Rainfall Change May Give Earlier Signal Of El Niño

Because some El Niño-induced climate changes can be devastating, it is important to find its earliest signs to better prepare nations that will most likely be affected by the event. The research by Curtis and Adler may eventually provide a more advanced notice on forthcoming El Niños to those countries.

Curtis will present these findings in his paper entitled "The Evolution of Tropical and Extratropical Precipitation During ENSO Events" at the 81st Annual meeting of the American Meteorological Society at the Albuquerque Convention Center on January 18th.

El Niño events occur when the trade winds that normally blow from east to west over the Pacific Ocean diminish and the waters of the eastern Pacific become warmer than normal. These events in the tropical waters of the Pacific have impacts on global weather patterns, including increased rainfall in the eastern Pacific, and drought conditions in Indonesia and Australia.

Scientists often use diminishing trade winds or warming water temperatures as clues to a coming El Niño. Changes in tropical precipitation have been viewed as secondary responses.

Curtis and Adler examined changes in precipitation patterns, amounts and intensities of precipitation of El Niño/Southern Oscillation (ENSO) events over the past 21 years. The global data sets they used viewed precipitation from a daily to monthly time scale, by combining the best satellite observations with rain gauge data. With this information they created the first ENSO indices based on precipitation over the open Pacific Ocean to identify and define interannual climate variations.

"Quantifying changes in the precipitation component of ENSO is important in understanding the distribution of heating in the atmosphere," Curtis said. "The heating drives the large-scale circulation of the atmosphere and affects weather patterns around the world."

During strong El Niños, like the 1982-1983 and 1997-1998 events, the 21-year record indicated dry conditions around Indonesia about two months before other atmospheric/oceanic characteristics of El Niño emerged. In both events, before the sea surface temperatures warmed, there was a decrease in precipitation over the East Indies, first occurring over and to the west of the large island of Indonesia. Curtis saw these precipitation decreases in January and February 1982 and 1997. The normal benchmarks, diminishing trade winds, and warming water temperatures over the eastern Pacific, came afterward.

Curtis cautions that this finding is not likely to apply to all El Niños, especially the weaker ones, such as the 1986-87 and the 1991-92 events, which had very different life cycles. Also, changes in precipitation in the East Indies do not seem to precede La Niñas. January-February 1982 and 1997 stand out as the two years in the 21-year data set with the largest deficit in precipitation in the eastern equatorial Indian Ocean.

In November 1999, the United Nations urged residents of nations that are adversely affected by El Niños to prepare in advance. The findings from Curtis and Adler may provide a way to recognize the earliest signs of a strong El Niño and help nations around the world make better preparations.