A remarkable feature of winds near the equator at about 25 km elevation (where pressures are about 25 hPa) is their reversal between comparatively fast easterlies and slow westerlies and then back again, taking about two years in all. So it is called the Quasi-Biennial Oscillation (QBO). For instance, the easterly winds over Singapore reached a maximum of around 15 m/ s in 1982, 1985, 1987 and 1990, i.e. there is a cycle of variation lasting about twenty-six months. It was first discovered in 1952 over Canton Island at 3°S in the Pacific and later traced as far as 30 degrees from the equator. The QBO is the only clear rhythm in the atmosphere unrelated to diurnal or annual variations, though its regularity is upset occasionally by volcanic eruptions like that of Mt Agung (at 8°S in Bali) in 1963. The change of wind direction occurs simultaneously round the entire tropical belt, beginning at 30 km altitude and extending to 20 km within a year or so. Below that, the oscillation becomes less evident.
The QBO is presumably associated with the tendency for alternate years to be relatively wet (Section 10.4). A similar alternation has been seen in an approximately two-year variation of temperatures in America, noted first in 1885. The explanation involves increased uplift within the atmosphere above Australia and South Africa during the westerly phase of the QBO, and therefore more rain, fed by evaporation from the Indian ocean. Years when the upper equatorial winds are from the east are likely to be relatively dry because of descending air, warming and evaporating of any cloud. But this effect on rainfalls is feeble compared to that of the El Niño (Section 11.2), which appears to be independent of the QBO and much more whimsical.
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