Trough - Southern Hemisphere

F'M'A'M'J1J'A'S'O'N'D' Month

Fig. 2.5. Seasonal cycle of temperature at 50°S and 65°S (upper curves), and meridional temperature gradient 50°-65°S (lower curve), at the 500 hPa level. Based on station data over a 5-8-year period (after van Loon, 1967).

Fig. 2.5. Seasonal cycle of temperature at 50°S and 65°S (upper curves), and meridional temperature gradient 50°-65°S (lower curve), at the 500 hPa level. Based on station data over a 5-8-year period (after van Loon, 1967).

advance; see for example Fig. 2.11). Air temperatures at 65°S and poleward therefore decrease only slowly throughout the winter season, and there is a weaker temperature contrast between middle and high latitudes in winter than in the subsequent spring season.

The steeper temperature gradient in mid-troposphere between 50°S and Antarctica around equinoctial months (March and September) produces an increase in cyclone activity during these two periods. Van Loon (1967) argued that this modulation of cyclonicity over the high latitude oceans shows up in the semi-annual oscillation in the position of the Circumpolar Trough, which is closer to the Pole and somewhat stronger in the autumn and spring seasons. Travelling cyclonic disturbances are indeed a major mechanism for transporting heat into higher latitudes to balance the nett radiation deficit here.

The increased cyclone activity over the Southern Ocean in autumn, with its associated increase in advection of warm air southwards, is also held responsible for a peculiar phenomenon that shows up in Antarctic temperature records. Antarctic temperatures do not decrease regularly to reach a minimum just before the return of the sun. Instead, a slight rise in temperature often occurs about June before further cooling to the minimum in August (Fig. 2.6). Van Loon (1967) found this "coreless" winter pattern could occur in any winter month anywhere

Fig. 2.6. Seasonal cycle of maximum (dashed line), minimum (dotted line), and mean (solid line) temperatures at Scott Base, Antarctica. Based on daily data over the 30-year period 1957-86.

over Antarctica, although the event was strongest and most frequent in the Ross Sea sector.

Cyclones

The dominant feature on mid-latitude weather maps is the series of migratory high and low pressure centres, the anticyclones and cyclones. In the 1920s, a group of Norwegian meteorologists developed a simple model of the structure of a cyclone that is still often used today. This classical structure, redrawn for the Southern Hemisphere, is shown in Fig. 2.7. The model delineates the boundaries between air masses of different characteristics by cold and warm "fronts". At the warm front, there is an upsliding of warm moist air over a wedge of colder air, producing a cloud sheet ranging from high-level cirrus down through gradually thickening middle-level cloud to precipitating nimbostratus near where the front reaches the earth's surface. The cold front is a much steeper wedge of cold air undercutting the warmer moist air to produce a band of deep convective cloud.

Winds circulate around the cyclone centre in a clockwise sense in the Southern Hemisphere, and the associated frontal cloud bands often take on a vortex-like

Trough Southern Hemisphere
Fig. 2.7. The Norwegian cyclone model adapted to a Southern Hemisphere perspective, showing schematic plan view (above) and indication of associated cloud types and weather (below) (from Hill, 1980).

appearance when viewed from above. Cyclones generally have a well-defined life cycle, going from birth (cyclogenesis) through maturity to decay, during which the three-dimensional atmospheric temperature and pressure fields also undergo systematic changes. The difficulty in the data-sparse Southern Hemisphere is that these changes often cannot be monitored by the conventional meteorological observations of pressure, wind, temperature and moisture at various levels in the vertical (such as might be measured from ships or radiosonde balloon flights). This is where satellites have come to be an invaluable tool, along with models of cyclone structure. Streten and Troup (1973) developed a classification scheme for Southern Hemisphere cyclones using the appearance of cloud vortices in satellite photographs. The structure of the surface pressure field and upper-level temperature and pressure fields, typical of each vortex type, was determined from coincident conventional observations where these were available. These results could then be applied to regions where ground-truth measurements were absent. Such an approach has proved extremely useful over the Southern Ocean.

The original cyclone climatology as developed by Streten and Troup (1973) was based on data from the ESSA satellite series over the 3-year period 1966-69, but did not include "winter" (defined in this section as June to September inclusive) because of poor illumination in high latitudes at that time of year. Carleton (1979, 1981a) complemented the earlier work by developing a descriptive climatology of cyclone activity for the winter season from five years (1973-77) of satellite infrared imagery. This section briefly reviews the results of these studies. The reader could also consult Trenberth (1981) for further discussion.

The most common cloud vortex types of Streten and Troup's (1973) classification are shown in Fig. 2.8 and Table 2.1. The main sequence is one of development (vortex types W, A or B) through maturity (C) and ultimately decay (D). Cyclogenesis can occur either as a development on a pre-existing cloud band (all type W events are of this type), or in the absence of such a band. Streten and Troup estimated that some 55-60% of "comma cloud" development (types A and B) occurred in the absence of a major cloud band, indicating that such evolutions are more common in the Southern Hemisphere than in the Northern Hemisphere. The distinction between types A and B is mainly a matter of when the satellite passed overhead in the life cycle of the cyclone. Those cyclones that reach and reinforce the Circumpolar Trough (mainly types C and D) are likely to bring severe weather to coastal Antarctica, and may even penetrate far inland.

The frequency distribution of cloud vortex types as a function of latitude for five winter (June-September) seasons is shown in Fig. 2.9. There is a southward trend in the latitude of maximum frequency as we progress from type W through type D. This progression is a little less obvious in summer (not shown), where the wave development (type W) vortices exhibit an approximately constant frequency north of 55°S (Streten and Troup, 1973). The summer peak frequency of type A comma development is also concentrated a little further north at 44°-55°S than evident in the winter figures. The high frequency of lower-latitude cyclone types F and G (also known as "cut-off" lows because they are displaced north wards of the basic westerly current, and are therefore slow-moving) observed by Carleton (1979) seems to be a feature specific to the winter season, when these developments occur preferentially in Australian-New Zealand longitudes.

Fig. 2.8. Classification scheme for frontal cloud vortices appearing on Southern Hemisphere satellite imagery (see also Table 2.1). The cross indicates the vortex centre, and r its assumed "radius" (from Streten and Troup, 1973).
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