NASA satellites witnessed El Nino creep in from the Indian Ocean

December 01, 2004

El Nino has fascinated people for centuries, and continues to interest people around the world, because it changes global weather patterns. It was named by Peruvian fisherman after the Christ child because the warm waters that appear in the eastern Pacific Ocean off the coast of Peru usually appear in the wintertime before Christmas.

Just in time for this Christmas, an index created to see the development of El Nino events received the approval of the scientific community. Scott Curtis, a NASA-funded scientist from East Carolina University in Greenville, N.C. and colleagues, created an index using satellite data of rain and winds in the eastern Indian Ocean that accurately predicted the arrival of the 2002-2003 El Niño.

The article appeared in a recent issue of the Journal of Geophysical Research - Atmospheres, and the index is now available to the scientific community for analyzing possible future events.

During the winter of 2001-2002, the climate of the eastern Indian Ocean underwent a dramatic shift as westerly winds increased and the weather flip-flopped from dry to wet. Turning to NASA satellites for answers, researchers tracked two cases where wind, rainfall, and warmer sea surface temperatures moved from the Indian to the Pacific Ocean in early 2002 prior to the 2002-03 El Niño.

Scott Curtis, Assistant Professor at East Carolina University, and Robert Adler, George Huffman and Guojun Gu, all of NASA's Goddard Space Flight Center, Greenbelt, Md. used NASA's Tropical Rainfall Measuring Mission (TRMM) and QuikScat satellite data ranging from November 2001 to March 2002. This study appeared in a recent issue of the Journal of Geophysical Research - Atmospheres, and advances previous work published in May 28, 2002.

"This new study expands on recent work linking rain and wind changes over the last 25 years to the development of El Niños," Curtis said. Data prior to 2000 came from the Global Precipitation Climatology Project. In the 2002 study, the researchers examined fluctuations in rainfall from week to week, and the total amount of wintertime precipitation in key locations of the eastern Indian Ocean. They found these data points could be a sign of early shifts in climate leading to the development of El Niños since 1979. The researchers then explored computer modeled winds, but did not find the same connections.

The researchers developed the El Niño Onset Index (EOI) using the rainfall data alone. "Because the rainfall data has been a consistent indicator of an on-coming El Niño, as compared to the wind data, only the rainfall data was used to construct the EOI," said Curtis. "This was the first El Niño where TRMM data was available for the entire event."

The researchers used TRMM and QuikScat to track two instances of wind, rainfall, and warmer sea surface temperatures moving from the Indian to the Pacific Ocean in early 2002 prior to the 2002-03 El Niño.

The first organized rain event followed the Equator and the second traveled further south closer to Australia. In the latter example, warm waters appeared first, followed by heavy rainfall in the eastern Indian Ocean. Then, strong westerly winds and a cooling of the sea surface developed. This sequence of events moved through the ocean area between Indonesia and Australia, suggesting a connection between rising air, wind, and sea surface temperatures over a period of days. These studies provided a basis for how variations in the East Indian Ocean are linked to subsequent events in the Pacific Ocean, including the initiation of El Niño events. For example, these large organized weather systems may prefer to travel over water, affecting certain areas of the Maritime Continent and not others.

El Nino is signaled by a warming of the ocean surface off the western coast of South America that occurs every 4 to 12 years when cold, nutrient-rich water does not come up from the ocean bottom. It affects Pacific jet stream winds, altering storm tracks and creating unusual weather patterns in various parts of the world.

The National Oceanic and Atmospheric Administration stated there is currently a weak El Niño underway, which the EOI did not predict. Curtis explained that the EOI may not be sensitive enough to register weak episodes. After computing the EOI back to 1979, the only El Niño that was not "predicted" was the weakest event in 1993. Curtis also explained that the current El Niño is not basin-wide, as the far eastern Pacific is cooler than normal.

Curtis and Adler said that given the limited skill of statistical and ocean-atmosphere computer models in predicting El Niño, any new prediction system has applications for regions over the globe impacted by El Niño.

The National Oceanic and Atmospheric Administration is the lead agency responsible for forecasts relating to El Niños. NASA uses its unique Earth observing satellites to also study climate patterns and related conditions.

In the future, NASA will launch the Global Precipitation Measurement Mission, which will contribute to the EOI, as TRMM does currently.

NASA/Goddard Space Flight Center

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