Polar-front jet streams (PFJ) occur at about 50°S and 8 km altitude, directly above the polar front, whose position changes with season and from day to day. The PFJ is a thermal wind due to the temperature difference across the polar front. It often contains a series of rapidly evolving shortwaves, whereas long waves are more common in the STJ.
Sometimes the PFJ and STJ combine into a single jet, as in Figure 12.10. This fusion is common over large continents in winter. At other times, the PFJ ridges far to the south (e.g. to 65°S), at the same time as the STJ forms a broad trough near 20°S. Such a pattern obstructs frontal disturbances which are travelling from the west, and sends them southward around the ridge. This is known as blocking. It happens when the shifting pattern of upper ridges and troughs locks onto a large high at the surface, located south of its normal subtropical position (Figure 12.7). Such a coincidence may arrest the eastward movement of the waves for days or weeks, causing settled weather at ground level, so weather forecasters are keen to detect incipient blocking. It is most common in winter, and may lead to drought in places that receive their rain from frontal disturbances, e.g. south-western Australia and Tasmania.
Blocking is associated with large-amplitude Rossby waves, swinging over a wide range of latitudes, whereas a low amplitude corresponds to a more even westerly flow. The two cases are distinguished by the difference between the heights (averaged over all longitudes) at which pressures are 500 hPa, at 35° and 55° latitudes, respectively. This difference is called the zonal index. A high index implies a fairly straight, strong jet stream. A low value corresponds to large loops of the Rossby waves, which equalise conditions at the two latitudes, and implies a low zonal wind speed and (in the extreme) blocking. The index tends to wax and wane irregularly, sometimes over periods of 3-8 weeks, and this irregular vacillation is called the index cycle.
In addition to the westerly jets there is sometimes in mid-year an easterly jet stream high above the ITCZ affected by the QBO (Section 12.3). This is due to the elevation of equatorial air by convection to a height of 14 km or so. The air at ground level rotates eastwards with the Earth's surface at a velocity of 463 m/s, but moves about 2 m/s slower at a height of 14 km to maintain a constant 'angular momentum', in the same way that a skater spins more slowly on stretching out to increase his/ her radius. This small westward flow adds to the zonal average easterly to produce an equatorial jet stream of about 4 m/s. But it is patchy, reaching 10 m/s over the Indonesian archipelago but is non-existent over South America and the eastern Pacific.
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