Barotropic Aspects of ITCZ Breakdown

Abstract

In satellite images the ITCZ (intertropical convergence zone) is sometimes observed to undulate and break down into a series of tropical disturbances. Tropical cyclones may later develop within these disturbances and move into higher latitudes allowing the ITCZ to reform. It has been proposed that ITCZ breakdown results from a convectively modified form of combined barotropic and baroclinic instability of the mean flow. An unstable mean flow can be produced by ITCZ convection in just a couple of days. In this sense, the ITCZ produces favorable conditions for its own instability and breakdown. In this study, a nonlinear shallow water model on the sphere is used to simulate barotropic aspects of ITCZ breakdown. In the shallow-water model, the ITCZ is simulated by a prescribed zonally elongated mass sink near the equator. The mass sink produces a cyclonic potential vorticity (PV) anomaly that has a reversed meridional PV gradient on its poleward side. According to the Ripa theorem, a flow that has a reversal in its meridional gradient of PV may become unstable in the presence of small disturbances. In the model simulations the unstable PV strip either undulates and breaks down into several cyclones or axisymmetrizes into one large cyclone. The model results suggest that ITCZ breakdown may play a role in producing the observed tendencies for tropical storms to cluster in time and form poleward of the central latitude of the ITCZ and to the east of existing tropical storms. Additional experiments indicate that the observed higher frequency of tropical cyclogenesis just west of the Central American coast may be due to the horizontal morphology of the ITCZ in that region. In the eastern Pacific, the ITCZ is a zonally elongated line of convection that is climatologically wider on its eastern side. It is proposed that axisymmetrization of the PV strip produced by such an ITCZ is the cause of the increased frequency of tropical cyclogenesis just west of Central America. Finally, the results obtained in this study point to the importance of zonal asymmetries inherent to the ITCZ in determining the flow evolution, suggesting the need for further studies of this effect. The importance of using forced simulations in the study of flow stability is also discussed.