Atmospheric "Springiness" and Global Warming
Dargan Frierson, August 2011
Recent results
from computer models of global warming have suggested
that the static stability, a quantity
which could be described as the "springiness"
of the atmosphere, will increase over much of the globe with
rising temperatures. What does this mean, and what implications will it
have for the Earth's climate? Let's investigate.
So how is the atmosphere like a spring, anyway? Well, under average
atmospheric conditions, if you lift up a bit
of air (scientists call these "parcels" of air), it immediately starts
rushing back downward, in the direction from which it came. Then it
overshoots the place it started from, and goes downward almost as far as it
was picked up. It oscillates back and forth like this over and over,
taking around 10 minutes to do each lap.
What causes this spring-like motion? Is air composed of tiny Slinkies??
No, this behavior is due to the temperature structure of the
atmosphere. When our parcel of air is lifted, it goes to a place where
it's colder than the air around it. Since colder air is heavier, it then
falls back downward.
The opposite happens when air is pushed downward.
Below, the parcel is warmer than the air around it, which means it's
lighter, and is pushed back up. These back and forth motions are called
buoyancy oscillations. Check
out the movie below to see how this works.
In addition to the up-and-down motion, the disturbance can spread out,
like a wave in the ocean. These waves are called
gravity waves,
because it is gravity that causes the heavier air to fall back
downwards (these should not be confused with the gravity waves that
astronomers
sometimes talk about, that were predicted by Einstein in his theory of
general relativity, which are very different types of waves). Waves aren't
just an ocean phenomenon, they are just as ubiquitous in the atmosphere
too!
You can often detect the presence of gravity waves in the atmosphere from
patterns of clouds. Condensation and cloudiness tend to occur in air that's
been pushed upward, so ridges of cloudy and cloud-free air
often mark the presence of gravity waves (these
structures can also be caused by organized roll vortices).
Image courtesy of
NASA
Beautiful examples of clouds caused by gravity waves are often seen here in
Washington state just downwind of our big mountains, since mountains are so
effective at forcing air upwards.
Downwind of the mountains you can often see several
lenticular (lens-shaped)
clouds,
which occur as buoyancy oscillations alternately force air upward and
downward.
Static Stability and Global Warming
Static stability varies naturally with height, latitude, and longitude in the
atmosphere, and with season as well
(here's a
recent paper I wrote about the seasonal cycle of static stability).
Climate models predict a consistent increase in the springiness of the
atmosphere with global warming though. This happens in both the tropics
(which is well-known
among scientists)
and also outside the tropics, from 30 to 60 degrees latitude in the Northern
and Southern Hemispheres (the midlatitudes,
where most of the USA, Europe, Japan, New Zealand, etc are located). My
work focused on the midlatitudes in these two studies.
So, why would the atmospheric "springiness" increase with global warming?
Computer models have shown that within the weather layer of the atmosphere,
global warming is expected to be larger and larger the higher in the
atmosphere you go. A figure below from one of my papers illustrates this
(click it for a more technical description of this research).
So, when air parcels are pushed upwards, they will
encounter air that is even warmer, sending the parcels rushing back downwards
even faster. It's just like making a spring tighter, so it bounces back up
and down faster.
What effects might this increase in static stability have on our climate?
First of all, it's important to
recognize that if it wasn't for the temperatures changing like this,
global warming at the Earth's surface where we live would be much more
severe. Increased temperatures in the upper atmosphere allow our planet to
shed energy more effectively -- this is a type of planetary safety valve
that counteracts some of the effects of our emissions of greenhouse gases.
If it didn't happen, global warming would be even worse than the current
dire predictions.
Static stability is also linked to parts of the climate that can change in a
more disruptive way. For instance, static stability has an influence on
both the strength and the size of midlatitude weather systems, and on the
intensity of large-scale overturning circulations in the atmosphere.
Recent research has suggested that springiness may also be linked to the
predicted northward shift of storms over North America, Europe, and Asia
in global warming simulations. The northward shift of storms leads to a
northward shift of climate zones, precipitation regions, and deserts as
well. By studying static stability and its relation to other
climatological quantities, scientists improve their understanding and
prediction of global warming and its consequences.
For a more technical description of this research, you can follow
this link, which describes a paper that I wrote on static stability and global warming.
This work is funded by National Science Foundation grant ATM-0936059.
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