These days, weather forecasters are lucky if they can accurately predict the
weather a week into the future. But a new study, funded in part by NASA,
says shifting wind patterns in the stratosphere during the winter may help
forecasters predict weather on the surface two months ahead of time, because
they have an affect on where storms track in the northern hemisphere.

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 NiƱo 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.

For more information:

http://www.gsfc.nasa.gov/topstory/20011018windsurface.html