Abstract
In an attempt to understand the dynamics of the Intertropical Convergence Zone (ITCZ), this study primarily explores the extent to which the ITCZ is causally related to zonally (mostly westward) propagating synoptic-scale disturbances. The ITCZ, measured by its mean convection, is represented by mean outgoing longwave radiation (OLR). Two analysis methods (2-D wavelet analysis and cloud pattern tracking) are developed and applied to extract the deep convective signals associated with synoptic-scale disturbances from a 20-year dataset of daily OLR.Westward-propagating synoptic-scale disturbances, generally displaced poleward from the equator, are mainly observed in the southeastern Indian Ocean, the southwestern Pacific, the northwestern Pacific to the eastern Pacific along the ITCZ, and the Atlantic-African sector. Clear zonal variations exist not only in their spectral properties (dominant frequencies and zonal wavenumbers), and their seasonal and interannual variability, but also in their occurrence and characteristics of associated westward-moving deep-convective systems (size, strength, duration, etc.). Westward-propagating synoptic-scale disturbances become active from early summer or late spring in each hemisphere, and can hardly be seen during the winter season. Their modulation by the ENSO events can be readily found, though this modulation is opposite between the Pacific and Atlantic. It is shown that the appearance, intensity and even moving tracks of these disturbances are closely related to their embedded environment.The relationship between westward-propagating synoptic-scale disturbances and the ITCZ is thoroughly examined. In general, the two phenomena match each other very well in their distributions and seasonal cycles. However, discrepancies in the details can always be found, some of them substantial. Both the maximum spectral signals and tracked deep-convective systems tend to occur at the poleward side of the ITCZ. The seasonal cycle in these westward-propagating synoptic-scale disturbances is often greater than in the ITCZ. On interannual time scales, their relationship is even more limited. Further comparisons between the observations and the theories relating the ITCZ and westward-propagating synoptic-scale disturbances suggest that the observed ITCZ does not owe its existence to the zonally propagating synoptic-scale disturbances. In a sense, the ITCZ would still exist in the absence of these disturbances. Thus, on seasonal and interannual time scales, the ITCZ might be more effectively influenced by large-scale processes through various physical mechanisms.