In contrast to the clear structures and phases of the El Nino-Southern Oscillation, the spatial structure and evolution of the tropospheric biennial oscillation (TBO) are still unclear. Using an extended EOF technique, the authors found that the most pronounced feature of the TBO in the Asian-Australian monsoon region is the development of two anomalous anticyclone centers located in the southeast Indian Ocean (SIO) and western North Pacific (WNP), respectively.

The seasonal evolution of the two anomalous anticyclones is summarized as follows. From June to August of the first year of the TBO cycle, the low-level circulation anomalies are dominated by an elongated anticyclonic ridge extending from the maritime continent to India. Associated with this anticyclonic ridge is a tilted belt of pronounced anomalous westerlies extending from the Bay of Bengal to the WNP, suppressed convection over the maritime continent, and enhanced convection over the Philippine Sea. From September to November, the SIO anticyclone grows explosively, leading to a giant anticyclonic ridge that dominates the Indian Ocean and has its center at 10šS, 90šE. A weak anomalous low-level anticyclone forms in the South China Sea near the Philippines. From December to February, the low-level circulation anomalies are dominated by two subtropical anticyclonic systems located in the SIO and the WNP, respectively. The former is a result of the weakening of the fall SIO anticyclone, while the later results from the amplification and eastward migration of the Philippine anticyclone. The period from March to May has a similar anomaly pattern in the WNP, characterized by the pronounced WNP anomalous anticyclone. The intensity of the WNP anticyclone, however, decreases toward summer. From June to August of the second year in the TBO cycle, subsidence controls the region over the Philippine Sea and Southeast Asia, signifying a weakening of the summer monsoon over the WNP and South Asia. The anomaly circulation pattern is nearly opposite to the one in the previous summer.

Based on the observational features, a hypothesis is put forward to explain the observed structure and evolution of the TBO. The key premise is that the SIO SST anomalies (SSTA) influence the eastern Pacific through anomalous heating over the maritime continent, whereas the WNP monsoon impacts the SIO SSTA through anomalous cross-equatorial flows. An intermediate coupled atmosphere-ocean model was adopted to test this hypothesis. The El Nino delayed oscillator dynamics were excluded by eliminating the effect of thermocline depth anomalies on SSTA in the model. Nevertheless, the model reproduced a biennial oscillation in the tropical Indian and Pacific Oceans. The result suggests that the biennial component of ENSO may arise from an inter-basin teleconnection between anomalous convection and SST patterns in the tropical Indian and Pacific Oceans.