Breakdown of ITCZ-like PV Patterns

The Inter-Tropical Convergence Zone (ITCZ) is a zonal belt of intense convection, responsible for the genesis of over 80% of all tropical cyclones. This region of intense diabatic heating and shear results in a maximum of Ertel's potential vorticity (PV) meeting Rayleigh's necessary condition for barotropic instability. A fundamental issue is understanding the necessary precursor events leading to the breakdown of the ITCZ and subsequent formation of tropical cyclones. Our research examines the non-linear PV dynamics of the breakdown of both finite-length and infinite-length vorticity strips of varying widths and shapes, simulating the ITCZ found near the tropical eastern Pacific region. We have also introduced regularly and irregularly-spaced mass sinks embedded in the strips to simulate pockets of enhanced diabatic heating. To study the evolution, we have developed a shallow-water, normal-mode spectral model in Cartesian coordinates on the f-plane. Since the absolute vorticity divided by the fluid depth is materially conserved in the shallow water framework, we can draw an analogy to the evolution of Ertel's PV in a stratified fluid. While the analogy is not exact, it does offer insight into to the fundamental dynamics of PV rearrangement. Comparisons with linear stability theory and observed cases are made to determine the extent to which linear theory captures the non-linear dynamics.