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722 VOLUME 15J O U R N A L O F C L I M A T E 2002 American Meteorological Society

Summary: 722 VOLUME 15J O U R N A L O F C L I M A T E
2002 American Meteorological Society
The Tropospheric Biennial Oscillation and Asian­Australian Monsoon Rainfall
National Center for Atmospheric Research,* Boulder, Colorado
(Manuscript received 16 January 2001, in final form 26 September 2001)
In the context of the Asian­Australian 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 land­atmosphere­ocean 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 March­May (MAM) to June­September (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 2­3-yr period) encompasses most ENSO years (with their


Source: Arblaster, Julie - Bureau of Meteorology, Australia
Meehl, Gerald A. - Climate and Global Dynamics Division, National Center for Atmospheric Research


Collections: Geosciences