ITCZ Breakdown in Three-Dimensional Flows

Abstract

The intertropical convergence zone (ITCZ) is observed to undulate and at times break down into a series of tropical disturbances in several days. Some of these disturbances may develop into tropical cyclones and move to higher latitudes, while others dissipate, and the ITCZ may reform in the original region. It has been proposed that the ITCZ may break down because of its heating-induced potential vorticity (PV) anomalies. Here this process is examined in three-dimensional simulations using a primitive equation model. A simulation of the ITCZ in a background state of rest is compared to simulations in different background flows. The effect of different vertical structures of the prescribed heating is also examined. Deep heating induces a positive PV anomaly in the lower troposphere, leading to a reversal of the PV gradient on the poleward side of the heating, while the induced PV anomaly at upper levels is negative, leading to a reversal of the PV gradient on the equatorward side of the heating. The response at upper levels leads to a weaker PV gradient change, but the response is greater in areal extent than the lower-tropospheric response. For shallow heating, the lower-tropospheric PV response is greater than that for deep heating, and there is no upper-tropospheric PV response. The ITCZ lasts longer before breaking in this case than in the deep heating case. Effects of the background flow are mainly felt in the deep heating cases. When the background flow enforces the PV-induced wind field, ITCZ breakdown occurs more rapidly, whereas when the background flow is opposite to the PV-induced flow, ITCZ breakdown takes longer and the ITCZ may dissipate before breakdown.