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Environment

If La Niña follows the current super El Niño, it will probably be bad news for drought-plagued California

ImaGeo iconImaGeoBy Tom YulsmanFebruary 14, 2016 4:25 AM

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La Niña tends to cause drying in California, and it often persists — and deepens — for years afterward

ENSO-transition.gif

As the animation above shows, the strong La Niña of 1999/2000 followed the record El Niño of 1997/1998. El Niño is characterized by a large pool of warmer than average waters along the equator in the eastern Pacific. In a La Niña, the opposite occurs. (Source: NASA) The El Niño that has been helping to spawn wild and wacky weather in many parts of the world for months now is still very strong. But the latest analysis from the U.S. Climate Prediction Center suggests that it should start to weaken and transition to neutral conditions by late spring or summer. Then what? If the cooling of the eastern and central tropical Pacific characteristic of a weakening El Niño progresses enough, we could well find ourselves in a La Niña by next fall or winter. That's the opposite of an El Niño, and it typically brings dry winters to California. That, of course, would be bad news for the state — which is still struggling to emerge from an epic drought. Ok. Before I go any further, I need to emphasize an important caveat: Right now, the Climate Prediction Center puts the odds of a La Niña developing at 50/50. That's a coin flip. Still... I thought I'd tell the story of what could be coming over the next year or so with the help of animations and other imagery. The animation up top consists of two graphics based on data from the Topex/Poseidon mission showing the state of the tropical Pacific Ocean. The first image shows conditions during the record El Niño of 1997/1998. That gigantic spear of white, orange and yellow jutting out from South America along the equator is indicative of much warmer than average sea surface temperatures — a hallmark of El Niño. And by the way, the current El Niño ties with 1997/1998 as the strongest El Niño on record. The second image in the animation shows conditions during the strong La Niña of 1999/2000. That uneven blob of purple, magenta and blue along the equator is indicative of much cooler than average sea surface temperatures. This is pretty much the opposite of El Niño, and it is a hallmark of La Niña. The La Niña that ultimately developed after the "super" El Niño of 97/98 was a strong one, as this graphic shows:

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The Oceanic Niño Index, or ONI, shows warm El Niño (red) and cold La Niña (blue) phases of abnormal sea surface temperatures in the east-central tropical Pacific Ocean. (Source: Kevin Trenberth/National Center for Atmospheric Research. Note: I've added some labels to the original graphic. ) Does that mean that a strong La Niña will develop after our current El Niño? Not necessarily. In the graph above, have a look at the La Niña that developed after the 82/83 super El Niño. It's piddling by comparison. The moral here is that a more sophisticated statistical analysis is necessary to reveal anything useful. And at the excellent ENSO Blog, atmospheric scientist Anthony G. Barnston has done just that. Based on statistics of past events, he finds that stronger El Niño's do increase the odds of a La Niña the following year. Overall, Barnston concludes based on the historical record alone that the likeliest outcome is for a weak La Niña to follow our current super El Niño. The physics of strong El Niño's tend to support this view, he says. But he also emphasizes this caveat: uncertainties are high! As I mentioned at the outset, based on all of the evidence at hand right now, the current consensus forecast is for a 50 percent chance of some sort of La Niña during the September/October/November period.

After-La-Nina.gif

The first map in the animation above shows average winter conditions in years following a La Niña. California tends to experience drier than normal conditions. The next map shows what happened during the winter of 2012/2013 and 2013/2014, which followed the La Niña of 2011/2012: Drier still in California. Also in 2013/2014: a large blob of extremely warm water helped lock in a pattern that accentuated the drying. (Images: ENSO Blog on Climate.gov. Animation: Tom Yulsman) I created the animation above to provide a sense of what could happen if a La Niña actually does develop and then subsequently decay to neutral conditions. It is based on imagery that first appeared in another excellent post at the ENSO Blog. The first map in the animation shows average conditions in the year after a La Niña has gone away. Note the browns over California indicative of drier than normal conditions persisting there. Not good. And it gets worse. The subsequent maps in the animation show what happened in 2012/2013 — the year after a La Niña coincident with the start of California's drought — and then 2013/2014. Again, La Niña is long gone. Yet check out those spreading and deepening brown colors over California, and much of the western seaboard of North America too. They tell the tale of a truly epic drought. We can't blame the El Niño-La Niño cycle — which is known as "ENSO" — entirely. Other climatic factors clearly played a role. For example, there's "The Blob," a huge region of abnormally warm surface waters that lingered off the West Coast of North America, helping to lock in a pattern that accentuated California's drying. Will the current super El Niño give way to a La Niña this winter and fall? It's too soon to say for sure. But if it does, we should expect California to experience drying again. And if past experience is an accurate guide, it could get even worse in subsequent years. I probably don't need to tell you that this would be really, really bad news.

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