Tuesday, September 11, 2012
9:17 P.M.
It’s El Niño–Southern Oscillation…
…For Dummies!
Let me first start out by saying no copyright infringement is intended. I know that doesn’t change anything, but it makes people less likely to sue you for “stealing intellectual property.” Nobody likes a thief, but people are generally kinder to thieves with good intentions, like Robin Hood.
Since there’s not much in the way of immediate weather, I thought I’d give all my readers the latest on the El Niño developing in the tropical Pacific. But then, it occurred to me that I may have not previously written an article on what El Niño–Southern Oscillation (ENSO) actually is. It turns out I have, but it doesn’t have any fancy illustrations, and was written back in the days when I was doing this blog on Facebook (2008-2009). I posted all my previous blog entries from Facebook onto this website a couple years back. If you want to read it, you can find it here.
Also, if you want to join my group on Facebook, by all means, do so. Since there was that weird group format change thingy, I’ve been the only member. Even though I don’t write the blog there, I post links to this blog, and all you weather nerds can ask me questions on there, or praise me for being the best weather forecaster the world has ever seen, or (God forbid), launch an assault on my soul when snow doesn’t fall, and your term paper I said you could put off for a couple days is due. My Facebook group is here.
So, let’s get down to business.
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El Niño–Southern Oscillation (ENSO) is the name given to the cycle of the semi-periodic warming and cooling of the tropical eastern Pacific. There are three stages to the entire oscillation: El Niño, La Niña, and “neutral,” in which the tropical eastern Pacific is neither in an El Niño or La Niña state. El Niño events are characterized by warmer than average sea-surface temperatures in the eastern tropical Pacific. They generally become apparent in spring, peak around Christmas, occur every 3-5 years, and last for 9-12 months. La Niña events where the sea-surface temperatures are cooler than normal in the eastern tropical Pacific. They also become apparent in spring, peak around Christmas, occur every 3-5 years, and last for 9-12 months, but they can last for 1-3 years. El Niño is Spanish for “little boy,” which refers to the “Christ child” (baby Jesus) because El Niños peak around Christmastime. La Niña is Spanish for “little girl.”
Closely related to the fluctuations in oceanic temperatures are large-scale changes in atmospheric pressure, and resulting differences in trade wind patterns. During neutral events, the air pressure in the western tropical Pacific is lower than the air pressure in the eastern tropical Pacific, causing the trade winds to blow from east to west. During El Niño events, the air pressure in the western tropical Pacific rises, and the air pressure in the eastern tropical Pacific falls, which acts to reduce the pressure gradients between the two locations and decrease the strength of the trade winds as a result. In La Niña events, the normally lower pressure over the western tropical Pacific gets lower, and the normally higher pressure over the eastern tropical Pacific gets higher, which results in a strengthening of the trade winds. The trade winds are essential to the whole ENSO cycle, and they are the driving force behind the differences in water temperature in the eastern tropical Pacific during different respective events.
A picture’s worth a thousand words, so let’s take a look at some pictures of neutral, El Niño, and La Niña events.
Before we go any further, let me define the temperature thresholds for El Niño and La Niña events. An El Niño event is characterized by temperatures that are .5 degrees Celsius above average in Niño 3.4, and a La Niña event has temperatures that are .5 degrees Celsius below average in the same region. Neutral events have anomalies that are less than .5 degrees Celsius above or below average in Niño 3.4.
Now, here is a diagram for what the atmosphere and ocean look like during neutral events.
Air sinks over the eastern tropical Pacific, where there is higher pressure, and air rises over the western tropical Pacific, where there is lower pressure. The trade winds flow from east to west, and push the warm, surface water over towards Indonesia, while cooler, deeper, more nutrient-rich water upwells off the coast of Peru. The upwelling off the South American coast makes for some extremely productive fishing grounds. Because the trade winds are constantly pushing water to the west, the sea surface height in the west is about half a meter higher than it is in the east.
