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El Niño drives rainfall

For the first time, scientists have been able to quantify the causes of year-to-year variability in global rainfall. After seasonal variations such as monsoons, it seems the major culprit is the El Niño-Southern Oscillation (ENSO) — a phenomenon in which seesawing ocean temperatures and atmospheric pressures in the equatorial Pacific change weather patterns around the world.

This NASA-created index map condenses five years of data from 1998 to 2003 to show annual changes in rainfall. Areas in blue receive more rain whenever areas in red experience a shortfall in rain, and vice versa. The year-to-year change in rain is smallest in the green areas. The global pattern confirms that El Niño is the second largest factor, after seasons, in changing how and where rain falls around the world. Courtesy of TRMM/NASA/JAXA.

"We know rain fluctuates," says Ziad Haddad of the Jet Propulsion Laboratory (JPL) at Caltech in Pasadena, Calif. The question is: "What are the reasons behind these fluctuations?"

To find out, Haddad and colleagues at JPL and NASA's Goddard Space Flight Center in Greenbelt, Md., used satellite data to develop a rain anomaly index — an indicator of where in the world rainfall was departing most from the normal local monthly accumulation. The team then compared the index of rainfall fluctuation with the ENSO indices of sea-surface temperature and atmospheric pressure and found that wetter- or drier-than-normal weather correlated strongly with the intensity of ENSO phases, as reported in the Sept. 11 Journal of Geophysical Research.

"So far, we can assert that the main drivers of the global change in rainfall are, in order of importance, the seasons and ENSO," Haddad says.

The index is based on the most comprehensive dataset ever compiled of tropical rainfall measurements over both land and water. Collected between 1998 and 2003, the data come from microwave and radar instruments aboard the joint NASA and Japanese Aerospace Exploration Agency's Tropical Rainfall Measuring Mission (TRMM) satellite.

"For the first time, we were able to get the best possible estimate of what El Niño does to rainfall around the entire Earth," says Jeffrey Halverson, a TRMM researcher at NASA Goddard Space Flight Center who was not affiliated with the study, "and it is a dramatic shift in the location and intensity of the rainfall."

Using historic rain gauge data, some of which was the legacy of colonial empires that occupied the tropics, the researchers extended the index back 50 years and compared it to historic ENSO indices. "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 Niño is the principal driver of global year-to-year rainfall change," Haddad says.

Scientists have long recognized a connection between ENSO and precipitation. For example, during El Niño (the ocean-warming phase), parts of Indonesia and the Amazon basin suffer droughts, while during La Niña (the cool phase), they experience excessive rainfall. Likewise, the U.S. Southeast and California are wetter than normal during El Niño and drier than normal during La Niña. But until now, the data required to show quantitatively that ENSO is the driving force behind this variability had been lacking.

Until 1979, global rain data came from rain gauges, which were sparsely placed throughout the oceans. Since 1980, infrared radar has collected cloud-top temperatures from space. But these instruments did not function well over land and were not as accurate as microwave sensing is at measuring actual precipitation over both land and sea. Thus, before TRMM, researchers did not understand the amount of rainfall in the tropics "to within a factor of about two," Halverson says, "and that's a huge amount of error."

TRMM was launched in November 1997, just as one of the strongest El Niños of the century was beginning. From 400 miles above Earth, TRMM monitors the tropics and subtropics — the region between 35 degrees North latitude and 35 degrees South latitude, where two-thirds of the planet's rain falls from hurricanes, monsoons and tropical thunderstorms, mostly over the oceans.

The findings from this TRMM study could be important for tropical countries that deal with extreme variation in their water supply. "Tropical rainfall has a tremendous amount of variability from year to year," Halverson says. "You can have very dry years alternate with extremely wet years and that's very difficult for societies that are struggling, particularly societies that rely on agriculture or have limited freshwater access."

However, while the finding that ENSO is the driving force behind variability applies across most of the tropics, there are regions, such as the Bay of Bengal and the western Indian Ocean, that exhibit rainfall extremes that do not seem to correlate strongly with ENSO. "In other words, the areas are affected by ENSO, but not in such a simple way as to say that they are wetter than normal during warm phases and drier than normal during cold phases or vice versa," Haddad says.

Understanding what factors other than ENSO are affecting rain globally is an area of continuing research, Haddad says.

Sara Pratt
Geotimes contributing writer

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