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

Observational studies have shown that the Asian-Australasian monsoon system exhibits variability over a wide-range of space and timescales. These variations range from intraseasonal (20-40 days), annual, biennial (about 2 years), longer-term interannual (3 to 5 years) and interdecadal. However, the amplitude of the interannual variability of the South Asian monsoon (at least as described by Indian precipitation) its summer pluvial phase is smaller than variability exhibited in other climate systems of the tropics. For example, drought or flood rarely extend to multiple years, with rainfall oscillating biennially from slightly above average to slightly below average precipitation. We argue that variability of the monsoon is regulated by negative feedbacks between the ocean and the atmosphere. The annual cycle of the heat balance of the Indian Ocean is such that there is an ocean heat transport from the summer hemisphere to the winter hemisphere resulting principally from wind-driven Ekman transport. Given the configuration of the low-level monsoon winds, the Ekman transport is in the opposite sense to the lower tropospheric divergent wind. The cross-equatorial ocean heat transport is large with amplitudes varying between +2 PW (northward) in winter and -2 PW (southward) in summer. Thus, the wind-induced heat transport works to cool the summer hemisphere upper ocean while warming the winter hemisphere. Similar regulation occurs on interannual timescales. For example, during anomalously strong Northern Hemisphere monsoon summers (typically a La Nina), strong winds induce a stronger than average southward flux of heat. When the monsoon is weak (typically an El Nino), the wind-driven ocean heat flux is reduced. In this manner, the monsoon regulates itself by reducing summer hemisphere sea-surface temperatures during strong monsoon years and increasing it during weak years. In this manner, the monsoon is self regulating. It is noted, however, that the ocean heat transport theory of monsoon regulation does not necessarily insist that heat anomalies persist from one year to the next. Furthermore, the theory does not include the Indian Ocean dipole as a dynamic entity. Finally, we develop a more general theory in which the slow dynamics of the dipole are integral components of a sequence of processes that regulate the monsoon, thus minimizing radical year-to-year departures of the monsoon from climatology.