Leading modes of synoptic scale convective activity over the central Pacific

The Pacific Intertropical Convergence Zone (ITCZ) and South Pacific Convergence Zone (SPCZ) are characterized by a broad spectrum of synoptic convective activity. An Empirical Orthogonal Function (EOF) analysis of global tropical (20S-20N) brightness temperature (Tb) data filtered to retain fluctuations from 2-6 days reveals out of phase variations in convection on either side of the equator within the Pacific, with a period of around 4 days. This antisymmetric pattern of convection is consistent with zonally propagating disturbances such as mixed Rossby-gravity (Yanai) and eastward inertio gravity waves. However, a lag-regression analysis of the EOF pattern shows that this convective signal propagates poleward in both hemispheres over time, with little zonal propagation evident. On the other hand, projection of the circulation data onto this convective pattern reveals westward propagation that is consisted with Yanai modes. In addition, spectral analyses of the EOF signal as well as long space-time series of Tb suggest that Yanai and EIG waves form a continuum, which should not necessarily be separated into westward versus eastward components. Theoretical shallow water modes are used to interpret the 2-6 days convective and associated circulation pattern, where it shown that an interference signal between Yanai and EIG waves results in EOF patterns similar to the observations.