Idealized Studies of the Intertropical Convergence Zone and the Shallow Meridional Circulation

David S. Nolan

Abstract

The Intertropical Convergence Zone is a preferred region of convection and precipitation that mostly parallels the equator. Convection is enhanced in the ITCZ due to the convergence of the trade winds and the abundance of heat and moisture available from the tropical oceans. It is taught in textbooks and courses around the world that the converging mass of the tradewinds is allowed to rise up through the atmosphere by the heating of deep convection, where it then diverges, transporting heat and energy to the subtropics.

With the advent of advanced observing systems and the success of various observational campaigns such as EPIC2001, this simplistic view has eroded, and it has been found that the ITCZ can have much more interesting structure and dynamics. In particular, there exists in some locations a shallow meridional circulation (SMC) embedded within the deep meridional circulation. A simple theory is proposed to explain the shallow circulation as a direct response to large-scale gradients in surface temperature and surface pressure. In the absence of deep convection, the warmer temperatures in the ITCZ region cause the meridional pressure gradient to reverse above the boundary layer, thus driving a shallow return flow out of the convergence region. This theory is further validated with idealized, high resolution, full physics simulations of the ITCZ using a tropical channel domain extending from the equator to 30N.

To better understand the ITCZ and SMC, this idealized modeling approach is continued with larger domains that extend across the equator, allowing for meridionally asymmetric SST distributions. As the SST maximum is moved off the equator, the SMC strengthens in the direction back across the equator, and becomes nearly non-existent in the poleward direction. This effect is even greater when a meridional SST profile modeled after the observed SST in the East Pacific is used. In these cases the robust convectively coupled Kelvin wave seen in the prior simulations disappears and the ITCZ is then modulated and disrupted by sporadic TC genesis on the poleward side.