**Space-time spectra of westward-propagating synoptic-scale disturbances in the ITCZ

The global properties of westward-propagating synoptic-scale perturbations along the Intertropical Convergence Zone (ITCZ) are examined using a 19-year daily record of satellite-observed outgoing longwave radiation (OLR). The seasonality of these perturbations is evident and depends on the longitude. Three main longitudinal regions are found where the perturbations are relatively strong: (1) the western Pacific-Indian Ocean, (2) the central-eastern Pacific and (3) the Atlantic-West Africa. Westward-propagating synoptic-scale perturbations in a specific region generally hold their own specific spectral characteristics. Global and local wavenumber-frequency spectral analysis clearly shows the difference. In the western Pacific, westward-propagating synoptic-scale perturbations mainly appear at frequencies -0.2 - -0.25 (1/day) and wavenumbers 7 - 14. In the eastern Pacific, relatively weak perturbations are observed. In the Atlantic-West African region, higher frequencies [around -0.3 (1/day)] and wavenumbers (10 - 18) perturbations dominate. The spectral difference for these perturbations probably implies the discrepancy in their origins, and dynamical development. These results can help us further examine the relationship between westward-propagating synoptic-scale perturbations and the ITCZ for their obvious seasonal evolution. Also the results point out some implications for the prediction of internal association with westward-propagating tropical cyclones through their synoptic-scale perturbations.