Thursday, April 19, 2012

[californiadisasters] La Nina Effect

La Nina effect

By Miguel Miller, graphics and research by James Thomas

We know that La Niña usually causes Southern Cali-fornia to receive rainfall below normal during a given winter. But is that always true? James Thomas, a meteorologist Intern here at the NWS in San Diego, did some research to find out. He used San Diego Lindbergh Field as the point of reference, although similar correlations can be inferred for anywhere in Southern California.

In this study, James used the strength of La Niña as defined from the Oceanic Niño Index (ONI), which uses a running three month average of sea surface temperatures. He broke these seasonal events into Weak, Moderate, and Strong events. The rainfall season is defined as July 1 to June 30, and the latter part of that season's year is annotated in the above graphic. Normal rainfall for San Diego Lindbergh Field is 10.34".

We found that San Diego has experienced lower than normal precipitation in 18 out of the last 20 La Niña events. That's a 90% dry bias!—Note how the moderate La Niña of 2010-11 sticks out like a sore thumb. In all six of the previous moderate La Niñas, San Diego ended the water year with well below normal precipitation.

During the water year of a weak, moderate, and strong La Niñas, San Diego averaged 7.37", 8.01", and 6.47" respectively. These averages all fall below San Diego's rainfall average of 10.34" (see chart at left).

We have a clear signature: La Niña produces below normal rainfall in San Diego. Recall that in the anomalous La Niña season of 2010-11, an"atmospheric river" setup brought copious amounts of rainfall in December 2010. It was a very wet fall and early winter in 2010 before a drier pattern followed.

This illustrates another interesting find. La Niñas seem to "kick in" and reduce rainfall most during the normally wettest part of the season from January to March. This was especially true for the 2010-11 season. And it has been true in this 2011-12 season. Conversely, this is something that has been noted about El Niño: it seems to enhance rainfall most from January to March.


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