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El
Niņo, La Niņa Rearrange South Pole Sea Ice
Goddard
Space Flight Center Press Release
During
the El Nino year of 1992, the Pacific Ocean from the Drake Passage to the Ross
Sea (about 70W to 180W) had less sea ice than in a normal year. Meanwhile, the
sea ice in the Weddell Sea (20E to 60W) extended further north.
In contrast, sea ice in the Pacific Ocean had a larger northward extension in
1999, a La Nina year, particularly east of the Ross Sea. Meanwhile, sea ice in
the Weddell Sea had a less than normal northern extent.
COLOR KEY: The light blue areas indicate open ocean water. All other areas show
the presence of sea ice.
Scientists have been mystified by
observations that when sea ice on one side of the South Pole recedes, it
advances farther out on the other side. New findings from NASA's Office of Polar
Programs suggests for the first time that this is the result of El Niņos and La
Niņas driving changes in the subtropical jet stream, which then alter the path
of storms that move sea ice around the South Pole.
The results have important
implications for understanding global climate change better because sea ice
contributes to the Earth's energy balance. The presence of sea ice, which is
generated around each pole when the water gets cold enough to freeze, reflects
solar energy back out to space, cooling the planet. When there is less sea ice,
the ocean absorbs the sun's heat and that amplifies climate warming.
By looking at the relationship
between temperature changes in the ocean, atmospheric winds, storms, and sea
ice, the new study pinpoints causes for retreating and advancing ice in the
Atlantic and Pacific ocean basins on either side of the South Pole, called the
"Antarctic dipole."
"El Niņos and La Niņas
appear to be the originating agents for helping generate the sea ice dipole
observed in the ocean basins around the Antarctic," said David Rind, lead
author of the study and a senior climate researcher at the NASA Goddard
Institute for Space Studies. The study appears in the September 17 issue of
Journal of Geophysical Research.
This map shows the
difference in sea ice cover between 1992 and 1999 around Antarctica. The red
color indicates areas where there was a higher concentration of sea ice in 1992
than in 1999, as a result of a 1992 El Nino event. The blue color indicates
places where ice concentrations were higher in 1999 than 1992, as a result of a
1999 La Nina event.
During El Niņo years, when the
waters of the Eastern Pacific heat up, warm air rises. As the air rises it
starts to move toward the South Pole, but the earth's rotation turns the winds
eastward. The Earth's rotation is just strong enough to cause this rising air to
strengthen the subtropical jet stream, a band of atmospheric wind near the
equator that also blows eastward.
When the subtropical jet stream
gets stronger over the Pacific basin, it diverts storms away from the Pacific
side of the South Pole. Since there are fewer storms near the Pacific-Antarctic
region during El Niņo years, there are less winds to blow sea ice farther out
into the ocean, and ice stays close to shore.
At the same time, the air in the
tropical Atlantic basin sinks instead of rising. That sinking air weakens the
subtropical jet stream over the Atlantic, guiding storms towards the South Pole.
The storms, which intensify as they meet the cooler Antarctic air, then blow sea
ice away from the pole farther into the Atlantic.
During La Niņa years, when the
Eastern and central Pacific waters cool, there is an opposite effect, where sea
ice subsides on the Atlantic side, and advances on the Pacific side.
The study is important because
the amount of sea ice that extends out into the ocean plays a key role in
amplifying or decreasing the warming effects of the sun on our climate. Also,
the study explains causes of the Antarctic sea ice dipole for the first time,
and provides researchers with a greater understanding of the effects of El Niņo
and La Niņa on sea ice.
Scientists may use these findings
in global climate models to gauge past, present and future climate changes.
"Understanding how changes
in the temperature in the different ocean basins will affect sea ice is an
important part of the puzzle in understanding climate sensitivity," Rind
said.
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