El Niño and La Niña may have small names, but anyone who watches climate news or weather reports knows that their impact around the world is not small. Like a two-faced mythical beast, the El Niño Southern Oscillation (ENSO) seems to have a mind of its own. Currently, we can’t predict which one we’ll get much more than a few months before it appears. So it’s interesting to look back and ask why an expected El Niño fizzled out in 2014. The answer also appears to help explain why today’s El Niño grew into such a monster.
The southern oscillation of El Niño relates to the pattern of sea surface temperatures along the equatorial Pacific Ocean. In neutral conditions, westward-blowing trade winds push surface waters aside, exposing deeper, colder water along the South American coastline that rises to the surface. Warmer surface water is accumulating on the other side of the Pacific Ocean, near the Philippines.
In a La Niña, more cold water is brought in from the east and the cool area expands to the west. Conversely, an El Niño occurs when warm water from the west sloshes eastward, putting a lid on the upwelling of cold water near South America. The different sizes and positions of these warm and cool stretches of ocean water shift weather patterns around the world and contribute to the global average surface temperature.
Simply put, the El Niño Southern Oscillation is essentially a precarious war push, which is what makes it so hard to predict. All it takes is a few favorable weather conditions for a few months to tip things in favor of El Niño or La Niña.
There are a number of requirements to reach the El Niño side of the spectrum: one on the surface and another below. Researchers keep an eye out along the equator, where the volume of water above the thermocline (the thermocline is the lower limit of the ocean’s warmer surface layer) is critical. As more and more warm water forms, the thermocline is pushed deeper, even if the warm surface water is not yet necessarily extend too far to the east. This volume of warmer water forms a reservoir that the El Niño will draw from as it grows. Without it, the El Niño will quickly dissipate.
On the surface, an El Niño hopeful must overcome the trade winds hampering its eastward advance. Once in a while, just the right weather comes along, and the wind temporarily reverses direction over a certain area in what’s called a “westerly gust.” (Westerly winds blow from west to east.) also add to that reservoir of warmer water just below the surface. The closer you get to El Niño conditions, the more likely these westerly gusts are to occur. One or two good ones can get the ball rolling, giving the transition some momentum.
A new study by NOAA researchers Aaron Levine and Michael McPhaden looks back at the failed 2014 El Niño in view of the one that exploded in 2015. In early 2014, the warm water reservoir built up massively, and some westerly wind bursts made an El Niño look quite likely. But a big one eastern wind gusts in the summer stopped the whole process and we never got beyond neutral conditions.
That’s not news, but the researchers claim it’s a major reason such a strong El Niño developed the following year in 2015. Here’s why: westerly wind gusts don’t just charge up the warm water reservoir, they consume it and breed an El Niño. The reservoir gradually spreads towards the poles until there is nothing left and the El Niño subsides. However, easterly winds do not really disperse the reservoir. They just bottle it up.
The researchers say the 2014 “false start” gave the 2015 El Niño a “head start.” The hot water reservoir charged and the strong easterly wind put the clamps on and locked it in place. When spring rolled around again, the reservoir was preloaded, so a few westerly wind gusts added to it and finally released it.
Something similar happened in 1990. An El Niño was broken up by an easterly wind, finishing 1991. But 2015 saw stronger westerly winds at the start of the year that really kicked things off to a big finish, while 1991 got pretty mundane. Still, these stalled El Niños likely make the next year more predictable — high chances of El Niño conditions with the potential for a big one.
By the way, strong El Niños tap their warm water reservoirs so hard (and leak out so much) that the oscillation normally swings pretty hard the other way right after. In keeping with that behavior, current predictions expect this El Niño to fade by the summer, while the other side of the two-faced beast is likely to emerge this fall.
Geophysical Survey Letters2016. DOI: 10.1002/2016GL069204 (About DOIs).