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September 2020 ENSO update: La Niña is here!

La Niña conditions were present in August, and there’s a 75% chance they’ll hang around through the winter. NOAA has issued a La Niña Advisory. Just how did we arrive at this conclusion, and what does a La Niña winter portend? Read on to find out!

Checking the boxes

Let’s revisit our La Niña decision tree.

Flowchart

Flowchart showing decision process for determining La Niña conditions. Figure by Fiona Martin, adapted by Climate.gov.

The answer to the first question, “Is the monthly Niño3.4 sea surface temperature anomaly equal to or less than -0.5°C?” is an easy “yes.” August’s value was -0.6°C according to our most consistent sea surface temperature dataset, the ERSSTv5 (though that is not the only SST dataset we monitor). For a quick refresher, the Niño3.4 sea surface temperature anomaly is the difference from the long-term average temperature of the surface of the Pacific Ocean in the Niño3.4 region. In this case, the long-term average is 1986-2015.

Monthly SST values

Monthly sea surface temperature in the Niño 3.4 region of the tropical Pacific for 2019-2020 (purple line) and all other years starting from neutral winters since 1950. Climate.gov graph based on ERSSTv5 temperature data

The second step is “Do you think it will stay more than half a degree cooler than average for the next several months?” and again, the answer is “yes.” Most of the dynamical computer models predict that the sea surface temperature will remain below the La Niña threshold of -0.5°C through the winter.

Plume graph

Climate model forecasts for the Niño3.4 Index. Dynamical model data (black line) from the North American Multi-Model Ensemble (NMME): darker gray envelope shows the range of 68% of all model forecasts; lighter gray shows the range of 95% of all model forecasts. NOAA Climate.gov image from University of Miami data

Now, on to the critical third step: “Is the atmosphere showing signs of a response to the cooler-than-average sea surface?” Another “yes!” La Niña intensifies the contrast between the warm far western Pacific and much cooler eastern Pacific, and so La Niña’s atmospheric response is a strengthening of the Walker circulation. This large-scale circulation pattern is characterized by air rising over the very warm waters of the far western Pacific and Indonesia, traveling eastward high in the atmosphere, sinking over the eastern Pacific, and traveling back westward near the surface. (Creating the trade winds—more on those in last month’s post.)

When the Walker circulation is stronger than average, the trade winds are stronger, which we observed in the end of August and early September. More rising air over the far western Pacific means lower air pressure, while descending air over the eastern Pacific means higher air pressure; the contrast between these two arms of the Walker circulation is measured using the Southern Oscillation Index and the Equatorial Southern Oscillation Index. Both indexes were positive in August, at 1.1 and 1.0 respectively. These values, which are in the top 20% of the 1950–present record, indicate a stronger-than-average Walker circulation.

La Niña impacts

La Niña’s altered atmospheric circulation over the Pacific Ocean affects global weather and climate. While every ENSO event (and every winter!) is different, La Niña can make certain outcomes more likely. This includes more rain than average through Indonesia, cooler and wetter weather in southern Africa, and drier weather in southeastern China, among other impacts.

December-February impacts

Typical winter (December–February) temperature and precipitation impacts from La Niña. Map by climate.gov.

One important global impact of La Niña is its effect on the Atlantic hurricane season. La Niña reduces wind shear—the change in winds between the surface and the upper levels of the atmosphere—allowing hurricanes to grow. The likelihood of La Niña was factored into NOAA’s August outlook for the Atlantic hurricane season, which favored an “extremely active” season.  As of September 8th, we have seen 17 named storms so far this season, and the forecast is for a total of 19-25 named storms (the hurricane season ends on Nov. 30th). 

La Niña affects US weather through its impact on the Asia-North Pacific jet stream, which is retracted to the west during a La Niña winter and often shifted northward of its average position. Tom wrote a great explanation of the La Niña/jet stream mechanics and impacts here. Generally, La Niña winters in the southern tier of the US tend to be warmer and drier, while the northern tier and Canada tend to be colder. Official seasonal outlooks are available from the Climate Prediction Center, and Nat will be writing about CPC’s winter outlook for the blog in November.

La Nina impacts

Average location of the jet stream and typical temperature and precipitation impacts during La Niña winter over North America. Map by Fiona Martin for NOAA Climate.gov.

