Weather Forecasters May Look Sky-High For Answers

Changes in the stratosphere, the atmospheric layer from six to 30 miles up, usually take a week or more to work their way down to where they affect weather, giving forecasters some lead-time. Once the changes affect the weather, they tend to last as long as two months.

"The study points weather forecasters towards a new source of information that hasn't been used before in weather prediction," said Mark Baldwin, Senior Research Scientist at the Northwest Research Associates (NWRA), and lead author in the study. The article appears in the October 19th issue of Science.

According to the study, the stratosphere plays an important role in how large-scale waves, which originate near Earth's surface, feed back to affect weather patterns in the Northern hemisphere.

In the winter, when stratospheric winds most often blow from the west, these waves tend to slow the winds in the stratosphere. This process starts with the higher stratospheric winds and over about a week's time, can work its way down to winds of the lower stratosphere, just above the levels of commercial air traffic.

Though the exact processes have yet to be fully understood, it has been observed that shifting wind patterns in the stratosphere precede changes in the Arctic Oscillation, a large-scale see-saw of atmospheric mass between the polar regions and mid-latitudes. The Arctic Oscillation, also called the North Atlantic Oscillation, is most pronounced over the Atlantic, and affects the strength of the winds through mid-latitudes, storm tracks, as well as extreme cold events in North America and Eurasia.

When the winds are weak in the stratosphere, the arctic oscillation is weak in the 60 days that follow, and that moves the paths of storms further south in the northern hemisphere. When the winds are strong in the stratosphere, the arctic oscillation is stronger, and the paths of storms are usually more northward.

"We can see it coming," said Baldwin. "It takes over a week to get to the surface. Once they reach the surface, once we are in one of these weather regimes, it lasts an average of two months."

Though the effects have been clearly observed, the exact interplay between the stratosphere and how surface weather patterns change is not fully understood.

"It is an initial step," said Dr. Timothy Dunkerton, Senior Research Scientist at NWRA, and co-author of the paper. "Our understanding of the role of the stratosphere in weather and climate could be compared to our knowledge of El Niño 20 years ago."

Baldwin added that there are two likely scenarios for practically applying this information to forecasts. One is to use a sophisticated forecasting computer model that includes the stratosphere. Current weather models usually do not include data from that high in the atmosphere. Still, this kind of modeling is a very involved process that requires large resources, and at present is not very practical.

The other option is to apply statistical measurements that use observational data to tell the likelihood that certain weather conditions will occur following shifts in stratospheric winds.

The study was jointly funded by NASA, the National Oceanic and Atmospheric Administration and the National Science Foundation.