The breakdown and reformation of the Inter-Tropical Convergence Zone in the moist atmosphere

The effects of moisture on the Inter-Tropical Convergence Zone (ITCZ) over the Eastern Pacific on the synoptic timescale are investigated using an intermediate complexity atmospheric circulation model, the quasi-equilibrium tropical circulation model (QTCM1), on an aqua-planet.

The dry simulation shows results consistent with those of simple dynamic models, except that a slightly stronger heating rate is needed due to different model designs. In the moist simulations, the most important result is the formation of a tail southwest of a vortex during and after the ITCZ breakdown. This tail may extend zonally more than 60 degree longitude and last for more than 2 weeks in an idealized simulation. In the Eastern North Pacific, this phenomenon is often observed in cases that involve easterly waves. In a sense, the formation of the tail suggests a possible mechanism that forms an ITCZ efficiently. Our study shows that the surface convergent flow induced by a disturbance initializes a positive wind-evaporation feedback that forms the tail. In the tail, the most important energy source is surface evaporation, and the latent heat is nicely balanced by an adiabatic cooling of the ascending motion. In other words, the energy is redistributed vertically by vertical energy convergence.

The lifespan of the tail is controlled by the propagation of tropical waves that modify the surface wind pattern, leading to a decrease in surface wind speed and corresponding surface fluxes. It may explain the absence of the tail in some of the events in the real atmosphere.