Tropospheric Biennial Oscillation (TBO) and Patterns of Asian-Australian Monsoon Rainfall

Various proposed indices of Asian-Australian monsoon strength often do not agree, and could be measures of competing phenomena associated with the dynamics of the monsoon circulation. Here we apply a technique to avoid the need to rely on area-averaged indices by relating the actual patterns of rainfall over the Asian-Australian monsoon region to two mechanisms involved with the tropospheric biennial oscillation (TBO) to account for interannual variations of monsoon rainfall. The two mechanisms are large scale forcing from the eastern tropical Pacific and regional forcing associated with meridional temperature gradients over south Asia. An SVD analysis relates patterns of 500 hPa height over Australasia and SSTs in the eastern tropical Pacific prior to the monsoon seasons to rainfall over Australasia during the monsoon seasons. Then a cumulative anomaly pattern correlation is calculated using the time-evolving SVD rainfall patterns and the observed rainfall anomalies year by year. Using information from the two hypothesized TBO mechanisms, it is shown that the cumulative anomaly pattern correlations account for a statistically significant portion of the observed monsoon rainfall pattern in about half the years considered in the period 1979-1997. This analysis provides a quantification of the contribution of the TBO mechanisms to interannual Asian-Australian monsoon rainfall in those years, leaving the possibility of different mechanisms or internal dynamics to account for the patterns of monsoon rainfall in the other years. Decadal modulation of these relationships is also examined, with implications for water resources over south Asia and Australia.