Explicit Simulations of the Intertropical Convergence Zone

Abstract

The intertropical convergence zone (ITCZ) is one of the most important components of the global circulation. In order to understand the dynamical processes that regulate its formation, latitudinal preference, and structure, explicit two-dimensional numerical modeling of convection on an equatorial beta plane was conducted with a nonhydrostatic cloud-system-resolving model. The model was forced by energy fluxes associated with constant sea surface temperature (SST) and by horizontally homogeneous radiative cooling. Two distinct patterns were identified for the spatial distribution of convective activity in the Tropics. The first was characteristic of enhanced off-equator convection, namely, a double ITCZ-like morphology (one more salient than the other) straddling the equator during the early period of the integration. The second featured enhanced equatorial convection, namely, a single ITCZ-like morphology on the equator during the later quasi-equilibrium period. Diagnostic analysis and two additional experiments, one excluding surface friction and the other having time- and space-independent surface fluxes, revealed that the wind-induced surface flux variability played an essential role in the development and maintenance of the equatorial maximum convection. Surface friction was largely responsible for the early asymmetric convective distribution with respect to the equator in the control simulation and acted to flatten the convective peaks. One important discrepancy from observations concerned the too-weak trade wind convergence around enhanced convective regions. This unrealistic feature suggested that, as well as the meridional dynamics, latitudinal SST gradients, large-scale forcing, and other physical processes regulate the observed ITCZs.