Mixing from barotropic instability of the QBO westerly jet

Evidence is presented from a hierarchy of models that indicates the westerly phase of the QBO is susceptible to barotropic instability. This has important implications mixing and transport in the tropical lower stratosphere. Results are presented from a shallow water equation model where the flow is relaxed to a Gaussian shaped jet at the equator, a mechanistic GCM where a equatorial westerly jet is produced by dissipating Kelvin waves, and data from ECMWF analyses. The mixing structure of the instability is investigated by using equivalent length diagnostics. This shows strong mixing regions at around 15 degrees in both hemispheres, with a region of weak mixing at the equator. Barotropic instability may have a role in the momentum distribution of the QBO by spreading out the westerly jet.