Single and double ITCZ in an aqua-planet model with constant sea surface temperature and solar angle

It has been known for more than a decade that an aqua-planet model with a globally- and temporally-uniform sea surface temperature and solar isolation angle can generate intertropical convergence zones (ITCZ). Employing such a model, previous studies have shown that one of several means can be used to change between a single ITCZ over the equator and a double ITCZ straddling the equator. These means include switching to a different cumulus parametrization scheme, making changes within the cumulus parametrization scheme, and changing other aspects of the model such as horizontal resolution. Here, an interpretation of these findings is offered. In an aqua-planet model with globally and temporally uniform sea surface temperature and solar isolation angle, the latitudinal location of an ITCZ is the latitude where a balance exists between two types of attraction, both resulting from the Earth’s rotation. The first attraction pulls the ITCZ towards the equator and is not sensitive to changes in model design. It is directly related to the Coriolis parameter, which provides stability to the atmosphere. The second ssattraction pulls the ITCZ poleward and is sensitive to changes in model design. It is related to the convective circulation, modified by the Coriolis force. A balance between the two types of attraction is reached either at the equator or more than 10(degree) north and south of the equator, depending on the shape and magnitude of the attractions. A balance at the equator yields a single ITCZ over the equator, whereas a balance north and south of the equator yields a double ITCZ straddling the equator.