Annual cycle and interannual variability of the Asian-Australian monsoon system

Annual cycle and interannual variability of the Asian- Australian monsoon precipitation and their relationship with El Nino - Southern Oscillation (ENSO) are studied by using the merged precipitation, outgoing long-wave radiation (OLR), velocity potential (VP) at 925hPa and 200hPa, Southern Oscillation index (SOI) and sea surface temperature anomaly (SSTA) in the NINO 3.4 region (5°N-5°S, 170-120°W) for 1979-1998. The main effort is devoted to exploring and understanding the association of monsoon precipitation of the southern Asia with that of the northern Australia and the possible physical mechanisms for the association. It is found that there is a significant correlation between the annual precipitation over the southern Asia and that over the northern Australia on basis of monsoon years, one of which starts from May of a calendar year and ends in April of the following calendar year. In a monsoon year, the monsoon precipitation and the corresponding convective activity appear first in the southern Asia in June, and then move southeastward, arrive to the southernmost end located in the northern Australia in the following February. An Asian-Australian tropical monsoon index (AAI) is proposed on the basis of year-to-year variation in precipitation for both the southern Asia and northern Australia, reflecting the coherence of Asian-Australian monsoon activities. Using the AAI, we examine year-to-year variation of the Asian- Australian monsoon and find that the AAI has a high positive correlation (0.69) with SOI and a high negative correlation (-0.59) with SST. The in-depth analysis shows that it is the SSTA persistence that causes the Walker circulation and then the Asian-Australian monsoon variations. The persistent warm SSTA in the eastern Pacific (El Niño) can induce a weakened Walker circulation which changes with the seasonal variation. Thus the upward motion related with the Walker circulation decreased mainly over the southern Asia during June-September and over the northern Australia during December-March. As a result, precipitation reduces for both the southern Asia and northern Australia in a monsoon year with El Niño. The situation is reversed in a La Nina year. In addition, the possible effect of the SSTA on the seasonal propagation of strong convection from the southern Asia to the northern Australia is also discussed.