83rd Annual

Tuesday, 11 February 2003: 3:30 PM
The coupled nature of intraseasonal and interannual monsoon variability
Peter J. Webster, Georgia Institute of Technology, Atlanta, GA
The traditional view of the Asian-Australian monsoon system a planetary-scale cross-equatorial sea-breeze system driven by land-sea heating differences and modified by rotational effects. The ocean in this model is essentially passive providing, in addition to heating differences, a supply of water through evaporation. We pose here an entirely different perspective where the ocean, in tandem with the atmosphere, plays a critical role in the evolution, maintenance and regulation of the monsoon annual cycle and intraseasonal and interannual variability.

Diagnostic studies and numerical experiments have shown that the oceans transport heat northward across the equator during the winter and southward during the summer with magnitudes between 2 PW with an annual average of about -0.2 PW. These fluxes are sufficient to cool the North Indian Ocean by about 3K in the summer and warm it by the same amount in winter. This heat transport is wind driven as a combination of Ekman transport and boundary currents. Because oceanic Ekman transport is to the right of the prevailing flow in the northern hemisphere and to the left in the southern, the oceanic heat transport is in the opposite sense to the atmospheric heat transport. In fact, although opposite in sign the magnitudes of the oceanic and atmospheric transports are almost equal. The coupling between the atmosphere and the ocean leads to a coupled regulation process: strong summer monsoon winds will lead to enhanced southward transport (cooler North Indian ocean) while a weak monsoon will produce a reduced meridional oceanic heat flux (warmer North Indian ocean). In fact, diagnostic studies show an extremely close correspondence between the strength of the monsoon and the net annually averaged meridional heat flux. The coupled processes also lead to a bienniality in monsoon strength which has also been observed.

Strong and weak monsoons are also associated with anomalies in upwelling in the eastern and western margins of the Indian Ocean. It is argued that SST anomalies, so produced, allow another ocean-atmosphere feedback to develop and the production of one phase or another of the Indian Ocean Zonal Mode (IOZM) or dipole. It is shown that the anomalous transports associated with the IOZM also act as feedbacks to either enhance a weak monsoon or reduce the intensity of a strong monsoon.

It will be shown that the combined effect of the processes described above is to limit the interannual variability of the monsoon. For example, El Nino may create an initially weak monsoon. However, the subsequently reduced monsoon winds will limit the seasonal southward transport of heat by the ocean. This fact is born out by the observation that there are rarely multiple flood or drought years in a row or that an ElNino is initially associated with a weak monsoon but a strong monsoon the following year.

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