The Tropospheric Biennial Oscillation and Asian–Australian Monsoon Rainfall

Abstract

In the context of the Asian‐Australian monsoon, the tropospheric biennial oscillation (TBO) is defined as the tendency for a relatively strong monsoon to be followed by a relatively weak one, and vice versa. Therefore the TBO is not so much an oscillation, but a tendency for the system to flip-flop back and forth from year to year. The more of these interannual flip-flops or transitions, the more biennial the system. The transitions occur in northern spring for the south Asian or Indian monsoon and in northern fall for the Australian monsoon involving coupled land‐atmosphere‐ocean processes over a large area of the Indo-Pacific region. There is considerable seasonal persistence from the south Asian to Australian monsoon as noted in previous studies, with a strong south Asian or Indian monsoon tending to precede a strong Australian monsoon and vice versa for weak monsoons. Therefore, transitions from March‐May (MAM) to June‐September (JJAS) tend to set the system for the next year, with a transition to the opposite sign the following year. Quantifying the role of the conditions that contribute to these transitions in the TBO and their relationship to ENSO is crucial for verifying their accurate representation in models, which should lead to improved seasonal forecast skill. An analysis of observed data shows that the TBO (with roughly a 2‐3-yr period) encompasses most ENSO years (with their well-known biennial tendency) as well as additional years that contribute to biennial transitions. Thus the TBO ←