Leading modes of synoptic scale convective activity over the west Pacific warm pool

The west Pacific Intertropical Convergence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ) are characterized by a broad spectrum of convective activity. This region forms a true "double ITCZ", and it turns out that they are closely coupled to each other on synoptic (less than 6 day) time scales. An Empirical Orthogonal Function analysis of global tropical (20S-20N) brightness temperature (Tb) data filtered to retain westward moving fluctuations on synoptic time scales reveals out of phase variations in convection on either side of the equator within the west Pacific ITCZ and SPCZ, with a period of around 4 days. These signals are linked to disturbances that can be characterized as mixed Rossby-gravity (Yanai) waves, which frequently then transform into easterly waves north of the equator as they approach the region of New Guinea, often initiating typhoon formation. In contrast, the leading mode of synoptic filtered Tb data without regard to propagation direction reveals a similar antisymmetric pattern in the same region, but in this case the convective signal propagates poleward in both hemispheres over time, with little zonal propagation evident. Projection of the circulation data onto this pattern reveals a combined Yanai and eastward inertio gravity (EIG) mode. Spectral analyses of the EOF signal as well as long time series of Tb suggest that Yanai and EIG waves form a continuum, which should not necessarily be separated into westward versus eastward components. It appears that these equatorial modes can be excited by higher latitude forcing, with Rossby wavetrains appearing within the extratropical storm tracks prior to the peak convective activity near the equator.