A Theory for the Tropical Tropospheric Biennial Oscillation
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Authors
Chang, C.-P.
Li, Tim
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Date of Issue
2000-07-15
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Abstract
The key questions of how the tropospheric biennial oscillation (TBO) maintains the same phase from northern
summer in South Asia to southern summer in Australia, and how the reversed phase can last through three
locally inactive seasons to the next monsoon, are studied by a simple tropical atmosphere–ocean–land model.
The model has five boxes representing the South Asian and Australian monsoon regions and the equatorial
Indian and western and eastern Pacific Oceans. The five regions interact with each other through the SST–
monsoon, evaporation–wind, monsoon–Walker circulation, and wind stress–ocean thermocline feedbacks.
A biennial oscillation emerges in a reasonable parameter regime, with model SST and wind variations resembling
many aspects of the observed TBO. Warm SST anomalies (SSTA) in July in the equatorial Indian
Ocean cause an increase of surface moisture convergence into South Asia, leading to a stronger monsoon. The
monsoon heating on one hand induces a westerly wind anomaly in the Indian Ocean, and on the other hand
intensifies a planetary-scale east–west circulation leading to anomalous easterlies over the western and central
Pacific. The westerly anomaly over the Indian Ocean decreases the local SST, primarily by evaporation–wind
feedback. The easterly anomaly in the central Pacific causes a deepening of the ocean thermocline in the western
Pacific therefore increasing the subsurface and surface temperatures. In addition, a modest easterly anomaly in
the western Pacific opposes the seasonal mean westerlies so evaporation is reduced. These effects overwhelm
those of the cold zonal advection and anomalous upwelling. The net result is warm SSTA persisting in the
western Pacific through northern fall, leading to a stronger Australian monsoon.
Meanwhile, the warming in the western Pacific also induces a stronger local Walker cell and thus a surface
westerly anomaly over the Indian Ocean. This westerly anomaly helps the cold SSTA to persist through the
succeeding seasons, leading to a weaker Asian monsoon in the following summer. During northern winter the
westerly anomaly associated with the stronger Australian monsoon, through anomalous ocean downwelling and
reduction of evaporation (when the seasonal mean wind is easterly), reinvigorates the warm SSTA in the western
Pacific, which has been weakened by the slow cold advection from the eastern Pacific. This further intensifies
the eastern Walker cell and helps to keep the eastern Pacific cold. The authors’ theory indicates that the TBO is an inherent result of the interactions between northern summer
and winter monsoon and the tropical Indian and Pacific Oceans. Thus, it is an important component of the
tropical ocean–atmosphere interaction system, separate from the El Nin˜o–Southern Oscillation. While the eastern
Pacific plays only a passive role in this mechanism, the western Pacific–Maritime Continent region is crucially
important. It serves as a bridge in space and time, both in connecting the convection anomaly from the northern
summer to the northern winter monsoon and in channeling the feedback of the northern winter monsoon to the
Indian Ocean.
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Meteorology
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Journal of the Atmospheric Sciences, Vol. 57, No. 14, 15 July 2000, pp. 2209-2224
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This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.