Sunday, April 22, 2012

This was never published in the Sun; I guess the editor didn't like it.  It had some great graphics to go along with it, but for some reason I can't get them to cut and paste on this blog.

What Exactly Is La Niña and Why Do They Keep Talking About It?

Some weather events are easy to understand.  Everybody knows what it means when we have 90 days over 100 degrees in one season.  If a tropical storm drops 15 inches of rain on central Texas in one night, we totally get the concept.  But what exactly is La Niña and how is it related to droughts in Texas?  Here is a short answer, with some pictures, so you can be ready if somebody brings up the topic at a dinner party.

El Niño and La Niña are the opposite ends of a naturally occurring cycle called the El Niño/Southern Oscillation (ENSO).  The oscillation refers to the year to year shifts in the temperature of the surface water of the Pacific Ocean in a spot called the Niño 3.4 region.  (See Figure A.)  Normally, tradewinds blow from South America along the Equator, pushing the warm surface water west toward Indonesia and Australia.  The warm water collects there in what is called the “West Pacific Warm Pool.”  This westward movement of water causes cold water to be pulled up from the depths on the eastern side of the Pacific, near Peru and Ecuador.

La Niña is said to exist when the tradewinds blow even more water from east to west, pulling up even more cold water near South America.  The cool water extends along the Equator like a big finger pointing toward the warm pool of water on the other side of the Pacific.  (Figure B.)  During La Niña, the Niño 3.4 region is cooler than normal.

The upwelling of cold water is beneficial to the fisheries off the coast of Peru, because nutrients are brought up from the depths.  Unfortunately for Texans, however, the presence of La Niña causes the jet stream air currents carrying moist Pacific air toward North America to deviate northward, taking much needed moisture away from the southern United States and depositing it in the Northwest and Canada.  (Figure C.)

La Niña conditions are typically associated with drought in Texas, but not always.  During the drought of record from 1950 to 1956, La Niña conditions existed for all of 1950, and then disappeared in the spring of 1951.  A very strong La Niña recurred in 1954 and persisted until spring of 1957, when the drought ended.  The two lowest annual rainfalls ever recorded for Austin (Camp Mabry) were 11.42 inches in 1954, and 15.41 inches in 1956.  On the other hand, the strongest and longest La Niña ever recorded occurred from the summer of 1973 until the summer of 1976.  In spite of La Niña conditions, annual rainfall in Austin for those four years was 38.7 inches, above average for the region.  (Figure D.)

Typically La Niña alternates with El Niño every three to five years.  An individual La Niña lasts 9-12 months, but occasionally a La Niña episode has persisted for more than 2 years.  La Niña conditions are usually strongest during the winter months of the Northern Hemisphere.

La Niña began during the summer of 2010 and continued until spring of 2011.  It then abated for the summer but came back in the fall.  It is not as strong this year as it was last year.  The National Weather Service predicts that La Niña will persist through April of 2012 with higher than average temperatures in Texas through the summer and fall.  They won’t commit themselves about the prospect of rain.

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