Researchers Link Midwest Flooding to El Nino Effect : Science: Report says Pacific Ocean warming changed weather patterns, bringing waves of storms to Mississippi Valley.

WASHINGTON — A new analysis of the record flooding in the Midwest this summer says that the unusual Pacific Ocean warming known as El Nino was probably a leading cause.

El Nino is part of a complex phenomenon known as ENSO, which causes the water in the central Pacific Ocean to warm up every three to seven years.

The ultimate fault of the flooding may lie in a combination of factors, but the El Nino anomaly was “a direct influence,” according to a report Thursday by the federal Climate Analysis Center in Camp Springs, Md.

Flood damage this summer totaled more than $10 billion, and 48 people died along the Mississippi, the Missouri and other rivers as persistent heavy rains deluged a region already soaked from winter snowmelt.

Wet soils continue to pose a problem for farmers in the region, and they raise the specter of more flooding next year, officials say.

This year’s wetness means that the ground will have a limited capacity to absorb snowmelt next spring, so heavy winter snows or spring rains next year could mean a recurring threat, the report says.

The direct cause of the heavy flooding was a persistent and unusual weather pattern causing storm after storm to inundate the region. The pattern that brought those storms to the Midwest was encouraged by the ENSO, according to the report.

ENSO stands for El Nino-Southern Oscillation. El Nino is the warming of the Pacific waters, while southern oscillation refers to changes in air pressure over the Pacific Ocean that occur at the same time.

Under normal summer conditions, high altitude winds that move weather patterns across the nation flow more or less directly from west to east. But the El Nino’s warm water caused rising air in the Pacific, deflecting the high-level winds to the north. Those winds then arced back to the south over the Western states and looped north again near the Mississippi Valley.

The southward-moving wind brought cool air down from Canada and the loop back north pulled moisture into the Midwest from the Gulf of Mexico. The warm, wet Gulf air met the cool Canadian air over the northern Mississippi Valley, and the result was seemingly endless rain, the report says.

In the words of the scientists: “The front supported production of widespread areas of prolonged and excessive precipitation . . . and initiated the worst flooding in more than a century throughout the northern Mississippi River basin.”

El Ninos, from the Spanish word for child, occur every few years on an irregular basis. Most are relatively mild, but stronger events can affect weather worldwide. Their causes are not known, but they have occurred for centuries.

The 1982-83 El Nino, the worst of this century, was eventually blamed for 1,500 deaths and up to $8 billion in damage in the United States, Australia, Indonesia, Africa and other areas.

Contributing factors to this year’s flooding, the report concluded, were excessive winter snowpack in the Rockies, saturated soil and dangerous runoff conditions in the Midwest.

There has been speculation that global warming was also associated with the unusual weather, but the researchers could find no direct relationship.