Vortex statistics

Climatologies of the stratospheric polar vortex in both hemispheres have been prepared based on areal extent (Baldwin and Holton, 1988) and on elliptical diagnostics (Waugh, 1997; Waugh and Randel, 1999). Fitting an ellipse to a specified contour allows one to summarize various measures of the vortex. These include the equivalent latitude of the vortex edge (defining its area), the offset of the vortex from the pole, the longitude

Figure 4.3 Schematic of the configuration of the winter stratospheric polar vortex as diagnosed from a hypothetical positive region of PV (i.e., a potential vorticity anomaly). The positive columnar region of PV is at the center of the vortex. Note the strong jet surrounding the region of PV, which weakens as one goes into the quasi-stationary core or outside the vortex altogether. Transparent, upward arrows conceptualize relative gravity wave activity (from Gerrard et al., 2002, by permission of AGU).

Figure 4.3 Schematic of the configuration of the winter stratospheric polar vortex as diagnosed from a hypothetical positive region of PV (i.e., a potential vorticity anomaly). The positive columnar region of PV is at the center of the vortex. Note the strong jet surrounding the region of PV, which weakens as one goes into the quasi-stationary core or outside the vortex altogether. Transparent, upward arrows conceptualize relative gravity wave activity (from Gerrard et al., 2002, by permission of AGU).

of its center and its elongation (aspect ratio or ellipticity). Waugh and Randel (1999) define these parameters on the basis of selected PV contours for the 440 K (lower stratosphere) to 1300 K (upper stratosphere) potential temperature surfaces, October 1978 through April 1998. The mean annual cycles of vortex characteristics are given in Figure 4.4.

The equivalent latitude of the vortex edge is lower (i.e., the vortex covers a larger area) in winter as compared to autumn and spring. Area increases with height - for January, the equivalent latitude is around 70° N in the lower stratosphere, compared to about 60° N in the upper stratosphere. Depending on height and season, the vortex is centered 6-18 degrees off the Pole (offset is clearly evident in the mean January field in Figure 4.2). As seen on the longitude of the center, the vortex tilts westward with height between 500 K and 1300 K by about 60 degrees. The vortex also tends to be elongated in shape (again see Figure 4.2). In November-December, there may be rapid eastward and then westward shifts of the vortex center in the lower stratosphere between

Figure 4.4 Mean annual cycle of the elliptical diagnostics of the Arctic stratospheric vortex from 1300 K (-41 km) to 500 K (-20 km): (a) equivalent latitude of the vortex edge; (b) latitudinal displacement of the center from the pole; (c) longitude of the center; (d) ellipticity (aspect ratio); 1 = circular. The contour intervals are: (a) 5 deg. Latitude; (b) 2 deg. Latitude; (c) 10 deg. Longitude; (d) 0.1 (adapted from Waugh and Randel, 1999, by permission of AMS).

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60-120° E and 60-90° W, apparently related to changes in the planetary waves. Similar but lesser shifts occur in late January to early February.

The Arctic vortex is smaller and breaks down earlier in summer than its Antarctic counterpart. It is also highly variable throughout its lifecycle, especially in late winter. This variability is partly caused by extreme events distorting the vortex. Waugh and Randel (1999) show that a distorted vortex is present in about half of the winters from 1990-1 to 1997-8. In all four cases, the vortex center shifted south of 65° N. There were also other periods when the vortex was symmetrical and quiescent. Interannual variability in the stratospheric polar vortex has been linked to the phase of the equatorial quasi-biennial oscillation (QBO), as first pointed out by Labitzke (1982). The QBO refers to an oscillation in the zonal winds of the equatorial stratosphere having a period that fluctuates between about 24 and 30 months. Stratospheric equatorial easterlies associated with the QBO favor a weaker, warmer vortex than equatorial westerlies, but the relationship breaks down in some winters.

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