Artist's concept of NASA's TRMM satellite operating in Earth orbit. Photo: NASA

Researchers at NASA and the University of Maryland, Baltimore County (UMBC), studying changes in tropical precipitation patterns, have noted a higher frequency of El Ninos and La Ninas over the last 21 years. In addition, when either of those events occur, the world can expect more months with unusually high or low precipitation with droughts more common than floods over land areas.

Scott Curtis of UMBC and Robert Adler of NASA/Goddard Space Flight Center are co-authors of a paper titled "The Magnitude and Variability of Global and Regional Precipitation Based on the 21 Year Global Precipitation Climatology Project (GPCP) and 3 Year Tropical Rainfall Measuring Mission (TRMM) Data Sets" being presented January 15 at the 2001 Annual Meeting of the American Meteorological Society in Albuquerque, N.M.

El Nino events occur irregularly at intervals of 2-7 years, although the average is about once every 3-4 years. Curtis said the global precipitation database shows El Ninos or La Ninas occurring almost every 2 years throughout the last two decades. These events typically last 12-18 months and are accompanied by swings in the Southern Oscillation, an interannual seesaw in tropical sea level pressure between the eastern and western Pacific. El Nino/Southern Oscillation (ENSO) events are characterized by large changes in precipitation over the tropics, which extend around the globe.

Curtis and Adler used data from the World Climate Research Project's Global Precipitation Climatology Project (GPCP), a database of monthly precipitation around the world, to study global and regional variations in rainfall. They compared the GPCP data to rainfall data from NASA's TRMM satellite over the tropics.

They confirmed that during El Ninos, such as the one that occurred during 1997-98, there is an increase in precipitation in the central equatorial Pacific and a decrease over the East Indies. More surprisingly, they noticed that changes in precipitation in the equatorial belt extend in a horseshoe shape toward the polar regions. These patterns depend on the season of the year.

Globally, results showed that during years with El Nino it is wetter over the ocean and drier over the land, compared to "neutral" years with neither El Nino nor La Nina. Research indicates that this decrease in rainfall can be up to 7% over tropical land areas. Curtis found that rainfall over the ocean doesn't change much during a La Niņa event compared to neutral periods, but the data does indicate wetter land areas.

Curtis said, "Over the entire 21-year span of the global rainfall database, results indicate an increase in rainfall in the Indian monsoon region and a decrease over central Africa. The change in precipitation over the tropics looks like a combination of El Nino and La Nina, which is not too surprising since we just experienced a strong El Nino in 1997-98 followed by a strong La Niņa in 1998-99." Also during the period 1979 to 1999, there has been an increase in rain in the monsoon region around India and a decrease over central Africa.

Curtis cautions that because the data only covers 21 years, which in climatic terms is a very short time, more data is needed before a trend in frequency of El Ninos and La Ninas can be seen.

TRMM is a joint U.S.-Japanese mission and part of NASA's Earth Science Enterprise; a long-term research program designed to study the Earth's land, oceans, air, ice and life as a total system.