Hemispheric (a-)symmetries in lower stratospheric dynamics and tracers

Hemispheric asymmetries in land-ocean distributions and the seasonal evolution of insolation would suggest strong, seasonally evolving asymmetries about the equator. However, water vapour in the - broadly speaking - lower stratosphere varies remarkably symmetric about the equator which, as will be shown with model calculations, is even more true when dehydration in the Antarctic polar vortex is suppressed. It is understood that hemispheric asymmetries in the forcing of the stratospheric residual circulation produce an annual cycle in the tropical lower stratosphere and tropopause layer, with minimum temperatures during boreal winter. In this period, the water vapour minima is observed to propagate out of the inner tropics into the extratropics in both hemispheres. During boreal summer, the Indian/Southeast Asian and American monsoons suggest a very strong hemispheric asymmetry in dynamics and transport, and correspondingly these features have received much attention recently. However, the two hemispheres are, with respect to water vapour, also in this season surprisingly symmetric about the equator. We discuss the dynamics and transport of this `lower branch' of the Brewer-Dobson circulation with an emphasis on the subtropics based on (re-)analysis data, observations of trace gases, and model calculations.