Interdecadal variation of Australian summer monsoon during late 1990s

In this study, the variations of Australian summer monsoons (AUSMs) over the last 30 years (1983–2012) were examined. The analysis period for a total of 30 years was divided into 15 years for 1983–1997 (8397) and 15 years for 1998–2012 (9812) to analyse interdecadal variations of AUSMs. The AUSM index (AUSMI) generally showed negative values in the 8397 period while showing positive values in the 9812 period, indicating that clear interdecadal variations exist in the AUSMI over the last 30 years. Recently, AUSMs have been trending towards being reinforced. The precipitable water averaged in the area for defining the AUSMI has also shown a tendency to increase, until recently, along with the characteristics of interdecadal variations. To examine the cause of the recent reinforcement of AUSMs, differences in 850 hPa stream flows between the average of the 9812 period and the average of the 8397 period were analysed. Anomalous cyclones were located in the west of Australia and the South China Sea, and anomalous anticyclones existed in the central and eastern Pacific in both hemispheres. Because of these patterns of anomalous pressure systems, anomalous equatorial easterlies (anomalous trade winds) were reinforced. The 200 hPa stream flows showed patterns opposite to those of the pressure systems shown by 850 hPa stream flows. Because of these patterns of anomalous pressure systems, anomalous equatorial westerlies were reinforced. These results mean that the Walker circulations – in which air currents ascend in the northern region of Australia and the Maritime Continent and descend in the equatorial central Pacific – were reinforced further in the 9812 period. These changes may be related to the interdecadal changes in the Pacific Ocean, including the Pacific Decadal Oscillation and El Niño-Southern Oscillation.