The Structure and Poleward Propagation Mechanism of the Boreal Summer Intraseasonal Oscillation

The equatorial Indian Ocean and western Pacific Ocean are characterized by envelopes of convection propagating poleward and/or eastward at intraseasonal timescales during boreal summer. In particular, multivariate EOF analysis depicts the northeastward migration of the boreal summer intraseasonal oscillation (BSISO) as the evolution of a pair of leading EOFs, which feature zonally elongated northwest-southeast-tilted bands of convection. However, the structure and propagation of individual events are more complicated than the simple picture painted by the composite life cycle of the BSISO, and in fact, it has been reported that only about half of the poleward propagating BSISO events over the Indian and western Pacific monsoon region are moving eastward concurrently. A critical question remains whether these leading EOFs are statistical features. To be specific, do the EOFs represent zonally stretched individual events per se, or a spatially elongated distribution of localized individual events that strongly project onto the EOFs? Here, as an alternative approach, a one-point correlation map technique is employed to characterize the structure and evolution of the intraseasonal variability. In addition, a tracking algorithm is used to identify BSISO events, which are then classified into different categories based on their geographic and propagation characteristics. For each category, composites are constructed to examine the structure of the BSISO. Results from different methods are compared. A better understanding of BSISO structures has important theoretical implications for the poleward propagation mechanism of this phenomenon. Historically most of the simple theories proposed to explain the mechanism of the northward propagating BSISO are based on axisymmetric models, assuming zonally elongated structures, while Boos and Kuang (2010) argued that the northward propagation is in fact beta drift of spatially localized disturbances. The present study seeks to explore how simple theories relate to the realistic BSISO events.