What is El Niño?

BOSTON — You've likely heard talk about the very strong El Niño this season, but do you know what that is? Here's a quick explanation. 

El Niño is a term coined by fishermen along the northwest South American coastline. They recognized times when the water in their fishing grounds would warm enough that it would impact the amount and kinds of fish they could catch.

Since they also noticed it seemed to happen near Christmas, they named the phenomenon after the baby Christ child true. They had no other science, but scientists later recognized a periodic shift in the trade winds over the equatorial Pacific. The connection was made between this wind shift, called the Southern Oscillation, and the change in water temperature observed with El Niño. So, the entire pattern is actually called ENSO, or El Niño Southern Oscillation. 

For reasons not fully understood, the trade winds in the Equatorial Pacific, which normally blow to the west, weaken and shift to blowing east.

Normally, the westerly-blowing winds push warm water at the surface to also move west. This allows nutrient-rich cold water from below to replace it near the surface. That's great for fishing and is called upwelling. The water temperature will cool the air above it, which will, in turn, change the air pressure. Cooler air sinks, exerting more pressure on the surface. So, an area of high pressure develops.

You know from listening to us on FOX25 that high pressure is typically dry weather. That's a typical pattern in northwest South America, like Peru.

Well, turn that around. When the trade winds shift easterly, the warm water sloshes back to the east. Also, upwelling won't occur because the warm surface water isn't being blown west.

Not only is that bad for fishing, but now the air above the water is being heated. That warm air will rise, exerting less pressure on the surface. This rising air becomes stormy low pressure. Much more rain happens in places like Peru in those years.

The big problem is not knowing when to expect the shift. Since measurements have begun, the period is every two to seven years. Clearly that's a lot of variability.

On top of that, the best computer models aren't very good at predicting El Niño. One year ago, almost every model had no El Niño happening this winter. By the summer, they did have it, but weaker than what we are seeing. 

Bottom line is we are in a strong ENSO now.

Looking back over the years, the strength of this one rivals two others, 1982-83 and 1997-98. It was forecast to be closer to '97-98 though is running behind it so far.

The U.S. weather pattern is closely resembling that year - wet and cool in the west and mild and less snowy in New England. If that continues, we would be hurting for snow this winter.

The winter of '97-'98 only gave us 25.6 inches. So, now into the spring, this may end up being more like '82-83. That year we had 32.7 inches with 22" of that in February. Still below normal, but shows there is hope if you love snow. 

Of course, there is more to look at than just ENSO, but that's another discussion.

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