The Origins of ITCZ, Monsoon and Monsoon Onset

Dr. Winston C. Chao

The intertropical convergence zone (ITCZ), monsoons and monsoon onset are among the most prominent features of the atmospheric general circulation. Understanding their origins is fundamental to a full understanding of the atmospheric general circulation, and has challenged meteorologists for a very long time. There have been some important advances in recent years in understanding these origins, however. In this presentation, these advances, including the speaker's own contributions, will be reviewed. These advances are: 1) land-sea thermal contrast is not necessary for monsoons to arise; 2) the ITCZ and the monsoon are not two separate phenomena as often thought; 3) monsoon onset is a sudden poleward jump of the ITCZ during its annual cycle of latitudinal movement; and 4) the SST latitudinal maximum is not a predominant, or even a necessary, factor in the formation of the ITCZ. The monsoon is an ITCZ located more than 10 degrees away from the equator and its associated circulation field. A monsoon, in fact, is an ITCZ after its poleward jump. Therefore, ITCZs and monsoons should be studied together. Finally, there are other important, if not more important, factors than the SST latitudinal maximum in the formation of the ITCZ. These other factors are the interaction between convection and surface fluxes, the interaction between convection and radiation, and the earth's rotation.


Chao, W. C., and B. Chen, 2004: Single and double ITCZ in an aqua-planet model with constant SST and solar angle. Climate Dynamics, 22, 447-459.
Chao, W. C., and B. Chen, 2001: The origin of monsoons. J. Atmos. Sci., 58, 3497-3507.