Tuesday, 11 January 2005: 11:45 AM
Scale interactions within the Madden-Julian Oscillation
The low frequency structure of the Madden-Julian Oscillation (MJO) is known to be the result of the projection of a multitude of smaller scale, higher frequency disturbances within the MJO envelope onto the large scale. In this presentation the statistical link between the MJO and other equatorially-trapped waves will be documented. The MJO appears to substantially modulate the activity and amplitude of Kelvin, equatorial Rossby (ER), mixed-Rossby gravity (MRG), and westward inertio-gravity (WIG) waves coupled to convection. While the occurrence of some of these waves is obviously increased within the convective envelope of the MJO itself, there are substantial signals of far-field modulation as well. In particular, the location of Kelvin wave activity is systematically altered throughout the MJO lifecycle. Kelvin waves appear in many instances to be linked to the initiation of the MJO in the Indian Ocean, and also to emanate into the Pacific from the MJO envelope once it reaches the Indonesian region. A close link between Kelvin waves and the onset of the Indian monsoon is observed during many years. The impact of scale interactions between the MJO and other modes is a crucial link to understanding the dynamics of the MJO itself. Observational evidence points to the important roles played by the vertical heating distribution of individual convective elements embedded within the large scale disturbances. Satellite and ground based data reveal a distinct evolution of cloudiness within the envelope of WIG and Kelvin waves as well as the MJO. The role of stratiform versus deep convective precipitation appears to be a key element of the energetics of the large scale organization of convection. A vertical mode decomposition of large scale heating profiles for the MJO, Kelvin, and WIG waves all reveal a characteristic progression from shallow to deep convective and then to stratiform heating, with a reversal in sign of the "second mode" profile accounting for both the shallow and stratiform modes. The implications of this progression suggest a fundamental invariance across a wide range of scales. Proper representation of these interactions will likely be a prerequisite to the realistic simulation of both the MJO and the other equatorial waves in numerical models.