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Figure 1: During the El Niño 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 Niña year, particularly east of the Ross Sea. Meanwhile, sea ice in the Weddell Sea had a less than normal northern extent. The light blue areas indicate open ocean water. All other areas show the presence of sea ice. (Click figures to see larger versions.) |
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, a phenomena referred to as the Antarctic sea ice dipole. New findings utilizing the GISS GCM suggest 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 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. Understanding why variations occur in the current climate can help us estimate how they may change as climate warms.
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Figure 2: The difference in sea ice cover between 1992 and 1999 around Antarctica. The red areas indicates areas where there was a higher concentration of sea ice in 1992 than in 1999, as a result of a 1992 El Niño event. The dark blue areas indicates places where ice concentrations were higher in 1999 than 1992, as a result of a 1999 La Niña event. |
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 rising air in the tropical Pacific forces the air in the tropical Atlantic to sink instead of rising. That sinking air weakens the subtropical jet stream over the Atlantic, allowing storm to progress 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 retreats on the Atlantic side, and advances on the Pacific side.
The results show that in order to understand how sea ice may change as climate warms, we must also learn what will happen to tropical temperatures, and El Niño / La Niña occurrences. This is one more example of how very disparate components of the climate system are connected, requiring that they all be understood before we can properly estimate future climate sensitivity.
Please address all inquiries about this research to Dr. David Rind.
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