Hemispheric differences in tropical lower stratospheric transport

Examination of the annual cycle of ozone and other tracers shows significant differences between the northern and southern tropics, with much larger annual amplitudes in the northern tropics and a 2-3 months shift in the phase of the annual cycle. It has been suggested that these observations imply hemispheric differences in the balance between upwelling and horizontal mixing transport in the tropical lower stratosphere, and that there is a need to revise the paradigm of well-mixed tropics (both for composition and transport properties) to consider latitudinal variations within the tropics. Here we use chemistry-climate models, together with satellite measurements, to further explore differences in lower stratospheric composition, and transport between the northern and southern tropics. The models also produce a contrast between tracer annual cycles in the northern and southern tropics. Analysis of simulated tracers using the Transformed Eulerian Mean (TEM) framework confirms that the balance between processes varies between hemispheres. In the southern tropics vertical advection is the dominant cause of tracer annual cycles, whereas horizontal mixing dominates in the northern tropics. We will also present analysis of the tracer budgets in climate change simulations to examine how tropical lower stratospheric composition and transport may respond to increases in greenhouse gases and other changes in climate.