We have a bunch of information on La Niña impacts here on the ENSO Blog! In the early stages of our last La Niña, 2018–2019, Tom mapped out the temperature and precipitation during every La Niña winter on record. Also, guest posts have covered La Niña’s effect on snow in the US, a potential link with tornado season, and how La Niña can interact with other climate variability patterns. And of course, we’ll be here every month, updating you on how ENSO conditions are evolving.

Comments

It's been a very dry fall and winter. Paraguay and parana river are at half their volumen. This plus covid is bringing hughe impact on our economy. Waiting for the rains to come before the soybean planting season. Where can I see precipitation forecast for this region with la niña impact for the YTG? Thank you!

Many thanks for this discussion. Does not bode well for more rain for us in the US SW. Stay well.

Que buena información sobre LA NIÑA muy valiosa para nuestros paises

Hello, so with upcoming La Nina how is Europe affected? Does it bring colder temperatures and more snow like I read on a newsite? I´m especially interested how it affects weather in germany, spain and UK

Since Europe doesn’t border the Pacific Ocean the effect is probably negligible. I imagine the temp of the Atlantic and Mediterranean have more to do with Europe.

In reply to by Martin

Your latest blog update was again very informative and entertainingly readable. So thank you very much for that and please go in that way. Even not experts can understand those phenomenons and like your blogging.

Does La Nina has any effect on european weather especially around uk and germany?

Was hoping for a wet cold winter after this dry dry dry and hot summer.

Is it possible for the Pacific Cold Tongue to be getting cooler in general? The idea is that equatorial heat moving to higher latitudes via increasing tidal strength. (This would be due to post-Einstein gravity theory. It could be confirmed by Venus's climate change attributable to recent bio-signature instead of microbial life)

Anecdotally, the Pacific northwest is actually seeing dramatic warming, although this year is a little cooler, we have seen dramatic warming in northern CA and OR. I live in an area that 40 years ago had over 300 days a year in the fog - great for the Redwoods, but now we see less than 100 days a year with fog, which generally burns off by noon. Not so good for the Redwoods. Even ten years ago a trip to the beach in the winter included ski jackets and gloves, now a light jacket and a hat for the sun.

Will Aruba get more rain in the coming months affected by la niña???

According to a Climate Prediction Center analysis, the southern Caribbean has received more rain than average during past La Niña winters. (You'll need to mentally reverse the colors in that map, since it's for El Niño).

In reply to by Remick Ras

What will this mean for the mountains of West Virginia north to New England? I remember 2010-2011 was a strong La Niña and was a great winter but I also remember other la ninas that lead to bad ski seasons in the East. Does the strength of the La Niña impact how Far East the cold air will reach?

Check out our guest post on snow during La Niña. Also, the Climate Prediction Center has maps showing what temperature, precipitation, and snow patterns have looked like during past La Niña winters here. In general, La Niña winters tend to have more snow in New England.

In reply to by Clayman

Yes, La Nina has significant effects on agriculture due to its impacts on rainfall, snowpack, and temperature patterns. This study shows that La Nina generally causes global declines in maize, soybean, rice, and wheat production, but you can see that the relationships vary across regions. This link provides a discussion of US agricultural impacts during our last La Nina, and this one describes the impacts over the Southeast US in more detail.

Hello, which is the impact between Seychelles archipelago and South Africa. We are sailors so it’s Important for us to know. And what about the cyclones?

During austral summer, La Nina tends to produce cooler- and wetter-than-average conditions from around Madagascar to locations south and west. You can see these impacts on the climate.gov ENSO impacts page. This relationship tends to break down by around March. 

 

Regarding tropical cyclones, the relationship with La Nina is a bit challenging to discern. This study suggests that La Nina causes less frequent than normal cyclone occurrences, but this one also suggests that La Nina leads to stronger and longer-lived cyclones in the region. Therefore, the relationship between cyclones and La Nina over the southwest Indian Ocean may be complex and not fully understood.

I have doubts about the different approaches to when the ENSO phase really is. We are in La Niña, however through the website: "https://www.ncdc.noaa.gov/teleconnections/enso/indicators/sst/" "El Niño (La Niña) is a phenomenon in the equatorial Pacific Ocean characterized by a five consecutive 3-month running mean of sea surface temperature (SST) anomalies in the Niño 3.4 region that is above (below) the threshold of + 0.5 ° C (-0.5 ° C). This standard of measure is known as the Oceanic Niño Index (ONI). " To which refer to "... five consecutive 3-month running mean ..."

