El Niño, La Niña
Rearrange South Pole 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 Niños and La Niñas 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
Niños and La Niñas 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. During
El Niño 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 Niño 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 Niña 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 Niño and La Niña 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.
Back
to Top |