The dynamical and transport properties of the extratropical tropopause (with Professor P.H. Haynes)

The tropopause is among the most fundamental structures in the atmosphere. It is the interface between the water vapour-rich and dynamically active troposphere below and the ozone-rich and relatively quiescent stratosphere above. Traditional radiative-dynamical theories suggest that the position of the tropopause is determined by radiative-convective equilibrium in tropical latitudes. But this theory is not really consistent in the extratropics and we require a new theory to explain the position of the tropopause here. Dr I.Held (GFDL, Princeton) has already suggested that the position of the extratropical tropopause is determined by baroclinic eddies, which are the predominant form of large-scale motion in the extratropical troposphere. Professor Haynes and I are using numerical models, new diagnostic tools, and analytical techniques to try to assess Held's suggestion. I have developed a numerical model which solves the three-dimensional quasigeostrophic equations in a channel geometry, which is the simplest realistic model for baroclinic eddy activity in the extratropical atmosphere. I have applied a new transport diagnostic (the effective diffusivity of an advected-diffused tracer field, introduced recently by Nakamura) to this model. There is a direct connection between the transport properties of the flow and its dynamical structure because the key dynamical quantity, potential vorticity (PV), from which all other dynamical fields can be derived, is nearly materially conserved. The action of the baroclinic eddies can be seen as the stirring of PV, with the tropopause representing the upper limit of the region of stirring. In this sense the tropopause is a barrier to transport, separating a well-stirred region below from a less well-stirred region above. The effective diffusivity diagnostic in our simulations reveals a very robust barrier structure as the result of the baroclinic activity in our model. We believe that this barrier structure is analogous to the extratropical tropopause and that a study of its behaviour will lead to an improved understanding of tropopause dynamics. We have examined the dependence of the barrier structure on model parameters, including the radiative relaxation rate, the Ekman friction in the surface layer, the supercriticality of the flow, and the structure of the relaxation state. Our results are presented in a paper `Transport barriers and jets in multi-layer baroclinically unstable flows', a preprint of which will be available shortly.