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March 2020 ENSO update: puzzle time

The ocean surface in the central tropical Pacific has been warmer than the long-term average for a few months now, but overall the ocean-atmosphere system is still in neutral—neither El Niño nor La Niña. The El Niño/Southern Oscillation (ENSO) forecast team estimates about a 65% chance that the tropical Pacific will continue in ENSO-neutral this spring, and about a 55% chance neutral will remain through the summer. We’ll take a spin through the current situation, and lay out some of the puzzle pieces forecasters look at when assembling a picture of ENSO in the future.

Jigsaw

First, though—does anyone remember why we spend so much time and energy predicting the evolution of the temperature of the tropical Pacific Ocean? If you guessed “because El Niño and La Niña have impacts on weather and climate around the world and can be predicted in advance,” you get a gold star! While nothing is a sure bet when you’re making predictions, a developing El Niño or La Niña can provide some valuable clues about upcoming seasonal climate. For more on that, check out Tom’s post.

With that important piece of housekeeping out of the way, it’s on to current conditions. As I mentioned above, the tropical Pacific has been hovering a bit warmer than average through the winter, including in our primary monitoring region, Niño3.4. The February Niño3.4 index was 0.4°C above the 1986-2015 average, according to the ERSSTv5 dataset, our most consistent long-term record. This is just a hair below the El Niño threshold of 0.5°C warmer than average.

Monthly sea surface temperature graph

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 atmosphere in the tropical Pacific region actually looked a bit El Niño-y (El Niño-ish? These are not real words) last month, with greater-than-average amounts of rain and clouds over the central tropical Pacific, and less over Indonesia. Also, the near-surface winds over the western tropical Pacific were weaker than average.

However, some atmospheric features are not consistent with El Niño, such as the atmospheric pressure difference at sea level between Tahiti and Darwin, which is known as the Southern Oscillation Index.  Also, most computer models predict we’ll remain ENSO-neutral (neither El Niño nor La Niña) in the coming seasons. I went into more detail about the forecast process last month—this month was quite similar, so head over to that post if you would like more information.

Brain teasers

Getting into late summer and next fall, though, some models predict that the Niño3.4 index could drop below the La Niña threshold of 0.5°C cooler than the long-term average. So are we gearing up for La Niña next winter? Perhaps, but we’re giving it lower odds. Forecasters are putting the odds of La Niña at around 35-40% by next fall, but there are reasons why we’re not giving La Niña a big edge.

Elo Nino odds graph

Official CPC/IRI forecast of the odds of El Niño, neutral ENSO and La Niña conditions issued in mid-March.

First, check out that Niño3.4 graph a few paragraphs up. The gray lines show every year since 1950 that started from an ENSO-neutral winter like the one we’ve just had. In 22 examples, La Niña has not followed a neutral winter once! Does that mean it’s impossible? Definitely not! But it would be something we haven’t observed before.

Another consideration is the spring predictability barrier. Forecasts made in March, April, and May tend to be less reliable than forecasts made during the rest of the year. This is partly because spring is often a transition time between El Niño, La Niña, and neutral. Also, the tropical Pacific has a particularly narrow temperature range in the spring, with only about 2°C separating the warmest spring Niño3.4 and the coolest. For comparison, the range in winter is more than 5.5°C. Predicting a small change is tougher than predicting a larger one, contributing to the spring barrier.

Trivia night

We’re always looking for more context to interpret model predictions, especially in the spring. An interesting study by friend-of-the-Blog Mike Tippett, of Columbia University, shows that the tendency of the Niño3.4 index in the spring does not have a strong relationship with the tendency through the summer. By “tendency” I mean the direction of the month-to-month change in Niño3.4 temperature. So, if the March Niño3.4 sea surface is cooler than February, that would be a cooling tendency. Mike compared the tendency in February–March to that in April–July.

