Effective diffusivity as a diagnostic of atmospheric transport: 2. Troposphere and lower stratosphere

The effective diffusivity diagnostic is used to analyze the isentropic transport and mixing properties of observed winds in the upper troposphere and the lower stratosphere (300–450 K), following the approach described in part 1 [Haynes and Shuckburgh, this issue]. Local minima in effective diffusivity on isentropic surfaces in the range 330–400 K indicate transport barriers in each hemisphere associated with the extratropical tropopause. The strongest part of these “tropopause barriers” are coincident with the core of the subtropical jet at about 350 K. They are shown to have a seasonal evolution in which they are strongest in winter and considerably weakened by the monsoon circulations in summer. The barrier in the Southern Hemisphere is seen to be generally stronger than that in Northern Hemisphere during the same season. The minimum value of effective diffusivity on each isentropic surface is proposed as a new definition of the tropopause. This effective-diffusivity definition corresponds most closely to potential vorticity (PV) values of ±2 PVU at 330 K, ±2.5 PVU at 350 K, and ±4.5 PVU at 370 K (with larger values being more appropriate during the summer monsoon period), rather than to the conventional tropopause definition of a single PV value at all levels. It is also demonstrated that the lower limit of the barrier at the stratospheric polar-vortex edge, i.e., the “sub-vortex” transition, varies in altitude throughout the winter. In the Antarctic the transition generally occurs at 380 K and is sometimes as low as 350 K. In the Arctic the transition is higher, rarely occurring below 400 K and frequently occurring above 450 K.