The lowermost layer of Earth's atmosphere--the troposphere--is a forecaster's nightmare. Weather patterns shift without warning, especially in winter. But in the 19 October issue of Science, researchers show that changes in the stratosphere, the atmosphere's upper layer, occasionally push Northern Hemisphere weather toward one extreme regime or the other for a couple of months at a time. The finding could give forecasters an edge in long-range predictions.
More than 10 kilometers high, in the stratosphere, high-speed winds swirl around the North Pole. This vortex waxes and wanes, sometimes abruptly. Another changing pattern, the Arctic Oscillation (AO), takes place at the surface, where atmospheric pressure increases and decreases over the higher latitudes. The pressure oscillation causes the prevailing westerly winds to vary in strength and position, thus shifting weather patterns from cold and stormy to fair. When the shifts persist long enough, climate changes.
In a previous study, meteorologists Mark Baldwin and Timothy Dunkerton of Northwest Research Associates in Bellevue, Washington, found that there were connections between the AO and the polar vortex. A switch from a strong stratospheric vortex to a weak one, for instance, would impact the troposphere below it and then weaken the AO's westerly winds. Because this chain of events took a few weeks, predicting a switch a week or two ahead looked possible.
Now, the duo has taken a more detailed look at 42 winters of vortex and AO behavior and found that the connection can persist. Once a major switch reached the lower stratosphere, the vortex would remain unusually weak or strong for an average of 60 days, which should let forecasters predict extremes in the underlying AO and the accompanying likelihood of weather extremes out as far as a month or two.
The "analysis is very careful and very complete," says stratosphere meteorologist Karin Labitzke of the Free University Berlin. Predicting long-term weather regimes will still be difficult, the authors point out, because switches in the stratosphere and the AO sometimes occur independently, and no one understands how that works. The challenge now is to build models that predict where and when this interaction takes place, says Edward O'Lenic, a long-range forecaster at the National Weather Service's Climate Prediction Center in Camp Springs, Maryland. "This is a tall order and a challenge for modelers, but the payoff could be great."