Tropical Cyclogenesis Due to Breakdown of Intertropical Convergence Zone: An Idealized Numerical Experiment

Abstract

Intertropical convergence zone (ITCZ) occasionally breaks down to simultaneously form several tropical cyclones mainly due to barotropic instability of the zonal flow. This process is called ITCZ breakdown and is thought to be one of the mechanisms for a tropical cyclogenesis in the central and eastern Pacific. In the present paper, an idealized numerical simulation is performed by a non-hydrostatic meso-scale model on an f-plane at 10°N, and a detailed energy budget analysis of the ITCZ breakdown and associated genesis of tropical cyclones is made for the first time. A sea surface temperature (SST) forcing is used to generate an ITCZ-like structure in the atmosphere. This “ITCZ” starts to undulate mainly due to barotropic instability of the lower tropospheric zonal flow, and two vortices are formed in the present experiment. These vortices merge while mutually rotating anti-clockwise, and end up with a single tropical cyclone. The central pressure and maximum wind speed reach under 950 hPa and over 45 m s^-1, respectively. An energy budget analysis shows that the barotropic instability is mainly responsible for the genesis and initial intensification of the two vortices, but cumulus heating rapidly takes over to contribute to the genesis of the tropical cyclone.