|
ATMOSPHERIC
WAVE LINKED TO SEA ICE FLOW NEAR GREENLAND, STUDY FINDS
A
NASA researcher finds that the amount of sea ice that moves
 |
|
| |
|
Image
1
|
| |
between
Greenland and Spitsbergen, a group of islands north of Norway,
is dependent upon a "wave" of atmospheric pressure
at sea level. By being able to estimate how much sea ice is
exported through this region, called Fram Strait, scientists
may develop further insights into how the ice impacts global
climate.
This
export of sea ice helps control the thermohaline circulation,
a deep water ocean conveyor belt that moves warm, salty water
poleward and cold, fresh water toward the equator. The thermohaline
circulation is one of the primary mechanisms that maintains
the global heat balance.
Don
Cavalieri, a researcher at NASA's Goddard Space Flight Center
in Greenbelt, Md., discovered a link between the transport
of sea ice through this region and the position or phase of
the longest sea level pressure wave circling the Earth at
polar latitudes.
 |
|
| |
|
Image
2
|
|
|
Until
now, scientists have had inconsistent results when trying
to identify the mechanism behind this transport of sea ice.
The North Atlantic Oscillation, in particular, was unable
to explain the changes in sea ice transport through Fram Strait.
"The
significance of this work is the connection between the phase
of the longest atmospheric wave called 'wave 1' in sea level
pressure and long-term Arctic Ocean and sea ice variability,"
said Cavalieri.
Sea level pressure is made up of high and low pressure systems
as any weather map will show. The large-scale semi-permanent
highs and lows define the longest pressure waves which are
called planetary waves because they extend thousands of miles
and circle the world. The longest wave, called wave 1, is
made up of one ridge (high pressure) and one trough (low pressure).
It turns out that wave 1 is the dominant pattern at polar
latitudes. Because these planetary waves are so dominant in
wintertime atmospheric circulation their amplitudes (strength)
and phases (position) provide useful information on large-scale
wind patterns and thus on sea ice transport.
The
Icelandic Low is the primary weather system in the North Atlantic.
At times this low pressure system extends northeastward into
the Barents Sea. When this happens a secondary low pressure
system may develop in the Barents Sea region. It is the counterclockwise
circulation around this secondary low pressure system in the
Barents Sea that drives sea ice through the Fram Strait. Whenever
this secondary low pressure system exists, wave 1 shifts eastward
and is said to be in its eastward phase, as opposed to a westward
phase.
When
wave 1 is in its westward mode, the Icelandic Low is more
intense and localized, no longer extending to the Barents
Sea. Because of the position of the Low relative to the Strait,
the winds are more westerly and less ice is forced southward
through Fram Strait.
Variations
in the phase of wave 1 between these two extreme modes also
seem to control the cycle of Arctic Ocean circulation which
reverses from clockwise to counterclockwise (or anticyclonic
to cyclonic, respectively) every 6 or 7 years.
Cavalieri
used simulations for the 40 year period (1958-1997) from two
computer models to obtain a record of the volume of sea ice
that moved through Fram Strait. The two models each showed
a similar correlation between the eastward phase of wave 1
and movement of sea ice through the strait, with the exception
of two anomalous years between 1966 and 1967. When those years
were removed, one ice-ocean model, using monthly surface wind
and air temperature data, found that the wave 1 eastward phase
explained 70 percent of Arctic ice export through Fram Strait,
while the other model, which used daily surface wind and air
temperature data, accounted for 60 percent of the sea ice
export.
Cavalieri
also used Northern Hemisphere monthly sea level pressure grids
to obtain phase and amplitude information for wave 1.
The
paper appeared in a recent issue of Geophysical Research Letters.
The study was funded by NASA's Cryospheric Sciences Research
Program.
Back
to Top
|
