El Nino, La Nina 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 suggest for the first time that this is the result of El Ninos and La Ninas 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 Ninos and La Ninas 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 Nino 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 Nino 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 Nina 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 Nino and La Nina 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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