Niño tendency graph for February–March and April–July

The relationship between the February–March tendency of the Niño3.4 Index to the April–July tendency. Tendency is the change between February and March, or April to July. The blue line indicates the linear regression model that best fits the data. If the relationship between these two tendencies were very strong, the dots would lie close to the line. If there were no relationship at all, they would be scattered randomly all over the graph. Figure by climate.gov from Mike Tippett’s data.

He found that the relationship between spring and summer tendencies is not particularly strong (see footnote*). And, when the spring tendency is very small, it provides even less information about the summer. We don’t yet know what the March Niño3.4 index will be, but it’s likely that it will be very close to February, meaning a tendency close to zero. In the past, a February–March with little change has preceded a wide variety of April–July outcomes. So, while this may help us make predictions in future springs, currently we are not seeing a strong favorite for upcoming ENSO conditions.

The spring barrier will hang around through May, but we’re here year-round. Thanks for checking in. You can look forward to Michelle taking the helm for the April ENSO Update!


* Mike used linear least squares regression, which finds the closest fit of a line to the scatter points in the graph. He found a correlation between the Feb–Mar and Apr–Jul tendencies of 0.46. A correlation of 0.46 equals an explained variance of about 21%: meaning about 21% of the behavior of the Apr–Jul tendency can be predicted based on Feb–Mar.

Comments

An outstanding tour de force. The 0 for 22 stat for ENSO neutral winters being followed by a La Nina winter the next year is quite compelling. Will the current CFSv2 forecast correctly break that streak? https://www.cpc.ncep.noaa.gov/products/CFSv2/imagesInd3/nino34Sea.gif

If enough people use a made-up word long enough, it becomes real. Language be like that.

So I saw a documentary once and a few scientists got together and collected air temps from around the globe the week after 9/11, when virtually all air traffic had been halted. They granted that it was only a week but noted that the temperatures were on average 1.5 degrees Celsius higher, if my memory serves. It might be worth inputting a few less clouds from air travel(I've no idea just how fewer flights there are right now or the actual amount they might contribute to dimming) over the next few months and see if that nudges your models. I wish I remembered which Doc' I saw it in, it was just a very brief part, it might have been Gore's Inconvenient Truth.

I also saw that documentary years ago and recalled it recently in light of the global shutdown that is happening. It was a BBC Horizon documentary aptly named 'Global Dimming.' You can watch it here: https://vimeo.com/138779240 It was also re-packaged for the American audience later the same year in a Nova episode: https://www.pbs.org/wgbh/nova/sun/ If the next few months see continued drastic reductions in air and automobile traffic and other sources of particulate pollution, it will present an opportunity to see if the hypothesis in the documentary holds up. If it does, this might be a hotter and drier summer than it would have been otherwise.

In reply to by Bailey

I am a skier so I follow your blog to try and figure out where to spend the next winter! Western Canada had amazing snow this Nuetral year and La Nina is usually good too so I'm happy for 2021!!

Just a thanks of appreciation.

thanks for explain soo good

Hi Thanks for this, it’s accessible and just enough slapstick!!! If the global dimming figure of 1.5 degrees is right(or the James Hansen figure of 1 degree), we are already at 1.26 degrees for February, if there is a decent El Niño on top are we talking Arctic Blue Ocean Event, multiple bread basket failures and ultimately the end of civilisation as we know it?

My organization prepares an impact outlook (focusing mainly on the socio-economic and humanitarian aspects) for Asia-Pacific countries every time a strong ENSO is predicted. This ENSO blog is a very important resource for us. Thanks to you and your colleagues for packing so much knowledge in every blog, and yet making each one of them so much fun to read. Your 2019 Valentine Day post was particularly awesome! Looking forward to reading your future blogs.

We don’t yet know what the March Niño3.4 index will be, but it’s likely that it will be very close to February, meaning a tendency close to zero. In the past, a February–March with little change has preceded a wide variety of April–July outcomes.

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