asian and Australian summer monsoons are the most conspicuous. The study of the Australian summer monsoon reveals the role of land-sea thermal contrast in a pure form. Using the ECMWF re-analysis (ERA), and the daily Q₁ (apparent heat source) and Q₂ (apparent moisture sink) obtained as residuals of the large-scale heat and moisture budgets from ERA in 1979–93, four major factors contributing to the onset of the Australian summer monsoon are identified: (i) land-sea thermal contrast, (ii) barotropic instability, (iii) arrival of the Madden-Julian oscillation (MJO), and (iv) intrusion of midlatitude troughs.; The rainfall amounts associated with preceding Asian summer monsoons have significant correlation with the rainfalls due to succeeding Australian summer monsoons. However, there is no significant correlation in rainfalls between preceding Australian summer monsoons and succeeding Asian summer monsoons. The one-way relationship between the two monsoons is explained by the continuous propagation of the intertropical convergence zone (ITCZ) from the Asian to the Australian summer monsoon regions in northern hemisphere (NH) fall, and the discontinuity of the ITCZ movement in NH spring due to the subsidence along the coast of South Asia induced by continental heating.; The Asian-Australian monsoon system exhibits a biennial tendency. From strong-minus-weak monsoon composites in 1979–93, a mechanism of tropospheric biennial oscillation (TBO) is suggested. The TBO is a result of alternatingly dominant (i) east-west (Walker) circulation between the Australian summer monsoon and the Indian Ocean and (ii) meridional (Hadley) circulations between the Asian continent and the ITCZ over the Indian Ocean, acting to enhance/suppress recipitation by the accompanied ascent/descent.; The interdecadel variability of the TBO is investigated for a longer period (1900–98). The recent decades are highly active periods of the TBO. During some other decades, the Asian and Australian summer monsoons are uncoupled. Instead, the biennial tendency involving the yearly rainfalls in the Asian summer monsoon region and the east African October-November-December rainfalls is found.