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One forecaster’s view on extreme El Niño in the eastern Pacific

This is a guest post by Ken Takahashi, who is a research scientist at the Instituto Geofísico del Perú (IGP) and currently leads the national scientific committee ENFEN, which issues the official El Niño forecasts in Peru. This post does not necessarily reflect the views of IGP, ENFEN or NOAA.

El Niño was first identified by fisherman in the late 19th century off the coasts of Peru and Ecuador (Carranza, 1892; Carrillo, 1893). Unusually high Pacific Ocean temperatures depressed the region’s fisheries, and intense rainfall led to coastal flooding.  The most extreme El Niño events, in terms of the surface warming in the eastern and central Pacific, occurred during 1982-1983 and 1997-1998. During these two events, Piura, a city in the coastal desert in northern Peru, experienced annual rainfall amounts equivalent to the other 40 rainiest years combined! The economic loss due to extreme weather in Peru during those events is estimated as 7% and 4.5% of its GDP, respectively (CAF, 2000).

The desire to help society prepare for those kinds of disruptions has led to great scientific advances in understanding El Niño. Still, one of the most frustrating things about El Niño for forecasters is why it doesn’t have the same impacts in the same places every time. In the past decade, the scientific community began to focus research on the diversity or flavors of El Niño and La Niña (the cold phase) as a possible explanation for the variability of impacts.

Pattern of sea surface temperature deviation from average

Pattern of sea surface temperature deviation from average (°C) associated with a unit value of the C index (top) and the E index (bottom), based on Takahashi et al., 2011. The Niño 3.4 and 1+2 regions are indicated as dashed boxes. Most El Niño events can be described as a combination of these two patterns. Image from Ken Takahashi.

 

In particular, they've focused on where along the equator the surface warming is largest, which does affect how El Niño and La Niña impact the global climate (Larkin & Harrison, 2005). Especially in Peru, El Niño can lead to very different rainfall impacts depending on whether the warming occurs in the eastern (wetter) or central Pacific (drier) (Lavado-Casimiro & Espinoza, 2014).

There are many different ways of classifying El Niño, but it is most common to measure it using sea surface temperature (SST) anomalies (departures from average conditions).  In order to classify the different types of El Niño, however, we need at least two indices or time series (Trenberth and Stepaniak, 2001). Some colleagues and I introduced an E index and a C index (data here), which isolate the SST changes in the Eastern and Central Pacific, respectively, that are unique to each region (Takahashi et al, 2011).

How different are the extreme El Niño events from the regular ones?

Usually the SST warming in the central and eastern Pacific overlap, or correlate, during El Niño. But during the two “extreme” El Niño events (1997-98 and 1982-83), the warming in the east, near the coast of South America was much stronger than the warming farther west in the central Pacific (as can be seen in the left panel below).

Temperature deviation graphs

December-February average eastern and central Pacific sea surface temperature deviations from average: (left) Niño 1+2 (east Pacific; on x-axis) and Niño 3.4 (central Pacific; on y-axis); and (right): E (east Pacific; x-axis) and C (central Pacific; y-axis) departures from average. The year corresponding to December is indicated. Extraordinary El Niño events are indicated in red, while other eastern Pacific and central Pacific El Niño events are in green and blue, respectively. Gray indicates non-El Niño years. In both graphs, the dotted lines are an attempt to summarize the relationships shown by the dots, and the abrupt change of the slope of the dotted line highlights the uniquely different behavior shown by the 1982 and 1997 cases, and to a much smaller extent the 1972 case.

 

In fact, the values of the central Pacific Niño 3.4 index were only slightly greater than those of the 1972-1973 event, but the values were around 3 times greater in the eastern Pacific Niño 1+2 region. These geographic differences are also clearly depicted using the E and C indices (right panel), with very high E values during the two extreme El Niño events. This difference in central versus eastern Pacific warming during extreme events compared to regular ones is also evident in monthly C and E index values (see graphs below).

El Niño monthly SST indices

(top) Central Pacific (C) and (bottom) eastern Pacific (E) monthly SST indices during selected El Niño events and the current year. The estimated values for August 1-19, 2015, are indicated with an open circle. Graph by Ken Takahashi.

 

We found that this is because, once the normally cooler eastern Pacific warms enough for heavy precipitating storms, El Niño shifts to a faster gear: the Walker circulation shifts dramatically towards the eastern Pacific and the processes that lead to El Niño growth strengthen threefold (Takahashi & Dewitte, 2015).

