by Thomas Reichler and Lee J. Siegel
September 23, 2012
North Atlantic ‘Achilles Heel’
Lets Upper Atmosphere Affect
A University of Utah study suggests something amazing:
Periodic changes in winds 15 to 30
miles high in the stratosphere influence the seas by striking a
vulnerable “Achilles heel” in the North Atlantic and changing
mile-deep ocean circulation patterns, which in turn affect
“We found evidence that what
happens in the stratosphere matters for the ocean
circulation and therefore for climate,” says Thomas Reichler,
senior author of the study published online Sunday, Sept. 23
in the journal Nature Geoscience.
Scientists already knew that events in
the stratosphere, 6 miles to 30 miles above Earth, affect what
happens below in the troposphere, the part of the atmosphere from
Earth’s surface up to 6 miles or about 32,800 feet.
Weather occurs in the troposphere.
Researchers also knew that global circulation patterns in the oceans
- patterns caused mostly by variations in water temperature and
saltiness - affect global climate.
“It is not new that the stratosphere
impacts the troposphere,” says Reichler, an associate professor
of atmospheric sciences at the University of Utah.
“It also is not new that the
troposphere impacts the ocean. But now we actually demonstrated
an entire link between the stratosphere, the troposphere and the
Funded by the University of Utah,
Reichler conducted the study with University of Utah atmospheric
sciences doctoral student Junsu Kim, and with atmospheric
scientist Elisa Manzini and oceanographer Jürgen Kröger,
both with the Max Planck Institute for Meteorology in Hamburg,
Winds and Sea Circulation Show Similar Rhythms
Reichler and colleagues used weather observations and 4,000 years
worth of supercomputer simulations of weather to show a surprising
association between decade-scale, periodic changes in stratospheric
wind patterns known as the polar vortex, and similar rhythmic
changes in deep-sea circulation patterns.
The changes are:
“Stratospheric sudden warming”
events occur when temperatures rise and 80-mph “polar
vortex” winds encircling the Artic suddenly weaken or even
change direction. These winds extend from 15 miles elevation
in the stratosphere up beyond the top of the stratosphere at
30 miles. The changes last for up to 60 days, allowing time
for their effects to propagate down through the atmosphere
to the ocean.
Changes in the speed of the
Atlantic circulation pattern - known as Atlantic Meridional
Overturning Circulation - that influences the world’s oceans
because it acts like a conveyor belt moving water around the
Sometimes, both events happen several
years in a row in one decade, and then none occur in the next
So incorporating this decade-scale
effect of the stratosphere on the sea into supercomputer climate
simulations or “models” is important in forecasting decade-to-decade
climate changes that are distinct from global warming, Reichler
“If we as humans modify the
stratosphere, it may - through the chain of events we
demonstrate in this study - also impact the ocean circulation,”
“Good examples of how we modify the
stratosphere are the ozone hole and also fossil-fuel burning
that adds carbon dioxide to the stratosphere. These changes to
the stratosphere can alter the ocean, and any change to the
ocean is extremely important to global climate.”
Soft Spot in the North Atlantic
“The North Atlantic is particularly
important for global ocean circulation, and therefore for
climate worldwide,” Reichler says.
“In a region south of
Greenland, which is called the downwelling region, water can get
cold and salty enough - and thus dense enough - so the water
It is Earth’s most important region of
seawater downwelling, he adds.
That sinking of cold, salty water,
“drives the three-dimensional
oceanic conveyor belt circulation. What happens in the Atlantic
also affects the other oceans.”
“This area where downwelling occurs
is quite susceptible to cooling or warming from the troposphere.
If the water is close to becoming heavy enough to sink, then
even small additional amounts of heating or cooling from the
atmosphere may be imported to the ocean and either trigger
downwelling events or delay them.”
Because of that sensitivity, Reichler
calls the sea south of Greenland,
“the Achilles heel of the North
Stratosphere to the Sea
In winter, the stratospheric Arctic polar vortex whirls
counterclockwise around the North Pole, with the strongest, 80-mph
winds at about 60 degrees north latitude.
They are stronger than jet stream winds,
which are less than 70 mph in the troposphere below.
But every two years on average, the
stratospheric air suddenly is disrupted and the vortex gets warmer
and weaker, and sometimes even shifts direction to clockwise.
“These are catastrophic
rearrangements of circulation in the stratosphere,” and the
weaker or reversed polar vortex persists up to two months,
“Breakdown of the polar vortex can
affect circulation in the troposphere all the way down to the
Reichler’s study ventured into new
territory by asking if changes in stratospheric polar vortex winds
impart heat or cold to the sea, and how that affects the sea.
It already was known that that these stratospheric wind changes
affect the North Atlantic Oscillation - a pattern of low atmospheric
pressure centered over Greenland and high pressure over the Azores
to the south. The pattern can reverse or oscillate.
Because the oscillating pressure patterns are located above the
ocean downwelling area near Greenland, the question is whether that
pattern affects the downwelling and, in turn, the global oceanic
circulation conveyor belt.
The study’s computer simulations show a decadal on-off pattern of
correlated changes in the polar vortex, atmospheric pressure
oscillations over the North Atlantic and changes in sea circulation
more than one mile beneath the waves.
Observations are consistent with the
pattern revealed in computer simulations.
and Simulations of the Stratosphere-to-Sea Link
In the 1980s and 2000s, a series of stratospheric sudden warming
events weakened polar vortex winds. During the 1990s, the polar
vortex remained strong.
Reichler and colleagues used published worldwide ocean observations
from a dozen research groups to reconstruct behavior of the conveyor
belt ocean circulation during the same 30-year period.
“The weakening and strengthening of
the stratospheric circulation seems to correspond with changes
in ocean circulation in the North Atlantic,” Reichler says.
To reduce uncertainties about the
observations, the researchers used computers to simulate 4,000 years
worth of atmosphere and ocean circulation.
“The computer model showed that when
we have a series of these polar vortex changes, the ocean
circulation is susceptible to those stratospheric events,”
To further verify the findings, the
researchers combined 18 atmosphere and ocean models into one big
simulation, and “we see very similar outcomes.”
The study suggests there is,
“a significant stratospheric impact
on the ocean,” the researchers write.
“Recurring stratospheric vortex
events create long-lived perturbations at the ocean surface,
which penetrate into the deeper ocean and trigger multidecadal
variability in its circulation. This leads to the remarkable
fact that signals that emanate from the stratosphere cross the
entire atmosphere-ocean system.”