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NOAA Scientist Reviews El Nino

Bryce Anderson
By  Bryce Anderson , DTN Senior Ag Meteorologist
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This comparison of "typical" El Nino precipitation versus the 2015-16 event shows how El Nino moisture has deviated from the usual expectations. (NOAA graphic by Nick Scalise)

Following are excerpts from an interesting post by NOAA climate scientist Michelle L'Heureux on the NOAA Climate.Gov website. Ms. L'Heureux's perspective on how the 2015-16 El Nino has performed is worth attention since this winter, the crop weather discussion has basically been "All El Nino All The Time." --Bryce Twitter @BAndersonDTN

Is El Nino missing? What happened to the rainfall I was promised? What is going on? Some quick answers for those who don't want to read it all:

I. Is El Nino missing? No. It's still here. Based on measurements of sea surface temperature in the tropical Pacific Ocean, this El Nino is the biggest event we've seen in almost twenty years.

II. What happened to the rainfall I was promised? NOAA CPC (Climate Prediction Center) climate forecasters don't promise precipitation. For some regions of the U.S., we provide seasonal outlooks for an increased chance of precipitation over a span of a 3-month (seasonal) average.

III. What is going on? I really don't know. I ask myself this all the time (particularly during election years).

Here, I'm going to present some recent observations and show how they compare to a typical El Nino pattern. Hopefully you'll walk away with a couple impressions. The maps below are of the atmospheric flow (1) and precipitation anomalies, or departures, from the December & January average during El Nino (2). Keep in mind no single year ever perfectly matches the "normal" El Nino pattern (3). But how well is the pattern matching up so far this winter?

The warmth in the tropical Pacific related to El Nino fuels a more southward-shifted, eastward-extended Pacific jet stream. The jet helps to drive moisture and precipitation into California. For the December 2015 and January 2016 average, we also can see an eastward expansion of the Pacific jet stream and wetter conditions prevailed over the northern half of California and the Pacific Northwest.

El Nino is also linked to anomalous winds that extend from Central America to Florida and is linked to increased stormy weather and precipitation over the Gulf Coast and Florida. So far this winter, we see a similar flow pattern with wetter conditions over portions of the Southeast and an uptick in tornadoes over Florida.

Impression #1: Though this year's impacts do not perfectly mirror the typical pattern, we're already seeing El Nino's thumbprints. But he's like the temperamental kid in art class who can't quite stay in between the lines with his finger paint. Part of that is because there are other weather and climate patterns, which provide distractions. At a given time, El Nino is never the only influence on the atmosphere, but it is the most predictable, which is why we like to talk about it.

Though the Pacific jet stream is juiced up and roaring this year, it is shifted slightly north of its typical position. Likewise, the wetter-than-average West Coast signal has expanded northward into the Pacific Northwest as well. This brings us to Impression #2: The details matter. Even subtle changes in the overall circulation can result in noticeable impacts. It is difficult to anticipate some of these shifts well in advance.

While El Nino is generally best seen in averages across 3-months (seasons), month-to-month variations can be significant. Again, there can be other things going on in the atmosphere, which can override El Nino. After all, El Nino doesn't mean the jet stays locked in the exact same place all winter and spring, just that it tends to return to a particular location more frequently than usual. For example, the jet shifted north during December when it just so happened that we simultaneously saw a very positive Arctic Oscillation (AO) state, which might have contributed to this movement. Then in January, the jet shifted back southward, more closely resembling the typical El Nino pattern.

These unpredictable details, like the AO, are why CPC climate forecasters provide the odds (probabilities) for certain impacts, and they are never 100 percent over the U.S. For example, this fall, forecasters indicated there would be a 60 percent chance of increased precipitation over southern California during the winter due to the knowledge there would be a strong, potentially historic El Nino. A reliable probability forecast means that, for similar outlooks over a long record, 6 out of 10 years should end up wetter and 4 out of 10 years should be near average or drier.

All in all, El Nino is enthusiastically reporting to work, even if its impacts are not identical to the "normal" pattern, and will exert an influence through the spring (March-April-May). It has peaked by most atmospheric and oceanic measures, but it will continue to push the circulation around not only over North America, but also across the globe.

Some additional details worth noting from Ms. L'Heureux's post---

To emphasize, we have never ever seen a winter that looks exactly like the "normal" El Nino pattern. Folks (including me!) love collecting past states from history and averaging them up to see what to expect--and it can be useful summary information--but it's also misleading because we know the real world will be somewhat different. Just keep that in mind whenever you see the words "average El Nino pattern" or "El Nino composite."

Another reason for the lack of similarity between the El Nino pattern and these 1- and 2-month snapshots is that they are always noisier and more chaotic. This is akin to the idea of how global warming is best seen in the long-term trends over decades, but is harder to see with shorter-term bumps and wiggles. El Nino is generally best seen in seasonal (3-month) averages. Ideally I would have...the December-January-February average, but I don't have the February data yet.

The full article with additional graphics is at this link:…



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