September 18, 2001 – Original Source: Goddard Institute for Space Studies, NASA
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 Sep. 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.
El Niño and La Niña Rearrange Ice Cover in Antarctica
The light blue areas indicate open ocean water. All other areas show the presence of sea ice.
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.
Locations of Increased Sea Ice During El Niño and La Niña Years
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 Niño event. The blue color indicates places where ice concentrations were higher in 1999 than 1992, as a result of a 1999 La Niña event.
Credit: Claire Parkinson and Nick DiGirolamo, NASA Goddard Space Flight Center.
Rind, D., M. Chandler, J. Lerner, D.G. Martinson, and X. Yuan 2001. Climate response to basin-specific changes in latitudinal temperature gradients and implications for sea ice variability. J. Geophys. Res. 106, 20161-20173.