The Tropospheric Biennial Oscillation and Asian-Australian Monsoon
Rainfall
Gerald A. Meehl and Julie M. Arblaster
National Center for Atmospheric Research
P. O. Box 3000
Boulder, CO 80307
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 teh 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 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 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
MAM to 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 year 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. ENSO El Nino and La Nina
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. We quantify these interannual transition conditions
from 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 in the atmosphere. By omitting El Nino and
La Nina onset years from the analysis, there are similar but lower amplitude
relationships among the transition conditions and Asian-Australian monsoon
rainfal. 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 an 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.
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Hongjun Zhang:
zhangho@ucar.edu