Neutral conditions keep the Pacific Northwest near average in terms of temperature and precipitation, but our biggest storms generally come on neutral years. The rest of the diagram is self-explanatory… neutral conditions generally lead to typical (average) meteorological conditions throughout North America.
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Above are the sea surface temperatures during a neutral event. There looks to be a small pool of warm water off South America, but this still qualifies as neutral conditions, as the temperature anomalies in Niño 3.4 are less than .5 degrees Celsius.
Now, for El Niño!
In El Niño events, the trade winds are weaker, and in this picture, they even slightly reverse in the western Pacific. Since the trade winds are weaker because of less atmospheric pressure differences, there is less upwelling off the coast of Peru, and therefore less cold water is arising from the deep, leading to warmer water temperatures in the eastern tropical Pacific. The thermocline is “flatter” throughout the ocean, and the whole tropical Pacific is more homogeneous in terms of temperature and nutrient concentration. El Niños are bad news for the Peruvian anchovy fisheries, as fewer available nutrients means that there are fewer anchovies. This picture doesn’t show it well, but there is not much of a difference in sea surface height between the eastern and western tropical Pacific during an El Niño event.
During an El Niño winter, the northern half of the US is warmer and drier than normal, and the southern half is cooler and wetter than normal. Southern California is generally a boring place to be as far as weather is concerned, but they can have some pretty intense storms roll through there during El Niño years. I strongly dislike El Niño years because they typically lead to boring winter weather, bad skiing conditions, and, because there is often reduced upwelling off our coast, less salmon.
See that picture below? That’s from the Russian River in California during March 1998, when it was getting pounded by heavy rains that were a result of the 1997-1998 El Niño.
Here is a great picture of sea surface temperatures (SST) during an El Niño event.
During neutral years, SST are generally around 8 degrees Centigrade warmer in the western tropical Pacific than the eastern tropical Pacific, but in strong El Niño years like this one, the temperature difference can be less than 3 degrees C. 1997 was an exceptionally strong El Niño, most of them aren’t this strong.
Whew!!! Two down, one to go. Now presenting: La Niña.
In La Niña events, the trade winds are stronger than normal, which leads to decreased temperatures off the Peruvian coast due to increased upwelling. Atmospherically, the pressure difference (high pressure in the east, low pressure in the west) is greater in La Niña years than neutral years, and this is what increases the strength of the trade winds. The thermocline is sharper because the pool of warm water in the western tropical Pacific is deeper than normal and the pool of warm water in the eastern tropical Pacific is very shallow. There is also more of a sea surface height difference, with the western tropical Pacific being over a half a meter higher than the eastern tropical Pacific. These pictures don’t show the differences in sea surface height between neutral, El Niño, and La Niña events very well, but they are nice diagrams nonetheless. And, since there is a boatload of upwelling off Peru, boats generally have no problem with loading themselves up with copious amounts of anchovies.
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During a La Niña winter, there is often a big fat ridge of high pressure over the eastern Pacific, and this ridge is generally far enough off our coast that we are not under its warm, dry influence. Instead, the jet stream slips down on the eastern side of the ridge from the Gulf of Alaska into our area, giving us cooler and wetter weather than normal. The winter isn’t completely dominated by this northern branch of the jet stream though, as the jet stream coming off the Pacific sometimes sneaks in and gives us warm and wet weather. The southern part of the country is generally dry, and the Ohio Valley is wet. I’m a La Niña fanatic because La Niñas often bring gobs of snow to the mountains and they are good for salmon because there is usually increased upwelling off our coast.
2010-2011 was a big La Niña year, and this diagram above shows it well. The temperature departures in La Niñas usually aren’t as big as those in El Niños, but they are significant nonetheless.
What’s my favorite type of year? La Niña, of course! La Niña years are definitely the snowiest in the mountains and are often the snowiest in the lowlands. Here’s a piece of personal history… this was taken in April of 2008, a huge La Niña year, and there was a ton of snow in the mountains.
Even today, I stand by everything I said in that video. It was AWESOME!!!
Charlie