Elliptical Diagnostics of Polar Vortices

Tracer transport in the wintertime stratosphere is dominated by a strong, cyclonic, polar vortex. In this study a new set of diagnostics are used to quantify the structure and evolution of these polar vortices. From the spatial moments of isopleths of quasi-conservative tracers (e.g., potential vorticity or a long-lived tracer such as N2O) the area, center, aspect ratio and orientation of the polar vortex are defined. (See Waugh (1997) for details.) These "elliptical diagnostics" (EDs) enable the movement and elongation of the vortex to be quantified.

The figures below show an example of the application of the EDs to data from a general circulation model (GCM). The upper plots show maps of the the vortex edge as defined by the N2O = 60 ppb contour, together with the elliptical fit to the contour. The lower plots show the time series of two of the EDs: (i) latitude of center, (ii) aspect ratio, and (iii) mean-square departure from ellipse. These time series summarize, and quantify, the features shown in the N2O maps: (i) the vortex is centered off the pole over northern Europe (mean latitude of center is 80N), (ii) the vortex is roughly circular (aspect ratio approximately 1) at beginning and end of period but is elongated during the middle of the period (aspect ratio = 2.6), and (iii) the elliptical fit is poorest during the middle of the period.

[Click on highlighted images for larger view.]

The EDs have been applied to 18 years of satellite data, and climatologies of the observed stratospheric polar vortices formed. These climatologies show many differences in the structure, seasonal variability, and interannual of the Antarctic and Arctic vortices. For example, as shown in the plots below, the Arctic vortex is smaller, displaced further off the pole and more elongated than the Antarctic vortex. See Waugh and Randel (1998) for details.

Comparison of climatological structure of Arctic (solid) and Antarctic (dashed) vortices at 850K (middle stratosphere).

Comparison of vertical structure of Arctic and Antarctic vortices.

The Eds have also been used to assess the predictive skill of numerical weather prediction systems (Waugh et al. 1998), and to examine structure of vortices in idealized numerical calculations (Waugh and Dritschel 1998).

Darryn Waugh