Effective diffusivity as a diagnostic of atmospheric transport: 1. Stratosphere

Abstract

Abstract. The transport and mixing properties of the isentropic flow in the lower and middle stratosphere are analyzed by using observed winds to advect a tracer on isentropic surfaces in the range 400-850 K. The effective diffusivity diagnostic introduced by Nakamura and collaborators is applied to the tracer field in order to identify barriers to transport and mixing regions, and to follow their seasonal evolution. Large effective diffusivity corresponds to strong mixing, and small effective diffusivity corresponds to weak mixing, i.e., to barriers. The effective diffusivity shows, in the winter stratosphere of each hemisphere, the evolution of the vortex-edge barrier and the midlatitude surf zone, and also the extent of any mixing within the vortex. At low latitudes in the stratosphere there is a region of low effective diffusivity whose latitudinal width varies with height, broadening substantially from 400 K to 550 K. The low values of effective diffusivity in this •'tropical-reservoir" region imply little isentropic transport into or out of it. There is a strong seasonal cycle to the reservoir, which has different forms at 400 K, 450-600 K, and above 650 K, determined by the relative influences of tropospheric synoptic eddies and stratospheric planetary waves. Comparison of effective diffusivity between the Northern Hemisphere winters 1996/1997 and 1997/1998 shows strong differences at low latitudes according to the phase of the quasi-biennial oscillation (QBO). When there are QBO easterlies, there is a broad region of very low effective diffusivity at low latitudes. When there are QBO westerlies, there are very low values of effective diffusivity at low latitudes within the westerlies themselves but larger values at their edges.

Low-N2O Air Masses after the Breakdown of the Arctic Polar Vortex in 1997 Simulated by the CCSR/NIES Nudging CTM.

“…The estimated value of the eddy diffusion coefficient is 1.3-2:9 Â 10 5 m 2 s À1 for the 1.5-month lifetime, and 0.77-1:7 Â 10 5 m 2 s À1 for the 2.5-month lifetime. These values are consistent with the eddy diffusion values that have been used for global zonal-mean 2-D chemical transport models (Newman et al 1986;Plumb and Mahlman 1987;Yang et al 1990;Haynes and Shuckburgh 2000). The values are also close to 2:59 Â 10 5 m 2 s À1 , the effective horizontal diffusivity that gives a diffusion-limited horizontal scale length of 300 km (Tan et al 1998).…”

Changing transport processes in the stratosphere by radiative heating of sulfate aerosols

“…Positive (solid) streamlines describe clockwise circulation, negative (dashed) ones counter-clockwise circulation. The streamlines represent the BDC with the overturning circulation in the winter hemisphere, but they do not show wave induced mixing in the surf-zone and transport barriers (Haynes and Shuckburgh, 2000). The contour lines in Fig.…”

“…given in Haynes and Shuckburgh (2000). Butchart (2014) provides an overview of the stratospheric dynamic processes described above, as well as related references.…”

“…The phase of the QBO influences transport processes in the tropics (Plumb, 1996;Haynes and Shuckburgh, 2000) and extratropics (Punge et al, 2009). The impact of sulfur injections on the QBO should, therefore, also affect transport processes in the stratosphere in addition to the acceleration of the Brewer-Dobson Circulation (BDC) described by Aquila et al (2014).…”

Strategies for measuring canonical tracer relationships in the stratosphere

“…Dynamics and chemistry in the stratosphere are dominated by the existence of well-mixed regions, separated by transport barriers (Haynes and Shuckburgh, 2000). Within each well-mixed region, longlived tracers form "canonical correlations", i. e., unique, well-defined curves that can be used to infer one tracer, based on knowledge of another.…”