The Hindu Business Line : El Nino variable seen as main driver of change in rain pattern

El Nino, an abnormal warming of surface ocean waters in the eastern tropical Pacific, is one part of what's called the Southern Oscillation. The Southern Oscillation is the see-saw pattern of reversing surface air pressure between the eastern and western tropical Pacific.

When the surface pressure is high in the eastern tropical Pacific, it is low in the western tropical Pacific, and vice versa. Because the ocean warming and pressure reversals are, for the most part, simultaneous, scientists call this phenomenon ENSO.

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

Researchers 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 ENSO.

The researchers 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 North-East Amazon basin consistently suffer droughts during El Nino and excessive rains during La Nina. South-East US 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 largely 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 and Micronesia.

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.

TRMM rain gauge uses microwaves to see how much precipitation falls from clouds around the tropics. It is capable of probing clouds to reveal their vertical structure and precipitation they produce.

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 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.

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.

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