Multiple quasi equilibria of the ITCZ and the origin of monsoon onset. Part II: Rotational ITCZ attractors

Chao's numerical and theoretical work on multiple quasi equilibria of the i ntertropical convergence zone (ITCZ) and the origin of monsoon onset is ext ended to solve two additional puzzles. One is the highly nonlinear dependen ce on latitude of the "force'' acting on the ITCZ due to the earth's rotati on, which makes the multiple quasi equilibria of the ITCZ and monsoon onset possible. The other is the dramatic difference in such dependence when dif ferent cumulus parameterization schemes are used in a model. Such a differe nce can lead to a switch between a single ITCZ at the equator and a double ITCZ, when a different cumulus parameterization scheme is used. Sometimes o ne of the double ITCZ can diminish and only the other remains strong, but s till this can mean different latitudinal locations for the single ITCZ. A single idea based on two off-equator attractors for the ITCZ symmetric wi th respect to the equator, due to the earth's rotation, and the dependence of the strength and size of these attractors on the cumulus parameterizatio n scheme solves both puzzles. The origin of these rotational attractors, ex plained in Part I, is further discussed. Each attractor exerts on the ITCZ a force of simple shape in latitude; but the sum gives a shape highly varyi ng in latitude. Also the strength and the domain of influence of each attra ctor vary when change is made in the cumulus parameterization. This gives r ise to the high sensitivity of the force shape to cumulus parameterization. Numerical results, of experiments using Goddard's GEOS GCM, supporting thi s idea are presented. It is also found that the model results are sensitive to changes outside of the cumulus parameterization. The significance of th is study to El Nino forecast and to tropical forecast in general is discuss ed.