Earth’s intertropical convergence zone (ITCZ) is a narrow band of low-level mass convergence that migrates north and south across the equator following the seasonal cycle of insolation. The ITCZ dominates the tropical water cycle and there are suggestions that it also modulates global climate via its influence on Earth’s radiation budget. Much recent work has focused on the mechanisms determining the mean latitude (or position) of the ITCZ but comparatively little is known about the physics of ITCZ width.

In this talk I will discuss the processes controlling ITCZ width from two perspectives. An energetic perspective shows that ITCZ width depends on four quantities: the net energy input to the tropical atmosphere, the gross moist stability, the advection of moist static energy by the Hadley circulation, and the transport of moist static energy by transient eddies. A complementary dynamical perspective based on the boundary-layer momentum budget demonstrates how Ekman balance places a strong constraint on ITCZ width but horizontal and vertical momentum advection are also important. Interestingly, an extension to Lindzen-Nigam theory suggests that ITCZ width scales with the latitude where the Laplacian of sea-surface temperature is zero. Time permitting I will conclude with a discussion of how changes in ITCZ width (and other atmospheric circulation changes) affect cloud feedbacks and climate sensitivity.