What is La Niña anyway? Why does it matter? and What does it mean for our winter weather?
Let’s answer these one at a time:
- La Niña (literally the little girl in Spanish) is a sea surface temperature anomaly in which the upper 300 feet (100 meters) of the tropical sea surface is cooler than average along the equator between South America and Indonesia. (See the graphic below) It is the opposite of El Niño which is characterized by warmer than average equatorial sea surface temperatures along the Pacific Equatorial region. The La Niña forecast for this winter is for a weak anomaly peaking in late winter.
The diagram above shows a below average “blue-colored plume” of equatorial water between South America and Indonesia. This is the unmistakable signature of La Niña. North America is to in the upper right of the diagram.
- It matters to the Pacific Northwest because it causes changes in the tropical wind and moisture currents over much of the northern hemisphere. The changes in wind patterns cause a ripple effect that changes global wind patterns. The change usually allows cooler air to migrate southward from the arctic and nudges the jet stream over the Pacific Northwest where it translates into cooler, and generally wetter winters.
Having said that, there is of course a generalization made by many people equating bad winters with La Niña and warmer drier winters with La Niña’s brother, El Niño. The statistics do indicate there is a correlation but it is not a very good correlation. In fact, the winter of 2016-2017 was one of the coldest and wettest in 30 years for Western Washington. This occurred as a very strong El Niño was still in place over the Pacific equatorial zone. By all accounts the winter of 2016-2017 should have been very dry and warmer than average. It certainly was not.
There are many cases when this correlation is not evident because there are a lot other factors at play in the atmosphere that can have a more direct role in the winter weather in Western Washington.
There is enough data, however, to suggest that there is certainly an influence on winter weather generated by a La Niña. Even though the connection between these events is loose, there still is a statistical correlation between La Niña and a cooler, wetter winter.
- What this means is that we may see a cooler than average and wetter than average winter this year. But here’s the answer to the headline question “is this cold and snow we are seeing in the first week of November a result of La Nina”? La Niña and El Niño effects are not generally seen until mid to late winter and into early spring so the answer is probably not.
Don’t forget that only a couple of weeks ago we were in the mid to upper 60’s through most October and that was not caused by El Niño so there are more impactful patterns that effect the local weather and climate than just the tropical Pacific sea surface temperature anomalies. The old adage is somewhat true about the butterfly flapping its wings in the Pacific can cause a hurricane in the Atlantic. Not necessarily that dramatic but the input or withdrawal of energy in one part of the global weather system can have unpredictable (or difficult to calculate) outcomes throughout the whole climate pattern.
La Niña is not the bringer of a weekend of cold weather or an early snow but it could bring a cool trend later in the season. I’m still not going there. I think we will just about average for the winter.