TRMM satellite proves El Nino holds the reins on global rains

11/08/04

NASA scientists recently found the El Nino Southern Oscillation (ENSO) is the main driver of the change in rain patterns all around the world.

The NASA and Japan Aerospace Exploration Agency (JAXA) Tropical Rainfall Measuring Mission (TRMM) satellite has enabled scientists to look around the globe and determine where the year-to-year changes in rainfall are greatest. The TRMM is a joint mission between NASA and JAXA designed to monitor and study tropical rainfall.

Researchers Ziad Haddad and Jonathan Meagher of NASA's Jet Propulsion Laboratory, Pasadena, Calif., Robert Adler and Eric Smith of NASA's Goddard Space Flight Center, Greenbelt, Md., used TRMM data to identify areas where the year-to-year change in rainfall between 1998 and 2003 was greatest.

By studying the rain patterns in these areas over the past 50 years, with rain gauge data prior to 1998, they established the main component of this change in global rainfall is directly correlated with the El Nino Southern Oscillation. The study appeared in a recent issue of the Journal of Geophysical Research-Atmospheres.

Haddad and his colleagues compared local changes in worldwide rainfall. For years, scientists have known El Nino drastically modifies rainfall patterns in many regions. For example, Indonesia and the Northeastern Amazon basin consistently suffer droughts during El Nino and excessive rains during La Nina. The Southeastern United States and California are typically wetter than usual during El Nino and drier than usual during La Nina.

Scientists also have known several regions with abundant rain are not influenced by the El-Nino/La-Nina changes, including the Bay of Bengal and the vast expanse of the Western Pacific Ocean between the Marshall Islands, Micronesia and the Marianas.

Until the launch of TRMM in 1997, it was impossible to accurately measure change in tropical rainfall patterns, because no instruments were available to record global rainfall. TRMM uses microwave technology to probe through clouds and estimate how much rainfall they are producing. The TRMM data are invaluable over areas where there are no rain gauges, such as the open ocean.

Using TRMM's measurements, the researchers were able to condense the year-to-year change in rainfall patterns into a single rain-change index. The index is a color-coded map that shows areas of rainfall around the world that are influenced somewhat to greatly, during an ENSO event.

Rainfall data from land and island stations were used to extend this index back in time and to compare it with the ENSO sea-surface temperature and atmospheric pressure. The results showed a strong relationship between the rainfall patterns and ENSO. "The fact that the rain-change index, which comes directly from global measurements, tracks the ENSO indices from the 1950s to the present confirms that El Nino is the principal driver of global year-to-year rainfall change," Haddad said.

NASA plans the Global Precipitation Measurement mission (GPM), a future multi-national multi-satellite mission to expand the scope of TRMM. GPM will focus on producing three- dimensional maps of rain around the world every three hours.

TRMM is the first space-based rain gauge that uses microwaves to see how much precipitation falls from clouds around the tropics. The TRMM satellite's precipitation radar acts like a highly sensitive microwave camera. It is capable of probing clouds to reveal their vertical structure and precipitation they produce. It has enabled scientists to measure rainfall over the oceans and landmasses with unprecedented accuracy.

Source: Eurekalert & others

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