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3. Mean Fields

3.a Stationary Waves

In this section the basic structure of stationary waves is discussed. Figure 1a shows ozone stationary component, calculated as the difference between July 1994 mean field and the corresponding zonal mean (denoted by "*"). South of 50ºS, ozone field depicts the typical wave number 1 that characterises the winter season, as described by Vigliarolo et al. (2000). Nevertheless, some differences could be appreciated. The high latitude-strong negative center is located slightly to the west and north regarding its climatological position and with values that are about two times greater. On the other hand, the positive center near 50ºS, 115ºW is much weaker (about 0.5 times) compared to climatology. Vigliarolo et al. (2000) have suggested that winter ozone stationary pattern over middle and high latitudes of Southern Hemisphere (SH) depends critically on the three dimensional structure of atmospheric stationary waves, which in turn produce ozone anomalies via both horizontal and vertical transports (although they also caution about the main role of non-conservative processes in contributing to ozone pattern).

Figure 1: Zonal asymmetries of July 1994 mean fields for a) ozone and b) 100-hPa geopotential height. The contour interval is (a) 5 DU and (b) 50 m and the zero contour has been omitted.

The geopotential-height stationary wave at 100-hPa (fig.1b) also shows a wave number 1 structure over middle to high latitudes. But, although the location of the geopotential centers roughly coincides with the winter mean position (see Vigliarolo et al.2000, its fig.2b), the corresponding extremes for this particular July are more intense (about 1.8 times) and displaced towards southern SA. In agreement, a westward displacement of the maximum of the subpolar jet from the Indian Ocean to 45ºS, 15ºW and a jet weakening along the high latitudes of Pacific Ocean are observed.

Figure 2: (a) 300-hPa mean zonal wind (contour interval 5ms^-1; only values above 30 ms^-1 are shown) and (b) mean  at 850-hPa (contour interval 0.75*10⁴ ºK² s^-1).

At middle latitudes of both southeastern Pacific and Indian oceans, ozone stationary pattern is related to geopotential height via the "tropopause effect" (Vigliarolo et al. 2000 and references therein). Moreover, a nearly continuous band of negative ozone anomalies centered along 40ºS extends from the Atlantic to the Central Indian Ocean and is related to geopotential positive anomalies by the same location; in the Pacific sector, relatively high positive ozone values are found in connection with a negative geopotential height center near New Zealand (figs.1a,b).

3.b Transients

The standard deviation of submonthly perturbations (constructed as the daily departures from July 1994 time mean) was chosen to represent transient wave activity. Ozone perturbation standard deviation (fig.3a) shows large values along the 40º-55ºS-latitude band, that roughly coincides with maximum 300-hPa geopotential height perturbation standard deviation maximums (fig.3b), thus confirming the main role of transients on driving ozone variability (Salby and Callahan 1993). In addition, the high-variability ozone centers are located poleward and maximize downstream with respect to the geopotential-height ones (Vigliarolo et al. 2000). Over southern SA, ozone perturbation standard deviation attains a maximum well above the mean winter standard deviation values for the region, that is in close association with a maximum of the geopotential-height standard deviation located over the same area (fig.3). In agreement, a minimum of ozone over this region of maximum submonthly wave activity is observed (fig.1a) that persisted throughout the month in association with a quasi-stationary, equivalent barotropic ridge centered at 55ºS, 90ºW (Figs.not shown).

Figure 3: Standard deviation of daily departures from July 1994 time mean of: a) ozone (contour interval 5 DU) and b) 300-hPa geopotential height (contour interval 30 m).

Berbery and Vera (1996) have shown that during austral winter, low level baroclinicity attains its maximum over the subpolar jet latitudes with the highest values located between 30º and 60ºE.  The term (where the overbar denotes time mean and (') is the daily departure from time means, T is temperature and v is the meridional wind) is proportional to the mean baroclinic conversion and is shown for the 20º-65º latitude band at 850 hPa (fig.2b). Note that values south of this boundary are not plotted as it is difficult to assess the quality of the data around and over Antarctica, where both a combination of high terrain and steep slopes is found (Berbery and Vera 1996). In general this field show maximums located further west than the climatology. In particular, the displacement submonthly scale enhanced activity (fig. 3b) from its climatological position over the Indian Ocean to the central Atlantic Ocean seems to be associated with a conspicuous center of high baroclinicity near 55ºS, 25ºW that extends towards the Antarctica Peninsula (fig.2b). It is worth to point out that baroclinic conversion is not increased over southern SA, implying that other dynamical processes not considered here may account for the transient activity intensification over that region.

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