Diagnosing the permeability of dynamical barriers in the stratosphere from satellite observations of long-lived tracers
Elisa Palazzi(1), Federico Fierli(1), Slimane Bekki(2) and Francesco Cairo(3)
(1) ISAC-CNR, via Gobetti 101, 40129, Bologna, Italy
(2) Service d'aeronomie-CNRS, 4, Place Jussieu, 75252, Paris Cedex 05, France
(3) ISAC-CNR, via Fosso del Cavaliere 100, 00133, Roma, Italy
This study addresses the issue of quantifying the permeability of the dynamical edge regions in the lower stratosphere making use of the probability distribution functions (PDFs) of long-lived tracers. Previous studies show that a precise correspondence exists between the field of a passive tracer and the multi-modal structure of its PDF, which can be exploited to identify in an objective way the position and variability of the barriers to isentropic mixing (the subtropical and polar vortex barriers).
With respect to the previous works exploiting the properties of long-lived tracers PDFs to quantify the dynamical barriers, we move our attention from the determination of the barriers position to the quantification of their strength. To do so, we focus on the minima between modes of a tracer PDF since they are a good indicator of the barriers permeability. The minima of a tracer PDF and the maxima of its horizontal gradient are anti-correlated and the mathematical relationship between these two quantities can be established. The analysis of the tracers PDF to diagnose the stratospheric barriers has a twofold advantage: (1) it is a more self-consistent approach since it does not requires, in principle, additional information coming from atmospheric analyses, (2) it deals with statistical properties that are less dependent from absolute tracers concentrations, allowing for an easier instrumental merging than using other approaches.
The proposed diagnostics is applied to the available satellite observations of long-lived tracers (N2O, CH4) coming from different instruments (HALOE/UARS, MLS/UARS, MLS2/AURA, MIPAS/ENVISAT, ACE/FTS) in order to obtain as long as possible datasets. These data are projected in a potential temperature-equivalent latitude coordinate system that gives the advantage of leaving a more compact picture of the tracers distribution with respect to meridional mixing.
The observed inter-annual variations of the tracers PDFs and meridional gradients are used to study the inter-annual variability in the position and intensity of dynamical barriers, which will be correlated to the quasi biennial oscillation (QBO) in the zonal mean winds.