Seasonal migration of ITCZ precipitation across the equator: Why can’t GCMs simulate it?

[1] The secondary meridional circulation induced by convective momentum transport (CMT) within the ascending branch of the Hadley circulation is a missing dynamical mechanism that can cause common failure of general circulation models (GCMs) in simulating seasonal migration of the intertropical convergence zone (ITCZ) precipitation maximum across the equator. This failure is manifested by the model bias that the precipitation peak remains north of the equator during November–March. The CMT-induced secondary circulation, characterized by strong downward motion along the equatorial belt and upward motion south and north of the belt, tends to modify the meridional distribution of precipitation with the strongest impacts during boreal winter and spring. A 20-year GCM simulation with the CMT parameterization successfully reproduces the observed seasonal migration of the ITCZ precipitation across the equator with the peaks near 8 N during boreal summer and near 8 S during boreal winter.