Westward-Propagating Synoptic-Scale Disturbances and the ITCZ

Abstract

In an attempt to understand the dynamics of the intertropical convergence zone (ITCZ), this study explores the extent to which the ITCZ is causally related to zonally propagating synoptic-scale disturbances. The ITCZ, measured by its mean convection, is represented by mean outgoing longwave radiation (OLR). Synoptic-scale disturbances, measured by their deep convective signals, are represented by the spectral power of the OLR that is significantly above its red-noise background. Time-mean spatial distributions as well as annual and interannual variability of the ITCZ are compared with those of synoptic-scale disturbances, which are dominated by westward-propagating signals. In general, they match each other well in their mean distributions and annual cycles. But, in detail, discrepancies between the two fields exist, some of them substantial. The maximum disturbance activity tends to be located at the polar side of the ITCZ. The seasonal cycles of the two share many similarities, but the variations in the intensity and latitudinal locations of the disturbances are greater than those of the ITCZ. On interannual timescales, their relationship is even more limited. Comparisons are also made between the observations and theories relating the ITCZ and westward-propagating synoptic-scale disturbances. The results suggest that the observed ITCZ does not owe its existence to zonally propagating synoptic-scale disturbances, in the sense that it would still exist in the absence of the disturbances. But the similarities in their means and annual cycles imply that the disturbances alone can result in an ITCZ resembling the observed one in many respects. The observations, on the other hand, are consistent with the theories that view the dynamical instability of the ITCZ as a cause of some westward-propagating synoptic-scale disturbances.