Windswept_ The Story of Wind and Weather - Marq de Villiers [43]
As its name implies, the AO circles the Arctic and extends high into the stratosphere. Its timescale is shorter than El Nino's—only a few months, or even weeks—and it cycles through a negative or cold phase, which brings high pressure to Arctic regions, along with lower-than-normal pressure over midlatitudes, and a positive or warm phase, whose effects are the opposite.
Perversely, warm AOs result in extracold weather in America and Europe. By contrast, when the AO high-level circulation is cool, it inhibits cold surface air dipping southward, warming up cities from Moscow to Vancouver, Calgary to Boston, London to Warsaw.21 Similar oscillations exist in the southern hemisphere. Some studies suggest that the Antarctic Oscillation, the southern equivalent to the AO, was corrupted by the recent hole in the ozone layer, which resulted in extraordinarily cold winds, which in turn may explain why the southern polar regions were warming more slowly than northern ones before the ozone hole repaired itself. (Warming in Antarctica is once again accelerating.)
Two other cycles with implications for wind and weather are the Pacific Decadal Oscillation and the Quasi-Biennial Oscillation.
Climatologists rather like the PDO because it is a way of showing the general public that so-called normal climatic conditions can change, sometimes radically, over a period less than a human lifespan. The cycle often causes wild swings in Pacific marine species like salmon, and in local weather patterns.
It's possible the PDO is merely an El Niiio writ large, with much longer cycles; there have been only two full PDO cycles in the last one hundred years—"cool" ones from 1890 to 1924, and again from 1947 to 1976, and "warm" ones from 1925 to 1946, and again from 1977 to the mid-1990s. It's also possible that the oscillation has two cycles, and not one—one from fifteen to twenty-five years, the other from five to seventy years. Its causes are unclear; the closest scientists can come is to suggest that it arises from some air-ocean interactions, which at least suggests some lines of inquiry.
Perhaps the most curious cycle of interest to wind scientists is the Quasi-Biennial Oscillation, in which the lower stratospheric winds of the tropics abruptly change direction, about every twenty-eight months. It is an enigmatic phenomenon, even by global oscillation standards. Why should winds that are easterly one month abruptly become westerly a few weeks later? And why should the easterly phase be about twice as strong as the westerly phase?
The QBO was only discovered in the 1950s, because it is only detectable at higher (stratospheric) altitudes. In the 1970s it was found that the periodic abrupt switches were caused by atmospheric waves starting in the tropical troposphere that travel upward into the stratosphere, where they are dissipated by cooling. The nature of these waves remains mysterious. The current culprit of choice is gravity waves, but what causes those and in this periodicity is opaque.
But they are important for millions of people, because hurricane activity is more common when these stratospheric winds are westerly, a pattern that is true also for cyclones in the Pacific. In its easterly phases, the PDO tends to knock hurricanes off their balance before they can really get going, while westerlies seem to act as catalysts. Why this should be so is another of the many unknowns.22
Reinforcing these oscillations of air, and probably partly caused by them, is a similar periodicity in global ocean movement that scientists call the thermohaline circulation, the steady movement of the world's oceans—with the Gulf Stream, the world's most