7B.6
Origin of monsoon intraseasonal oscillation
B. Wang, University of Hawaii, Honolulu, HI; and P. J. Webster and H. Teng
The active and break periods of Indian summer monsoon are often associated with northward migrating intraseasonal oscillation that originates in the equatorial Indian Ocean (EIO). Analysis of recent satellite data reveals intraseasonal convection anomalies first occurring in the western EIO (60-70E) when the eastern EIO (75-100E) reaches its driest phase. A preconditioning phase and a shallow convection phase that together last about a week precede the development of deep convection in the central EIO. In the preconditioning phase, while subsidence prevails the free troposphere over the western EIO, the surface winds converge, which along with ocean surface warming, gradually builds up convective instability in the boundary layer, leading to growth of organized shallow convection. The wind convergence and ocean surface warming, which play a central role in re-initiating convection, are essentially induced by the subdued convection over the eastern EIO. This suggests a self-sustaining mechanism for the Asian summer monsoon intraseasonal oscillation. In an averaged scenario, the formation of deep convection in the central EIO is followed sequentially by an eastward extension and shift, a deformation and bifurcation near Sumatra Island, and a northeastward movement, causing an active Indian monsoon about 20 days latter. The mechanisms responsible for the preferred location of re-initiation and the role of air-sea coupling are discussed.
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Session 7B, intraseasonal variability I
Tuesday, 4 May 2004, 1:30 PM-3:15 PM, Napoleon I Room
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