Predicting extreme El Niño this year

If the physics of extreme El Niño events are different, then they should sometimes be analyzed separately from the rest; this also makes sense considering their large societal importance. Of great urgency this year: Are our scientific understanding and models good enough for the prediction of an extreme El Niño?

Although climate models provide objective predictions, models are far from perfect. They have common errors (particularly large in the eastern Pacific) and misrepresentation of slower changes in SST (decadal or 10-year timescales) or SST trends (2). By considering a collection of different models, or a multi-model ensemble (3), we hope that the errors cancel out among the different models. However, there are errors common to all models, such as the warm and rainy tendency in the cold and dry southeastern Pacific.

And we know that the models have a harder time making accurate predictions in the eastern Pacific. In particular, the models do not predict large enough SST anomalies in the far eastern Pacific during the extreme El Niño events (Takahashi et al, 2014). Even so, many models are predicting a strong El Niño in the central and eastern Pacific this year, similar to (or stronger than) 1972-1973, 1982-1983, and 1997-1998.

In addition to models, forecasters have other tools available, such as observational predictors and ideas based on physical common sense. The limitation in this case is the small number of events, with only two well-observed extremes, coupled with the fact that one El Niño is never a perfect mirror image of another El Niño, not even the extremes.

This year the ocean has accumulated a substantial amount of heat, a necessary condition for El Niño, but this does not tell us whether El Niño will be extreme or not in the eastern Pacific (Takahashi & Dewitte, 2015). Again, an extreme El Niño is a very different beast from the others in terms of impacts on weather and wildlife in the coastal regions of northern Peru and Ecuador, so El Niño strength is not just a detail.

One feature we found potentially useful is that if the trade (easterly) winds in the central Pacific become very weak around August, this allows the eastern Pacific to warm up a few months later, possibly enough to trigger strongly enhanced precipitation that could help El Niño become extreme (Takahashi & Dewitte, 2015). This did not happen in 1972, which is perhaps why that El Niño did not become as extreme.

Temperature and wind stress anomaly maps

Difference from average sea surface temperature (colors) and difference from average of surface wind stress (arrows showing direction and strength by the length of the arrow line) in August 1982 (top) and January 1983 (bottom). The red box outlines the averaging region for the wind stress predictor for judging the probability of occurrence of an extreme condition in the Eastern Pacific 5 months later in January. Images adapted from Ken Takahashi.

 

This year we are putting this tool to the test. So far, the trade winds in August have not weakened as much as in 1997 but more than in 1982, indicating the probability of an extreme El Niño in 2015-2016.  However, the eastern Pacific (E index) has been tracking the substantially weaker 1972 event and it would have to surge upwards, as in 1982, to become extreme (Fig. 3b). A quite different outcome could be that E keeps following 1972, remaining below the extreme threshold, while the central Pacific continues to warm into perhaps a larger version of the 2009-2010 El Niño (see bottom graph of Figure 3).

Predicted departure from average westerly wind stress

Predicted departure from average westerly wind stress (see footnote 1) in August (x-axis) vs. the eastern Pacific warming (E) in the following January (y-axis). Observations are in red, while the CM2.1 model ensemble forecasts (repeated model runs with different starting conditions) are grey, with their 10%, 50%, and 90% percentiles shown by the black sloping curves to summarize the positions of most of the gray dots. Adapted from Takahashi & Dewitte (2015).

 

As you can see, the chance of an extreme El Niño in the eastern Pacific is not straightforward to assess (5). Several factors will affect such a estimation. This year’s El Niño is already different from anything seen before. Furthermore, the rules of how the climate system works do not stay the same throughout time (e.g. climate change may affect El Niño), so statistical relationships found in a previous period might not be valid anymore.

Also, it is possible that random factors outside of the El Niño system could go against El Niño to keep it below the extreme threshold. Although several climate models are predicting a very strong El Niño, due to their common errors, we cannot fully trust them. Perhaps the only reliable rule is that El Niño can surprise us, and this year could be yet another example.

Anthony Barnston, lead reviewer

Footnotes

(1) The wind stress is based on the wind speed squared. Here, we are talking about the departure from average of the westerly wind stress. When the trades winds (winds from the east) become weaker, as they do during an El Niño event, the departure from average of the westerly wind becomes positive (because weaker trade winds mean stronger westerly winds, even if the actual wind is still from the east, but less strong than average). Then we square that departure from average. For example, if the westerly wind is usually -9 miles per hour, and now it is only -2 miles per hour, then the departure from average of the westerly wind is +7 miles per hour. And the departure from average of the westerly wind stress is the square of 7 miles per hour, which is 47 miles per hour.

