O

WEAK Indian Summer Monsoon

Pacific Ocean SSTA

cold cold cold monsoon forcing sign change

Figure 12. Schematic illustrating the relationship between the Indian—Australian summer monsoons and the Indo-Pacific Ocean SST anomalies (SSTA) during the TBO.

warm anomaly pattern typically associated with a strong Indian monsoon in June-July-August (JJA, Year 0) (Lau and Yang, 1996). The strong monsoon winds force the Indian Ocean SST anomalies to reverse sign. The Indian Ocean SST anomalies in September-October-November (SON, Year 0) are, therefore, in transition and have small amplitudes. During the same period, the large cold Pacific SST

anomalies persist and dominate the in-phase transition to a strong Australian summer monsoon in December-January-February (DJF, Year 0). This explains why the Pacific Ocean coupling is more important to the in-phase transition from the Indian summer monsoon to the Australian summer monsoon. The strong Australian summer monsoon winds then force the Pacific SST anomalies to change sign and to have small amplitudes in March-April-May (MAM, next year; +1). During this period, cold SST anomalies have been established in the Indian Ocean and have grown to large amplitudes. These cold Indian Ocean SST anomalies then dominate the out-of-phase transition and lead to a weak Asian summer monsoon in JJA of the next year. This explains why the Indian Ocean coupling is needed for the CGCM to produce the out-of-phase monsoon transition. The specific air-sea coupling processes that are involved in these monsoon-ocean interaction should be similar to the wind-evaporation/entrainment and cloud-radiating feedback processes discussed by Wang et al. (2003). They showed that these processes allow the remote forcing of ENSO in the Indo-Pacific warm pool to be amplified and maintained from the developing summer to the decaying summer of an ENSO event, to contribute to the formation of the TBO.

Yu et al. (2003) concluded that the Indian summer monsoon has a stronger impact on the Indian Ocean than on the Pacific Ocean, and that the Australian summer monsoon has a stronger impact on the Pacific Ocean than on the Indian Ocean. These seasonally dependent monsoon influences allow the Pacific and Indian Oceans to have different feedbacks during the in-phase and out-of-phase monsoon transitions, and thus lead to the TBO.

7. ENSO's Interactions with Indian

Ocean SST Variability

The recent interest in the observed east-west contrast pattern in Indian Ocean SST anomalies has prompted the suggestion that the Indian Ocean has its own unstable coupled atmosphere-ocean mode similar to ENSO (e.g. Saji et al., 1999; Webster et al., 1999). This interannual SST variability is often referred to as the Indian Ocean zonal mode (IOZM) or Indian Ocean dipole. The IOZM is characterized by opposite polarities of SST anomalies between the western and eastern parts of the equatorial Indian Ocean, and is always accompanied with zonal wind anomalies in the central Indian Ocean. The strong wind-SST coupling associated with the IOZM has been used to argue for the similarity of the phenomenon to the delayed oscillator of ENSO (Webster et al., 1999). The fact that the temporal correlation between the observed IOZM and ENSO events is not strong, and that several significant IOZM events have occurred in the absence of large ENSO events, have led to the suggestion that the IOZM is independent of ENSO (Saji et al., 1999). On the other hand, there are suggestions that the IOZM is not an independent phenomenon, but is forced by ENSO through changes in surface heat flux or Indian Ocean circulation (e.g. Klein et al., 1999; Chambers et al., 1999; Murtugudde and Busalacchi, 1999; Venzke et al., 2000; Schiller et al., 2000; Huang and Kinter, 2002; Xie et al., 2002). It has also been suggested that the IOZM is a weak natural coupled mode of the Indian Ocean that can be amplified by ENSO during a particular season (e.g. Gualdi et al., 2003; Annamalai et al., 2003). The IOZM is also suggested to be a natural part of the Indian summer monsoon and the TBO (e.g. Meehl and Arblaster, 2002; Loschnigg et al., 2003, Li et al., 2006). The IOZM is argued to arise from the ocean-atmosphere interactions within the Indian Ocean with links to the Pacific involved with the TBO.

Yu and Lau (2004) examined the intrinsic and forced SST variability in the Indian Ocean by contrasting the Indian ocean SST variability between the Indo-Pacific and Indian Ocean Runs. The former run includes ENSO influences, while the latter one excludes the influences. The M-SSA was applied to the interannual anomalies of Indian Ocean SST to extract leading oscillatory modes from the simulations. One major advantage of the M-SSA is that it easily identifies oscillatory behavior, even if it is not purely sinusoidal (Robertson et al., 1995). In the M-SSA, an oscillatory mode appears as a pair of M-SSA modes that have similar eigenvalues, similar sinusoidal principal components in quadrature with each other, and similar eigenvector structures.

In the Indo-Pacific Run, an oscillatory mode of the Indian Ocean SST variability was found with the M-SSA (not shown). The mode comprises two patterns that can be identified with the IOZM and a basinwide warming/cooling mode respectively. To link the oscillatory mode to the interannual SST variability in the Pacific Ocean, we calculated the time-lag correlation coefficients between the principal component of the leading M-SSA mode and SST anomalies in the entire Indo-Pacific Ocean. Figure 13 shows that the correlation is characterized by an IOZM pattern in the Indian Ocean and an ENSO pattern in the Pacific. The time sequence simulated in the Indo-Pacific Run appears close to the sequence observed during the 1997-98 ENSO event, although discrepancies exist. One discrepancy is that, in the Indo-Pacific Run, ENSO peaks earlier than does the IOZM. The

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