What Determines the ITCZ Position During Solsticial Seasons on Earth and Other Planets?

Studies dating to at least 1972 have suggested that the ITCZ's position roughly coincides with a transition in the role of relative vorticity advection in the boundary layer, from being of leading-order to lower-order importance. A recent study using an idealized, aquaplanet GCM indicates that this coincidence is insensitive to the presence of baroclinic eddies or thermal inertia and holds over a range of planetary rotation rates, with this transitional regime and the ITCZ extending farther poleward the slower the planet is rotating. However, this transition in the boundary layer in those simulations generally coincides with another transition, that of the thermal forcing from super- to sub-critical in terms of the convective quasi-equilibrium (CQE) condition for the existence of an angular momentum conserving meridional overturning circulation. Thus, whether the boundary layer or CQE constraints are independent -- and, if so, which is of greater importance -- is difficult to determine.

We use a dry GCM to address this issue, via simulations analogous to and extending the aforementioned moist cases. The importance of planetary rotation and lack thereof for both baroclinic eddies and thermal inertia emerge in the dry simulations also, implying base causes rooted in simpler, steady-state, solsticial, axisymmetric, dry dynamics. Simulations in which the equilibrium temperature profile being relaxed towards is explicitly sub- or super-critical at all latitudes up to a specified maximum enables a cleaner separation of the roles of the boundary layer v. CQE processes. We also derive a modified CQE criticality condition that accounts for meridionally varying static stability; criticality occurs at weaker temperature gradients in the new, varying stability case compared to the original, fixed stability version. We discuss the resulting implications for ITCZ dynamics on Earth and other planetary bodies.