Factors controlling the position of the intertropical convergence zone on an aquaplanet

Aqua planet experiments performed with fixed sea surface temperatures (SST) using the ECHAM6 GCM are studied to understand properties that influence the position of the inter tropical convergence zone (ITCZ). A single ITCZ develops when using the Nordeng scheme and a double ITCZ when using the Tiedtke scheme. The position of the ITCZ is found to depend on a feedback loop process wherein convective heating drives pressure gradients and winds, which determine the rate of surface evaporation, which influences the boundary layer moist static energy, which finally couples back to the pattern of convective heating. This feedback loop process is sensitive to the SST profile and the choice of the convection scheme. However SSTs are only important in so far as they control the boundary layer moist static energy. The feed-back loop can be broken by specifying the wind used to calculate surface fluxes, in so doing it is possible to control the magnitude of boundary layer moist static energy and hence the position of the ITCZ. The cloud top height and therefore the convective heating decisively depends on the entrainment rates and the free tropospheric humidity. In the double ITCZ case the humidity in the lower free troposphere is higher on the equatorward side of the double ITCZ compared to the poleward side. Therefore an increase of the entrainment rates favor convection on the equatorward side. This explains why the Nordeng scheme produces a single ITCZ, although the Tiedtke scheme produces a double ITCZ