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 is a fundamental feature of the coupled climate system over the entire Indian?Pacific region. El Niño and La Niña events as well as Indian Ocean SST dipole events are large amplitude excursions of the TBO in the tropical Pacific and Indian Oceans, respectively, associated with coupled ocean dynamics, upper-ocean temperature anomalies, and associated ocean heat content anomalies. Conditions postulated to contribute to TBO transitions involve anomalous Asian land surface temperatures, Pacific and Indian Ocean SST anomalies, and the associated strength of the convective maximum over Australasia. These interannual transition conditions are quantified from singular value decomposition (SVD) analyses on a year-by-year basis using single and cumulative anomaly pattern correlations. This technique takes into account intermittent influences and secular variations in the strength of any particular association in any given year. Anomalous Pacific and Indian Ocean SSTs are the dominant transition conditions in the TBO, with anomalous meridional temperature gradients over Asia a secondary factor. There is an intrinsic coupling of the anomalous strength of the convective maximum in the seasonal cycle over Australasia, surface wind forcing, ocean dynamical response, and associated SST anomalies that feed back to the strength of the convective maximum, and so on. All are tied together by the large-scale east?west circulation, the eastern and western Walker cells, in the atmosphere. By omitting El Niño and La Niña onset years from the analysis, there are similar but lower-amplitude relationships among the transition conditions and Asian?Australian monsoon rainfall. An SST transition in the Pacific is started by surface wind anomalies in the far western equatorial Pacific associated with the Australian monsoon, while an SST transition in the Indian Ocean is started by surface wind anomalies in the western equatorial Indian Ocean associated with the Indian monsoon. This provides successive forcing and response among Indian and Pacific SSTs and the Asian?Australian monsoons half a year apart. The consequent feedback to the monsoon circulations by the SST anomalies results in the TBO.