Subtropical Jet Streams

Two jet streams can sometimes be distinguished in the westerlies aloft. One is the subtropical jet (STJ), a chain of jet streaks at about 12 km altitude. The core velocity of a STJ averages 70

m/s (250 km/h) at 25°S over Australia in winter, but only 30 m/s at 31°S in summer. The average latitude and the mean speed of STJs over Australia vary in alternate years, with years of stronger jets around 27°S taking turns with years with weaker jets around 29°S. Presumably this is connected with the QBO (Section 12.3).

The STJ is partly due to the acceleration (relative to the ground) of upper westerlies, as the air moves polewards in the upper part of the Hadley cell. (Air from the equator moves eastward at 490 m/s, whilst the Earth's surface at 30 degrees latitude rotates 65 m/s slower.) Also, the troposphere is much colder at latitudes above 30°, and the STJ is partly a thermal wind resulting from the temperature gradient (Note 12.F).

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