Yes, that description is correct. I am not sure if I understand your question correctly, but the "five consecutive 3-month running mean" refers to overlapping 3-month seasons. For example, if July - September (JAS) satisfies the -0.5C ONI criterion, then the following four seasons (ASO, SON, OND, and NDJ) must also satisfy that criterion for La Nina conditions to occur. Because NOAA declared a La Nina Advisory, it means that they expect the ONI value to remain at or below -0.5C through the rest of the fall and into early winter (NDJ). 

In reply to by José Faccin

Exactly that and I thank you for the answer. Extending my question, what if, for example, OND and NDJ do not confirm an anomaly below -0.5 ° C? I really have this question and thank you for your attention in answering! Hug from Brazil. José Faccin josefaccin@gmail.com

In reply to by Nathaniel.Johnson

Glad that I could answer your question! If the sea surface temperatures in the Nino3.4 region go above the -0.5C threshold earlier than expected (and they are expected to remain above -0.5C), then NOAA will end the La Nina Advisory. If OND and NDJ ONI wind up above -0.5C, then this event will not be categorized as a La Nina based on the ONI historical record. 

I live in Colorado, where we have some HUGE wildfires going on. Couple that with the West Coast wildfires and La Nina seems to be just the OPPOSITE of what is needed. We usually wait for fall (snow) to finish putting out the spring/summer wildfires. It does not give me much hope. Man, 2020 has sucked. Any thoughts on La Nina and wildfires this year?

Hi Angela, 

I responded to this question on your other post, but pasting it here, too: 

We have post about this exact question: 

https://www.climate.gov/news-features/blogs/enso/what-about-snow-during-...

Although all events are somewhat different, on average, La Niña is good for snow in Northern CO, so if we see a typical La Niña, it should be good for fires in the northern mountains of the state. 

I live in Tucson, Southern AZ, this years wildfires and the weather impacted us a lot. Particularly with the smoke and my son's Asthma. I had stumbled across your Satellite and Product Operations with the Hazard Mapping System of Wildfires and Smoke Tracking back in June. I had noticed that almost the entire mountain spine of Mexico was engulfed in wildfires, practically to the US border, most of this summer. I was wondering if those wildfires also had an effect on the weather. We didn't have much of a monsoon this year. 2020 was a very bad year for wildfires as the fires where from Southern Mexico into Canada, across most of the southern US and even the Caribbean. Smoke from the California wildfires had reached Europe in September. Are there any discussions on the affects all that the smoke and ash might have on the weather and for how long? I know that volcano's tossing up massive amounts of smoke, ash and other gasses effect the weather. I'm sure that the amount of smoke and ash from the wildfires should have an effect on this years and early next years weather too. It would be interesting to know if the smoke and ash, combined with either El Nino, La Nina or the Enso conditions could cause significant weather pattern changes. Or bring heavy rains and floods to some regions. Do you have any thoughts on this?

How does La Nina typically affect water levels on the great lakes? Currently they are at record level highs, will La Nina be good, bad, or indifferent as it relates to the water levels.

Does La Niña mean lots of snowfall for central Idaho?

It's a little hard to tell where the heavier rainfall (green) area is. I'm trying to determine if we're likely to have greater rainfall than usual in northern CA, specifically the Sacramento region. Can you point me to any sites that would have this information? Thanks.

How much longer will we remain in this pattern of seeing such warm weather in northern Georgia. We seldom see the teens or snow any more. Is this just climate change due to la Nina?

Well, the combination of climate change and La Nina is likely tilting the odds in favor of warm weather in northern Georgia. The latest U.S. Winter Outlook from the CPC (more on that on the ENSO Blog in a couple weeks!) indicates that a warmer- and drier-than-normal winter is favored for the southern U.S., including Georgia, and La Nina is a major player in this forecast. However, that certainly doesn't rule out the possibility of some cold and snowy periods. For much of the South, all it takes is one storm to reach average snowfall for the season. That's likely why we do not see a significant impact of La Nina on winter snowfall for the region, as shown in Dr. Stephen Baxter's guest post.  

In reply to by Tami Clemmons

Do you think the pattern is showing much snow for Missouri this year?

Winter snowfall amounts are very hard to predict. The most NOAA does is predict whether the precipitation amounts (both rain, snow and ice) will be above-, below- or near-average. Find those forecasts here.
But you can check out what La Nina winters normally mean for snowfall in this post. 

https://www.climate.gov/news-features/blogs/enso/what-about-snow-during…

In reply to by Bonnie Randall

Itís hard to come by knowledgeable people on this subject, but you sound like you know what youíre talking about! Thanks 

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