Tropical cyclogenesis at and near the Equator

The formation of tropical cyclones within a few degrees latitude of the Equator is investigated using European Centre for Medium-Range Weather Forecasts (ECMWF) analyses of some prominent cyclogenesis events there. The possibility of formation at the Equator is demonstrated also using idealized model simulations, starting from a prescribed, weak (maximum wind speed 5m/s) initial counter-clockwise vortex in an otherwise quiescent, non-rotating environment. In the real events investigated, vortex formation occurred within a broadscale counter-clockwise flow that encompasses a region of predominantly positive absolute vertical vorticity typically extending more than 5 degrees south of the Equator. Patches of enhanced vertical vorticity form within this region as a result of vorticity stretching by deep convection. These vorticity patches are organized by the convection, the collective effects of which produce an overturning circulation that fluxes vorticity at low levels towards some centre within the convective region. By Stokes' theorem, the tangential circulation about circles of fixed radius around this centre increase and the vortex spins up. This process of spin-up is the same as that which occurs away from the Equator.