92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Tuesday, 24 January 2012: 3:45 PM
Exploring the Initiation of Intraseasonal Variability in the Indian Ocean (invited)
Room 245 (New Orleans Convention Center )
Peter J. Webster, Georgia Institute of Technology, Atlanta, GA; and V. E. Toma, G. Stephens, and R. H. Johnson

The summer monsoons of South Asia and North Australia are marked by slow oscillations between wet (active) and dry (break) periods. These oscillations are tied to 20-60 day oscillations of planetary scale modes often termed the Madden-Julian Oscillation. In both seasons intraseasonal variance has a global maximum in the equatorial Indian Ocean corresponding to the development of convection. There exist few explanations for the geographic locations of convection, nor are there explanations of the locations of these phenomena and also little explanation of why the phenomena possesses a frequency far lower than might be expected from a normal mode response. Anther peculiarity is that the MJO is initiated in a region of the strongest cross-equatorial pressure gradient on the planet. Yet, there is an absence of high frequency modes associated with inertial instability found in other regions such as the eastern Pacific Ocean (i). In fact, there is near absence of high frequency motion in the eastern equatorial Indian Ocean.

We analyze satellite data guided by a priori theoretical considerations. We find that while the cross-equatorial flows is inertially unstable (by linear considerations) it is extremely thermodynamically stable such that the cross-equatorial shear must reach values that are not observed to reach inertial instability. In this manner, the destabilization phase (ii) of intraseasonal variability is prolonged allowing the slow build up of both CAPE and the inertial oscillator (i) extends from periods of days to weeks. In essence, the location of the initiation of the MJO occurs in a region where there is exaggerated thermodynamical stability. The time scale is set by the time it takes for the coupled ocean-atmosphere dynamical system to destabilize the thermodynamical instability.

(i) Toma, V., P. J. Webster, 2010: Oscillations of the intertropical convergence zone and the genesis of easterly waves. I Theory and diagnostics. Clim. Dyn. doi: 10.1007/s00382-009-0584 (ii) Stephens, G. L., P. J. Webster, R. H. Johnson, R. Engelen and T. L'Ecuyer, 2004: Observational Evidence for the Mutual Regulation of the Tropical Hydrological Cycle and Tropical Sea Surface Temperatures. J. Climate: 17(11), 2213-2224.

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