The key questions of how the tropospheric biennial oscillation (TBO) maintains the same phase as the monsoon convection moves 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, respectively, 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 feedback.
A biennial oscillation emerges in the model in a reasonable parameter regime, with simulated SST and wind variations resembling many aspects of the observed TBO. In particular, the SST anomalies in the Indian Ocean and the eastern and central equatorial Pacific are approximately in phase in time, while they are out-of-phase with the SST in the western equatorial Pacific. Maximum westerly winds over the Indian Ocean appear in July-September, accompanied by a strong Indian monsoon. Over the western Pacific and the Maritime Continent, they occur in December-February and are associated with a stronger Australian monsoon.
In the model a warm SST anomaly starts in July in the equatorial Indian Ocean. This causes 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 due to evaporation-wind feedback. The easterly anomaly in the central Pacific has both positive and negative effects on the western Pacific SST. The deepening of the ocean thermocline increases the subsurface and thus surface temperature, whereas the zonal advection cools the ocean. However, even with the help of the upwelling effect due to the easterly anomaly over the western Pacific, the first effects dominates. The net result is an anomalous warm SST persisting in the western Pacific through the northern fall, leading to a stronger Australia 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 SST anomaly to persist through the succeeding seasons, leading to a weaker Asian monsoon in the following summer. During the northern winter the westerly anomaly associated with the stronger Australia monsoon, through anomalous ocean downwelling, reinvigorates the warm SST in the western Pacific, which has been weakened by the slow cold zonal advection from the eastern Pacific. This further intensifies the eastern Walker cell and helps to keep the eastern Pacific cold.
The proposed TBO theory is based on processes within the tropics only and describes a coherent, self-contained variation of the Asian-Australia monsoon system. The results also imply a potential role of the monsoon on the El Nino-La Nina variations.