(2) Changes in the entire North Pacific plus tropical Pacific on an approximately 10-year time scale, known as decadal variability, can change the backdrop behind El Niño and La Niña and encourage one of these at the expense of the other. As it turns out, much of the advances in El Niño science took place during a warm Pacific decadal phase, but we have been in a cold phase since approximately the year 1999 (although there are hints that we might be switching back to warm; we need to wait another year or two to make sure). Which decadal phase we are in can subtly, but noticeably, affect the strength of El Niño or La Niña, and our prediction models may not adequately take this decadal variability into account.

(3) A multi-model ensemble refers to the use of more than one model to make a forecast of deviations from average of climate or of sea surface temperature. Because each single model has its own biases or peculiarities, averaging the forecasts of several models tends to cancel these out and deliver a forecast having fewer specific biases. If several models have common biases, however, using more than one model does not help as much.

(4) The 1953-54 El Niño (leftmost green dot in both panels) had its largest warming in the eastern Pacific around mid-1953, but in DJF the eastern Pacific became relatively cool.

(5) Despite the large uncertainties in the eastern Pacific, Peru’s ENFEN will produce an estimate of the probabilities of the various strengths of El Niño, including the extreme type, later this week.

References

CAF, 2000: El Fenómeno El Niño 1997-1998. Memoria, Retos y Soluciones, V. Perú. Perú. Corporación Andina de Fomento, http://scioteca.caf.com/handle/123456789/676

Capotondi A, Wittenberg AT, Newman M, Lorenzo ED, Yu JY, Braconnot P, Cole P, Dewitte B, Giese B, Guilyardi E, Jin FF, Karnauskas K, Kirtman B, Lee T, Schneider N, Xue Y, Yeh SW, 2015: Understanding ENSO Diversity. Bull. Amer. Meteor. Soc.96, 921–938, doi:10.1175/BAMS-D-13-00117.1

Carranza, 1892: Contra-corriente maritima, observada en Paita y Pacasmayo. Boletín de la Sociedad Geográfica de Lima,  1, 344-345.

Carrillo, C.N., 1893: Hidrografía oceánica. Boletín de la Sociedad Geográfica de Lima, 2, 72-110.

Larkin NK, Harrison DE, 2005: Global seasonal temperature and precipitation anomalies during El Niño autumn and winter. Geophys. Res. Lett., doi: 10.1029/2005GL022860

Lavado-Casimiro, W and Espinoza J.C., 2014: Impact of El Niño and La Niña events on rainfall in Peru (1965-2007). Revista Brasileira de Meteorologia, doi:10.1590/S0102-77862014000200003

Takahashi K, Montecinos A, Goubanova K, Dewitte B, 2011: ENSO regimes: Reinterpreting the canonical and Modoki El Niño. Geophys. Res. Lett., doi: 10.1029/2011GL047364

Takahashi, K., Dewitte, B, 2015: Strong and moderate nonlinear El Niño regimes. Climate Dynamics, doi: 10.1007/s00382-015-2665-3.

Takahashi, K., Martínez, R., Montecinos, A., Dewitte, B., Gutiérrez, D., Rodríguez-Rubio, E., (2014), White Paper #8a Regional applications of observations in the eastern Pacific: Western South America, Report of the Tropical Pacific Observing System 2020 Workshop (TPOS 2020), La Jolla, United States, 27-30 January 2014, GCOS report 184, vol. 2, 171-205. http://www.wmo.int/pages/prog/gcos/Publications/gcos-184_II.pdf

Trenberth, K. E., Stepaniak, D. P., 2001: Indices of El Niño evolution. J. Climate, doi:10.1175/1520-0442(2001)014<1697:LIOENO>2.0.CO;2

 

 

Comments

This is an excellent work, a new way of thinking about el Niño. I do think, that if you ask the normal people, what is El Niño? all of them will say, is RAIN!!! I have already done in Ecuador, the reply was that 100%. Now what happens when the news talk about El Niño and there is not rain, like now and 2014, we the scientists lose credibility, don't we?. If El Niño stays in area 3.4, simply there is not El Niño or there is not any visible consequence. Therefore, el Niño got to be defined also in areas 1-2. I would think in the typical indexes such as: 1) Delta T, 2) Winds, 3) Thermocline depth, 4) ZCIT position, 5) Sea level. About what is going on 1-2 now, SST anomalies are decreasing quite consistently and getting away from those recorded on 1997. So then, can we say there a Godzilla on the way? Good article!!!

