The monsoon as a coupled ocean-atmosphere self-regulating system

The traditional view of the monsoon system is a planetary scale circulation driven by large-scale heating gradients between the summer and the winter hemispheres and modulated by external forcing such as ENSO and by changes in albedo and ground surface water associate with (possibly stochastic) variability in winter and spring snowfall. In this model, variability in the Asian monsoon is externally forced and the Indian Ocean appears as a passive entity changed by external perturbations and by the monsoon winds themselves. Whereas some intriguing relationships have been found between external forcing and monsoon precipitation variability, they rarely explain large amounts of variance and physical explanations of the relationships are only vaguely understood. Furthermore, the concept of the passive Indian Ocean fails because the heat budget of the Indian Ocean and the observed changes in sea surface temperature throughout the entire year cannot be explained by local processes. Furthermore, despite the existence of large scale influences (ENSO and etc.), the large scale and seasonal average monsoon rainfall (measured here in terms of Indian rainfall) is remarkably stable from one year to the next compared to other regions of the globe which are also forced by the same external phenomena. In addition, there is recent evidence that the Indian Ocean exhibits strong and reoccurring oscillations between the eastern and the western Indian Ocean. This Indian Ocean mode, or Indian Ocean dipole, appears as a coupled ocean-atmosphere phenomena of considerable magnitude with strong influences on the east African monsoon precipitation although seemingly less on the South Asian monsoon. During the cold phase of the mode (cool in the eastern Indian Ocean, warm in the west) the phenomena is referred to by the people of Sumatra as the "Papen Raya" (grand harvesting) in acknowledgment of the large fish yields associated with the cold upwelling. Furthermore, basin scale oscillations in sea surface height associate with the Indian Ocean mode may be related to extensive flooding in monsoon river deltas.

We present a new view of the Indian Ocean-monsoon system and one that is regulated through a negative feedback between the ocean and atmosphere. We show modeling and empirical evidence that the variability of the monsoon is held within relatively tight limits through a coupled regulatory system that operates on intraseasonal, annual and interannual timescales and may be associated with the Indian Ocean mode. In this manner, anomalies produced by the large scale external forcing of system, especially during the northern hemisphere summer, are modulated and extreme responses ameliorated. Differences in the magnitude of the monsoon and the location of rainfall maxima within a particular season result mainly from of the seasonal character of intraseasonal oscillations which are probably chaotic oscillations occurring within the annual cycle of the monsoon. Predictability of the seasonal details of the intraseasonal variability of the monsoon before a monsoon season is therefore unlikely. However, there appears to be solid precursors of individual intraseasonal modes within a particular monsoon season that will probably allow their prediction on the time scale of weeks.