A NASA researcher finds that the amount of sea ice that moves 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.

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.

For more information, please see: http://www.gsfc.nasa.gov/topstory/20020807seaice.html