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Joseph Biello

University of California

The Modulation and Rectification of the ITCZ (M-ITCZ) equations and matching to the midlatitudes

Abstract:

The intertropical convergence zone is the atmospheric center of heating and the location of the ascending branch of the Hadley circulation. Observations show that the (ITCZ) can undergo rapid changes wherein the zonally aligned structure breaks up into large scale cyclones. These phenomena occur rapidly (over the course of a few days) on the tropical mesoscales and, as such, may be understood using the mesoscale equatorial weak temperature gradient (MEWTG) equations of Majda and Klein.

I will discuss a generalization of the MEWTG equations which provides a closure for the large scale pressure gradient (which was left unresolved in the original theory). We show that the pressure gradient is carried by gravity waves zonally in the ITCZ and acts to make the meridional mean zonal winds uniform at all longitudes. In doing so it carries temperature perturbations which are favorable to exciting convection westward of the original convective source. This provides a mechanism for long range modulation of the ITCZ.

I will also show how the M-ITCZ equations can be derived from the same starting point as the quasi-geostrophic equations (QG) of midlatitude dynamics. This suggests an exciting, but as yet speculative prospect that a careful matching can be made in which tropical diurnal dynamics (M-ITCZ) can interact with midlatitude dynamics (QG). While many authors have focused on the inability of extending QG to the tropics because of the singularity of the Coriolis parameter, this work recongnizes that another, possibly more important mechanism is at work; the strength of the diabatic forcing is large in the tropics resulting in strong vertical motion. It is the transition from strong to weak vertical motion in addition to weak to strong Coriolis parameter that must be simultaneously considered in order to carefully match the two regimes.