 | |  |
|
| Enlarge ImageThe severity of wintertime climate over North America and Europe is strongly linked to the most prominent atmospheric pattern in the Northern Hemisphere, a seesaw exchange of air massed called the “northern annular mode.” (Illustration by E. Paul Oberlander, Woods Hole Oceanographic Institution) |
|
|  |
|
| Enlarge ImageThis graph depicts a simulated "random walk," in which one repeatedly flips a coin: If it comes up heads, take a step forward; if it's tails, step backward. This portion of the "walk" seems to show a periodic oscillation, with peaks or valleys every 500 coin flips, but no real cyclic process is occuring, just random chance. (Jason Goodman, Woods Hole Oceaonographic Institution) |
|
|  |
|
| Enlarge ImageBlue lines chart daily variations in both the northern annular mode (upper panel) and a similar pattern that occurs in the Southern Hemisphere called the southern annular mode (lower panel). The patterns resemble those from the output (red line)from a random "coin-flipping" model (see graph above). (Jason Goodman, Woods Hole Oceanographic Institution) |
|
|
Related Links |
|
|
By Jason Goodman, Assistant Scientist
Department of Physical Oceanography
Woods Hole Oceanographic Institution
Winds and temperatures in Earth's atmosphere vary from month to month
and year to year in countless ways. Decades of monitoring the weather
and climate have revealed a few simple patterns that explain much of
this variability.
The severity of wintertime climate over North America and Europe, for
example, has been strongly linked to the most prominent atmospheric
pattern in the Northern Hemisphere, which is called the “northern
annular mode.” It is a natural shift of air masses back and forth
between the North Pole and mid-latitudes. At some times, we see a
surplus of air mass and pressure over the pole and a deficit at around
45°N; at other times, the air mass is redistributed to create a deficit
at the pole and a surplus in mid-latitudes.
This seesaw exchange of air masses shifts temperature conditions and
storm patterns throughout the region. The pattern of this exchange may
change from one week to another, or it may recur for several winters in
a row. There even seem to be long-term trends in the pattern that
continue for decades.
So, in many cases, we know the what, where, and when of these
atmospheric patterns. But how and why these patterns happen is less
clear. Many scientists have investigated the driving forces behind
changes in the northern annular mode and a similar pattern of changes,
called the “southern annular mode,” that occurs over Antarctica and the
Southern Hemisphere. If we can comprehend the physics behind these
changes, we might someday reach a point where we can predict not just
the weather, but some aspects of longer-term climate.
But there are more basic questions to answer first: How much of the
annular mode fluctuations are a purely random result of atmospheric
weather, and therefore inherently unpredictable? Can we discern true
patterns in annular mode fluctuations that occur more frequently than
can be explained by simple chance, and therefore could be predicted?
Random walks
We often begin our conceptual leaps by making models. When we simplify complicated
observations down to a few basic physical mechanisms—like the spare
lines of a cartoonist’s sketch—we can often see
the world more clearly. Even better, by looking at where our simple
models fail, we can more easily figure out what parts of our
understanding are lacking.
Suppose I stand in a long hallway and flip a coin over and over again.
When the coin comes up heads, I take a step forward. When it's tails, I
take a step backward. My position over time would naturally fluctuate
back and forth.
This sort of cumulative randomness is known as a "stochastic process."
Similar processes occur widely in nature, as well as in engineering and
economics, and can be very misleading. They often appear to show
long-term trends, or hints of periodic oscillation, even though nothing
is going on except purely random behavior.
As we try to analyze the fluctuations in the annular modes of the
atmosphere, we have the same problem: What is random, stochastic
fluctuation masquerading as a pattern, and what is truly a pattern with
identifiable and certain causes?
For instance, the winter atmosphere is full of storms that last a few
days. These storms can be predicted a few days in advance, but over the
duration of an entire winter, they are random and unpredictable.
The winds in these storms move air around as a random process. If we
imagine a line of latitude encircling the pole, we would observe that
global storm activity sometimes will move an excess of air north across
the line, and sometimes it will move more air south across the line.
That means our winter storm activity randomly adds or removes air mass
from the polar atmosphere.
This is a stochastic process, just like my random walk down the hall.
Twists in the wind
To understand what the annular modes mean to the bigger picture of
global climate, we built a simple mathematical model of a stochastic
process, not much more complicated than our coin-flipping hall walker.
We input the random north-south motion of air caused by daily storm
activity, ran a series of mathematical equations, and tried to produce
an output as similar as possible to the annual mode fluctuations we
observe in reality.
There are some unknowns in this model, such as: At what latitudes, and
what altitudes, should we measure the poleward motion of air? And, if,
by chance, we get a long series of storms with net poleward air motion,
is there some "leakage" of air back to the south? (After all, air mass
can't build up at the pole forever.) These unknowns can be thought of
as control knobs that we can adjust and "tune" to get the best match
between the model's output and the real-world annular mode changes.
After running our model, we found that a great deal of the fluctuation
of both the northern and southern annular modes (and the weather
patterns they spawn) can be explained using the "random walk" idea. But
there were some noticeable departures. Intriguingly, we found that the
air mass motions that drive the annular mode are not just like flipping
a coin. In some cases they're slightly non-random, depending on the
state of the annular mode; it's as if my coin was more likely to come
up "heads" when I stood at one end of the hallway, and more likely
"tails" at the other end. That sort of behavior can lead to long-term,
semi-random oscillations in the climate that, in principle, may be
predictable.
This means that the annular mode is a complicated mixture of both
random wobbles caused by weather events and of other, possibly more
predictable, interactions. Our simple model cannot tell us exactly what
these interactions are, but it does indicate that there may be
interesting climate-changing phenomena out there that we're only
beginning to investigate.
This research is funded by the WHOI Ocean and Climate Change Institute.
Posted: February 8, 2007 [top] |
