The southern westerlies

The strong zonal airflow in the belt of the southern westerlies, which is apparent only on mean monthly maps, is associated with a major frontal zone characterized

Figure 10.33 Main climatological features of Australasia and the southwest Pacific. Areas with >100 mm (January) and >50 mm (July) mean monthly precipitation for Australia are also shown.

Source: After Steiner, from Salinger et al. (1995), copyright © John Wiley & Sons Ltd. Reproduced with permission.

Figure 10.33 Main climatological features of Australasia and the southwest Pacific. Areas with >100 mm (January) and >50 mm (July) mean monthly precipitation for Australia are also shown.

Source: After Steiner, from Salinger et al. (1995), copyright © John Wiley & Sons Ltd. Reproduced with permission.

by the continual passage of depressions and ridges of higher pressure. Throughout the Southern Ocean, this belt extends southward from about 30°S in July and 40°S in January (see Figures 9.18 and 10.35B) to the Antarctic trough which fluctuates between 60° and 72°S. The Antarctic trough is a region of cyclonic stagnation and decay that tends to be located furthest south at the equinoxes. Around New Zealand, the westerly airflow at an elevation of 3 to 15 km in the belt 20 to 50°S persists throughout the year. It becomes a jet stream at 150 mb (13.5 km), over 25 to 30°S, with a velocity of 60 ms-1 in May to August, decreasing to 26 ms-1 in February. In the Pacific, the strength of the westerlies depends on the meridional pressure difference between 40 and 60°S, being on average greatest all the year south of western Australia and west of southern Chile.

Many depressions form as waves on the inter-anticyclonic fronts, which move southeastward into the belt of the westerlies. Others form in the westerlies at preferred locations such as south of Cape Horn, and at around 45°S in the Indian Ocean in summer and in the South Atlantic off the South American coast and around 50°S in the Indian Ocean in winter. The polar front (see Figure 9.20) is associated most closely with the sea-surface temperature gradient across the Antarctic

Figure 10.34 The synoptic situation at 00:00 hours on 1 September 1982, resulting in heavy rainfall in the Southern Alps of New Zealand.

Sources: After Hessell; from Wratt et al. (1996). From Bulletin of the American Meteorological Society, by permission of the American Meteorological Society.

Figure 10.35 (A) Surface currents in the Arctic, together with average autumn minimum and spring maximum sea ice extent. (B) Southern Ocean surface circulation, convergence zones and seasonal ice limits in March and September.

Sources: (A) Maytham (1993), Barry (1983). (B) After Barry (1986), copyright © Plenum Publishing Corporation, New York. Published by permission.

Heavy Rainfall West Southern Alps

Figure 10.34 The synoptic situation at 00:00 hours on 1 September 1982, resulting in heavy rainfall in the Southern Alps of New Zealand.

Sources: After Hessell; from Wratt et al. (1996). From Bulletin of the American Meteorological Society, by permission of the American Meteorological Society.

convergence, whereas the sea ice boundaries further south are surrounded by equally cold surface water (Figure 10.35B).

In the South Atlantic, depressions travel at about 1300 km/day near the northern edge of the belt, slowing to 450 to 850 km/day within 5 to 10° latitude of the Antarctic trough. In the Indian Ocean, eastward velocities range from 1000 to 1300 km/day in the belt 40 to 60°S, reaching a maximum in a core at 45 to 50°S. Pacific depressions tend to be similarly located and generally form, travel and decay within a period of about a week. As in the northern hemisphere, high zonal index results from a strong meridional pressure gradient and is associated with wave disturbances propagated eastward at high speed with irregular and often violent winds and zonally oriented fronts. Low zonal index results in high-pressure ridges extending further south and low-pressure centres located further north. However, breakup of the flow, leading to blocking, is less common and less persistent in the southern than in the northern hemisphere.

The southern westerlies are linked to the belt of travelling anticyclones and troughs by cold fronts, which connect the inter-anticyclonic troughs of the latter with the wave depressions of the former. Although storm tracks of the westerlies are usually well to the south of Australia (Figure 10.33), fronts may extend north into the continent, particularly from May, when the first rains occur in the southwest. On average, in midwinter (July), three depression centres skirt the southwest coast. When a deep depression moves to the south of New Zealand, the passage of the cold front causes that country to be covered first by a warm, moist westerly or northerly airflow and then by cooler southerly air. A series of such depressions may follow at intervals of twelve to thirty-six hours, each cold front being followed by progressively colder air. Further east over the South Pacific, the northern fringe of the southern westerlies is influenced by northwesterly winds, changing to west or southwest as depressions move to the south. This weather pattern is interrupted by periods of easterly winds if depression systems track along lower latitudes than usual.

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