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Figure 10. Time-lagged correlation coefficients between the monthly IMRI and AMRI calculated from the (a) CMAP observation, (b) Indo-Pacific Run, (c) Pacific Run, and (d) Indian Ocean Run. (From Yu et al., 2003.)

The negative one represents the out-of-phase transition from Australian to Asian summer monsoons. The CGCM produces only the large positive correlations (i.e. the in-phase transition) in the Pacific Run, and produces only the large negative correlation (i.e. the out-of-phase transition) in the Indian Ocean Run. These results suggest that the Pacific Ocean coupling is crucial to the in-phase transition from the Indian summer monsoon to the Australian summer monsoon. The Indian Ocean coupling is crucial to the out-of-phase transition from the Australian summer monsoon back to the Indian summer monsoon.

By analyzing SST evolutions during the monsoon transitions, Yu et al. (2003) noticed that the Indian and Pacific Oceans showed different and interesting relationships with the inphase and out-of-phase monsoon transitions. Figure 11 is constructed to summarize these different SST evolutions and their relationships with the monsoon transitions. In this figure, the evolutions of SST anomalies in the central Indian Ocean (20°S-20°N; 40°E-80°E) and in the central Pacific Ocean (10°S-10°N; 150°E-170°W) are composited for the in-phase and out-of-phase monsoon transitions. The corresponding IMRI and AMRI values composited during those seasons are also shown in the figure. Figure 11(a) shows that after a strong (weak) Indian summer monsoon occurs, SST anomalies change sign in the Indian Ocean. SST anomalies are small throughout this in-phase monsoon transition. During the same period, SST anomalies in the Pacific Ocean are large and contribute to the in-phase monsoon transition. Figure 11(b) shows that, after changing sign in the Indian summer monsoon season, Indian Ocean SST anomalies continue to grow and reach large amplitudes during the out-of-phase monsoon transition. During this transition period, the Pacific SST anomalies change sign. Therefore, SST anomalies in the Pacific are small. The large Indian Ocean SST anomalies contribute to the out-of-phase monsoon transition from a strong (weak) Australian summer monsoon to a weak (strong) Indian summer monsoon.

The different evolutions of the Indian and Pacific Ocean SST anomalies are the key that reveals the different roles of these two oceans in the transition phases of the TBO. Figure 12 illustrates how the different monsoon-ocean interactions lead to the TBO. For the sake of discussion, the process begins with warm Indian Ocean SST anomalies and cold (La Nina-type) Pacific SST anomalies, an SST

(a) In-Phase Transition

(b) Out-of-Phase Transition

(a) In-Phase Transition

(b) Out-of-Phase Transition

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