the 1.2 region is decreasing but will ramp up big time when the next kelvin wave hits which should be in next couple of weeks

In reply to by Franklin Ormaza

Thanks. The definition of El Niño is a tricky thing. In the end, it is guided by practical reasons. In Peru, we (ENFEN) opted to define the "coastal El Niño" in terms of the Niño 1+2 SST anomaly because this is closely linked to the impacts (e.g. coastal rain, fisheries, etc.) and is also part of the large-scale ocean-atmosphere dynamics that provides the predictability. But Peru (presumably also Ecuador) is also impacted by the "central Pacific El Niño" (Niño 3.4 warming), which reduces precipitation in our Andes and Amazon. So, we need to work with the two El Niños at the same time! (Needless to say, this has been a challenge to communicate.)

In reply to by Franklin Ormaza

So refreshing to read a sober minded, scholarly article about El Nino instead of the journalistic equivalent someone hyperventilating into a brown paper bag. That said, I'm a Californian and praying for rain!

Yes, for everyone involved on this poorly treated Planet...hopefully we will all pray together. And BTW, it is an excellent article.

In reply to by Noah Zimmerman

En primer lugar, agradecer al Sr. Ken Takahashi por el articulo y la manera tan sencilla de ilustrar el arduo trabajo que realiza. Soy ex trabajadora del area de OCI del IGP y puedo dar fe de la calidad de trabajo de todos los cientificos que conforman esta digna institucion. Actualmente radico en Piura y mi preocupacion es por el desarrollo de este fenomeno, por ser esta zona una de las mas afectadas, quisiera que siga escribiendo sobre este tema. Por lo pronto gobierno peruano esta tomando precauciones para minimizar el efecto, esperemos que el niño no se porte tan mal en el transcurso del año. Saludos.

Congratulations Ken and colaborators, very interesting post

Dear Ken, Thank you for the interesting read. Most of the latest news on El Nino are predicting for a Godzilla on the way but how much of that is going to take shape in reality is something that we have to wait and watch. Let us hope that El Nino brings some relief to the drought in the west coast. I read elsewhere that California would need 1.5 times it normal rainfall to get out of the extended drought. Can all the precipitation events be linked to El Nino alone? How can we tell what amount / percent of precipitation events are as a result of the El Nino effect? Last year we had a cold winter thanks to the Polar Vortex.. so this year can we say that El Nino is going to keep the polar vortex at bay? Any thoughts on how much of correlation exists between them? Would be interested in knowing your thoughts. Thanks!

Thanks for reading. In summary, El Niño is a large and predictable signal and is therefore its is helpful for climate forecasting but, as you also suggest, it is not the only player. The problem is that the other potentially important influences are not necessarily as predictable. I'm not an expert on US climate impacts, so I refer you to the posts by Tom Di Liberto that discuss these issues for the US and California in particular (https://www.climate.gov/news-features/blogs/enso/fun-statistics-el-ni%C3%B1o-and-california-rainfall, https://www.climate.gov/news-features/blogs/enso/no-you-can%E2%80%99t-blame-it-all-el-ni%C3%B1o), and Professor Hartmann's post, which discusses how the tropical Pacific influences the polar vortex (https://www.climate.gov/news-features/blogs/enso/tropics-prime-suspect-behind-warm-cold-split-over-north-america-during).

In reply to by Lavanya Partha…

Science and common sense beautyfully combined!

Un artículo muy útil para todos aquellos que no somos expertos en el tema y nos ayuda a entender mejor el fenómeno del Niño y a prepararnos para posibles sorpresas. Precisamente, estamos trabajando con los alcaldes de Piura y seguiremos insistiendo en que es mejor prepararse para el Niño este fin de año.

