Dynamical constraints on the ITCZ extent in planetary atmospheres

We study a wide range of atmospheric circulations with an idealized moist general circulation model to evaluate the dynamical mechanisms controlling intertropical convergence zone (ITCZ) migrations. We employ a zonally symmetric aquaplanet slab ocean of fixed depth and force top-of-atmosphere insolation to vary seasonally as well as remain fixed at the pole in “eternal solstice" runs. We explore a range of surface heat capacities and rotation rates, keeping all other parameters Earth-like. For rotation rates ΩE/8 and slower, the seasonal ITCZ reaches the summer pole. Additionally, in contrast to previous thermodynamic arguments, we find that the ITCZ does not follow the maximum MSE, remaining at low latitudes in the eternal solstice case for Earth's rotation rate. Furthermore, we find that significantly decreasing heat capacity does little to extend the ITCZ's summer migration off the equator. These results suggest that the ITCZ may be more controlled by dynamical mechanisms than previously thought; however, we also find that baroclinic instability, often invoked as a limiter on the extent of the summer Hadley cell, appears to play little to no role in limiting the ITCZ's migration. We develop an understanding of the ITCZ's position based on top-of-atmosphere energetics and boundary layer dynamics and argue that friction and pressure gradient forces determine the region of maximum convergence, offering a new perspective on the seasonal weather patterns of terrestrial planets.