Montana Earth Science Picture of the Week

Blame it on El Nino

Map courtesy of NOAA

Oceans do affect Montana . . .
The map shows how El Nino events affect precipitation amounts throughout the lower 48 states. According to the information above the map, there were 11 "strong" El Ninos from 1895 to 1997. The various colors show which areas tend to experience more winter precipitation during El Nino years and which ones tend to be drier. For instance, Montana and Wyoming tend to be drier during El Nino years, whereas New Mexico tends to receive more precipitation as a result of El Nino. The darkest brown color in south-central Montana tells that 10 of the 11 El Nino winters were "dry" for that part of the state. . . In order to be considered "dry" the winter had to be among the driest one-third of all winters from 1895 to 1997. The brown that covers most of the state indicates that 9 of those 11 El Nino winters were dry in that area of Montana. The "1" in northeastern Montana indicates that this part of Montana did have one "wet" winter during an El Nino year. In contrast to Montana, parts of New Mexico experienced 10 "wet" winters during the 11 El Nino years.

So what's the connection? . . .
The El Nino is a change in the distribution of warm water in the equatorial Pacific Ocean. Normally the Trade Winds push the warm surface waters near South America toward Australia and Indonesia. This causes an upwelling of cold water along the west coast of South America, so surface water there is normally cold. During an El Nino, the Trade Winds falter or may even reverse, allowing the warm surface waters to move back toward South America. During extreme El Nino events such as the one in 1997-98, the humidity above these warm waters can cause tremendous rainfall amounts and flooding in places such as Peru. On the other side of the ocean, Autralia and Indonesia may experience droughts and wild fires. The El Nino also alters the position of the jet streams, causing storms to track through the the southern states more often. With more warm water in the eastern Pacific, these storms cause higher precipitation amounts for the southern states. On the other hand, El Nino tends to leave Montana out of the loop, as the jet stream diverts storms to south of the Treasure State.

Term: upwelling, jet stream


*More maps that illustrate El Nino trends
El Nino Demonstration
El Nino Explanation
*El Nino Theme Page (NOAA)
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By Rod Benson
Earth Science Teacher at Helena High School

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