Dear Ken & colleagues, thanks for this very nice dissection of CP vs EP El Nino events. When you calculate the indices, how do you treat the large-scale background warming trends? Since these indices are anomalies relative to a fixed baseline period, a long-term warming trend (i.e., decadal) would project into the index by making it more 'extreme' for El Nino events at least. Is this accounted for? What impact does it have on the ENSO indices and their 'extremity' in recent years, is it important? thanks, Felix Landerer

Dear Felix, We looked at the trends in our 2011 paper (see Fig AM3 under "Supporting Information" in http://dx.doi.org/10.1029/2011GL047364). You're right in that there is a warming trend in E that makes the two extreme El Niño larger relative to older El Niño, but this does not change the fact that they stand out, way outside of the cluster of the other El Niño. On the other hand, trends in the variability (standard deviation) and warm/cold asymmetry (skewness) in E change sign if we recalculate them after removing these two events. They are so large that the statistics of 60 years are very affected by only these two. A more robust and perhaps more interesting result is that the positive trend in the mean C index and its variability is accompanied by an increasingly negative asymmetry. That is, the central Pacific is warming but the extreme La Niñas are getting bigger there. This is consistent with a recent study I coauthored (http://dx.doi.org/10.1038/nclimate2492) that shows this trend in future climate scenarios. Cheers Ken

This year's hurricane season in the Pacific seems unusual (anecdotal observation of mine). For instance, there are three (3) hurricanes of category four (4) in the Pacific basin. Since the trade winds are somewhere between 1997's and 1982's El Ninos, what were the Pacific hurricane seasons like for those respective years and how do they compare to this year's? Also, the Atlantic seems unusually passive. http://www.weather.com/storms/hurricane/news/three-category-4-hurricanes-pacific-kilo-ignacio-jimena

It's a good question. I think (and spoke to Ken about it also) that the short answer is no. First of all, the El Nino tends to influence the tropical cyclone activity, through the energy provided by the warmed SST and its increased atmospheric moisture, rather than the storms significantly influencing the El Nino. El Nino has much more anomalous energy than a few extra tropical cyclones or hurricanes in the Pacific. The circulation around these cyclones cyclones tends to reinforce the wind anomalies associated with El Nino: westerly low level wind anomalies in the tropical latitudes of the Pacific due to the circulations around the storms in both hemispheres. (In the northern hemisphere, the counterclockwise circulation around the cyclones contributes to stronger than average westerly winds on the equator side of the cyclone, and in the southern hemisphere the clockwise circulation has the same effect.) So an excess of cyclones appears to be in concert with El Nino rather than opposed to it. There may be other mechanisms through which the cyclones may inhibit the El Nino slightly, such as the north-south (or south-north) temperature gradient spanning the subtropics of each hemisphere through the equator. I cannot comment on that -- other scientists are welcome to. The extra cloudiness from the cyclone likely does not extend into the deep tropics where the El Nino is occurring, as the cyclones stay well outside of the equatorial belt.

Now that we are in October and have the final September Nino 1+2 and 3.4 values, I'd like to hear what you now think of the current ENSO evolution. Please consider revisiting your blog post.

Dear Ken It is very interesting your interpretation however I think that it will be more real to consider that El Nino began in 2014 in your second graph wherein you compare monthly SST indices. Your interpretation can change if you compare actual El Nino 2014-15 with 1982-83 and 2014-15 Ninos. Also your comparison will be better if you include 1991-92 data when El Nino finished in 1993 like actual El Nino that had a duration of more than 2 years. It is interesting also to see that trade winds persist stronger than normal near south america during extreme El Nino such as David Halpern explained in a conference in Peru on june 2015; he concluded that not statistical significance was observed in winds intensity in the buoys located east of 140 W during Nino or not Nino years; but he found big changes in the high atmosphere more than 20 kms higher than sea level. El Nino 2014-16 is between the 3 strongest of the last 80 years and the end of the actual will be in Peruvian coast weaker than 82-82 and 97-98 due that part of the energy was liberated between april-june 2014. Actually in the faculty of fisheries of the Universidad Nacional Agraria La Molina, we are writing a paper that will demonstrate that El Nino Mechanism and the beginning is very similar in ALL NINOS, however due to the numerous factors that influence its advance (winds intensity and DIRECTION, Cromwell Current,Salinity, Radiation, etc) the end of El Nino it is not predictable actually because all the factors are not considered in the different models. At last 2009-10 El Nino near Peru?

As Peruvian for centuries El Nino is a weather event just focused on the Peruvian coast.It was named by Peruvians (native fishermen) right after Spaniards conquerors arrived because it usually appears near Christmas. The event that modern climatologist nominate El Nino for warming waters on the Central Pacific is a complete different event. Last year there was a big "Nino" for the Central Pacific but for us nothing happened. We prepared for a big event spending millions of dollars and nothing affected us. Naming the events as Nino 3-4, Nino 1-2 are confusing instead of preventing and teaching people. You shouldn't call El Nino for The Central Pacific event.

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