A Spectrum Analysis of Synoptic-Scale Disturbances in the ITCZ

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Abstract
Spectral characteristics of convective signals in synoptic-scale disturbances along the intertropical convergence zone (ITCZ) are examined using a 20-yr daily outgoing longwave radiation dataset. A new analysis method, which combines conventional wavenumber–frequency spectrum analysis and wavelet analysis, is developed to explore the longitudinal, seasonal, and interannual variations in these disturbances within the ITCZ whose seasonal migration varies in different parts of the Tropics. The analysis focuses on three longitudinal sectors where the ITCZ can be clearly identified: the western-central Pacific, the central-eastern Pacific, and the Atlantic–West Africa. The most striking results are the evident zonal variability in the spectral properties of westward-propagating synoptic-scale disturbances. The zonal variability exists not only in their dominant frequencies and zonal wavenumbers, but also in their seasonal and interannual variations. Eastward-propagating synoptic-scale disturbances in the ITCZ, in contrast, exhibit much less zonal variability. The results suggest that dynamical relationships between the ITCZ and its embedded westward-propagating synoptic-scale disturbances, if they exist as predicted by theories and numerical simulations of the ITCZ, are likely to vary in longitude.