El Niño Southern Oscillation

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 Niños, 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. As a result, the upwelling that normally occurs off the Peruvian Coast is decreased and the water warms and becomes more sterile, often having disastrous effects on the local anchovy population. During La Niñas, 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.

Before we delve any further, let’s take a look at our Nino “regions.”

Atmospheric scientists and oceanographers refer to certain regions when talking about SST in the tropical Pacific. For example, they might say “temperatures were 1 degree Celsius above average in Nino 1+2, and 0.7 degrees Celsius above average in Nino 3.4.” Nino 3.4 is the most commonly cited region, and El Nino conditions are often classified as anomalies of 0.5 degrees C or above in Nino 3.4. Likewise, La Ninas are defined as having anomalies of at least -0.5 degrees C in Nino 3.4. If temperatures are within 0.5 degrees of normal, we are in the “Neutral” phase, or, as some like to call it, “La Nada.”

A picture’s worth a thousand words, so let’s take a look at some Neutral, El Niño, and La Niña 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.

Taken as an overall average, Neutral conditions generally lead to “average” meteorological conditions throughout North America. However, individual years can be highly variable. Additionally, as the graphic below shows, most of the Pacific Northwest’s biggest storms occur during Neutral years.

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, there is a large area of low pressure off the coast, causing the jet stream to drag further south. As a result, the northern half of the US is warmer and drier than normal, and the southern half is cooler and wetter than normal.

Credit: NWS

Credit: NWS

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. However, as the most recent strong El Nino (2015-2016) has shown us, that is not always the case, as much of the Pacific Northwest had the wettest winter on record while Southern California suffered through yet another drier-than-normal winter.

We’ve had three particularly strong El Ninos since 1950: one from 1982-1983, one from 1997-1998, and one from 2015-2016. Take a look at these pressure anomalies below and the large area of low pressure over the Pacific. Graphics created with NCAR dataset reanalysis tool from ESRL.

During Neutral years, SST are generally around 8 degrees Centigrade warmer in the western tropical Pacific than the eastern tropical Pacific, but in exceptionally strong El Niño years like 1997-1998 or 2015-2016, the temperature difference can be less than 3 degrees C.

Now, let’s move on to La Nina.

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.

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. 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.

Credit: UW Atmospheric Sciences

Credit: UW Atmospheric Sciences

Here are some upper-level charts for the three largest La Ninas since 1950, and you can clearly see how there is a large area of high pressure in the Eastern Pacific with them. Again, these graphics were created with the NCAR dataset reanalysis tool from the ESRL.

2007-2008 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.

That’s all I have for now! I’ll expand on this and further organize it as time goes on.