Could El Nino Return This Summer?

Could El Nino Return This Summer?

Rumors of ol’ El returning for summer 2017 have been percolating through meteorological echo chambers for the past few months, but in the past few days, mainstream media outlets like the New York Times have spilled the beans to the bourgeoisie and proletariat alike, alerting Americans of all walks of life that another El Nino may be in store for summer 2017.


As a forecaster, it’s always a bit risky to go sounding the El Nino alarms in spring due to the “spring predictability barrier,” which I will touch on later in this blog. But climate models are in reasonable agreement that a weak-moderate El Nino will develop over the next few months. If this were to happen, it would be the most rapid switch from El Nino to La Nina and then back to El Nino since the July 1963- April 1966 time period, when we swung from El Nino to La Nina and back again over three consecutive winters, only spending a combined 6 months over that time period in Neutral, or as I prefer to call them, “La Nada” conditions.

The tropical Pacific, which can be thought as ground zero for El Nino, is currently giving us mixed signals as a whole. While the Eastern Pacific appears to be in full-on El Nino mode, the Central Pacific is still firmly in Neutral conditions, and the atmospheric circulation over the most of the tropical Pacific still looks more reminiscent of a La Nina!

Let’s start by taking a look at some of the current sea-surface-temperatures (SST) in the tropical Pacific.

Tropical Pacific Sea-Surface Temperatures
Tropical Pacific Sea-Surface Temperatures
Credit: Climate Prediction Center

The animation above clearly shows temperatures getting warmer. Part of this is because we are transitioning out of a La Nina, but part of it is simply because we are transitioning away from winter to spring, when the sun is more directly positioned over the equator. To get a better idea of the status of El Nino/La Nina in the Tropical Pacific, we really need to be looking at SST anomalies. Let’s take a look at those now.

Tropical Pacific Sea-Surface Temperature Anomalies
Tropical Pacific Sea-Surface Temperature Anomalies
Credit: Climate Prediction Center

As you can see, temperatures in the Tropical Pacific are near-normal in the Central Pacific, but are well-above normal in the far Eastern Pacific. Thus, it would appear that a “coastal El Nino” is taking shape. However, the far Eastern Pacific (Nino 1+2 in the graphic below) has cooled over the past couple weeks.

Taken from the Climate Prediction Center’s most recent El Nino/Southern Oscillation (ENSO) Diagnostic Discussion

Meanwhile, the atmosphere actually still appears to be in a configuration more reminiscent of something we would expect with a La Nina pattern! During a La Nina, the Walker Circulation, which is name given to the large-scale circulation over the tropical Pacific where easterly trade winds move water and air to the Western Pacific, is stronger than normal. This results in wetter weather over Indonesia/Northern Australia and drier weather off the Peruvian Coast. During an El Nino, the trade winds weaken, resulting in a less robust Walker Circulation with a flatter east-west sea-surface-height distribution, a more gentle-sloping thermocline, relatively drier weather over Indonesia/Northern Australia (and often devastating forest fires in Indonesia due to the widespread practice of slash and burn agriculture there), and relatively wetter weather over the Peruvian Coast.

Walker Circulation
Credit: Wikipedia User PAR

The map below shows the outgoing longwave radiation anomalies, where “outgoing longwave radiation” is just fancy-talk for the amount of heat that escapes from the surface of Earth to space. Clouds help prevent some of this heat from escaping, so where you see negative anomalies, there are cloudier skies than usual. As you can see, there’s a LOT of blue over the Western Pacific! But then again, there is some blue over the Peruvian Coast as well where our “Coastal El Nino” is brewing. As is often the case, one is left with more questions than answers after looking at maps like these.

Credit: Climate.gov
Retrieved from Anthony Barnston’s 4/13/2017 blog on the current status of El Nino

Since we just came out of a La Nina regime in the Central Pacific, I decided to look into whether it is common for the Walker Circulation to lag sea-surface-temperatures in the tropical Pacific. I discovered a paper by Kumar and Hoerling (2003) that found a 1-3 month lag in upper-level atmospheric heights in the tropical Pacific and a peak in rainfall anomalies over the Western Pacific during the late winter over the 1950-1999 time period. This would lead me to believe that this type of disconnect we are seeing between the ocean and the atmosphere isn’t all that uncommon. However, some of the top climatologists around are very perplexed with the uniqueness of the current situation. Perhaps Mike Halbert, deputy director of NOAA’s Climate Prediction Center in College Park, MD, summed it up best when he said, “if you just look at the current state of the ocean and the atmosphere, it doesn’t really look like what we typically expect to see as we head into El Niño. There’s been a little bit of head scratching.” (New York Times)

The Spring Predictability Barrier:

For some reason, it is much harder to accurately forecast El Nino and La Nina events before spring than after spring. There are many hypothesized reasons for this – one is that spring is the time when El Nino/La Nina are decaying, and that it is easier to predict the evolution of an El Nino/La Nina when one is already underway. Evidence for this theory is supported by the fact during the springtime, dynamical models – models that predict the future state of the land, ocean, and atmosphere by plugging in a set of “initial conditions” into a given suite of equations, outperform statistical models – models that predict the future by drawing statistical correlations between the present and historical conditions. This is likely because there is a relative dearth of statistical correlation in the springtime compared to other times of the year, particularly during a time like this when the atmosphere and ocean are doing different things.

Let’s define an El Nino as a 5+ consecutive month period with running 3-month mean temperatures above 0.5 degrees C in the Nino 3.4 region. This is the threshold most commonly used by NOAA, as it is a good approximation of when the ocean and atmosphere become coupled, which in this case means that a link is established between the warming of the tropical Pacific and the decreasing strength of the Walker Circulation.

Not surprisingly, the models show quite a bit of variance in solutions over the next several months. Still, nearly all of the dynamic models show a weak-to-moderate El Nino developing, while the statistical models give us a weak El Nino or a La Nada. But most importantly, no model shows a La Nina event occurring. Given our current information, the chance for La Nina returning for next winter appears small – around 10%.

Nino 3.4 SST Anomaly Predictions from a variety of different dynamical and statistical models. Taken from the Climate Prediction Center’s most recent El Nino/Southern Oscillation (ENSO) Diagnostic Discussion

The fact that nearly all dynamical models show an El Nino developing is certainly telling, but the existence of a La Nina-esque Walker Circulation in the Western Pacific has thus far precluded the forecasters at the Climate Prediction Center from issuing an El Nino Watch. The current probabilistic forecast has a 50% chance of El Nino conditions developing by September.

ENSO Probabilistic Forecast
ENSO Probabilistic Forecast
Credit: Climate Prediction Center/International Research Institute for Climate and Society

Yup, all that writing just to tell you that our state of the art climate models are just as useful as a coin flip! But hopefully you learned something along the way.

In the more immediate future, be careful out there today! There are some heavy thundershowers roaming around. In the words of NOAA, “when thunder roars, head indoors!”

Enjoy your Tuesday, and thanks for reading.
Charlie

Resources:

Barnston, Anthony. “April 2017 ENSO update: Conflicting signals from the tropical Pacific Ocean.” April 2017 ENSO update: Conflicting signals from the tropical Pacific Ocean | NOAA Climate.gov. NOAA, 13 Apr. 2017. Web. 18 Apr. 2017.

Fountain, H. (2017, April 13). It’s Like It Never Left: Another El Niño May Be on the Way. The New York Times. Retrieved April 18, 2017, from https://www.nytimes.com/2017/04/13/climate/el-nino-climate-change.html?_r=1

Kumar, A., & Hoerling, M. P. (2003). The Nature and Causes for the Delayed Atmospheric Response to El Niño. Journal of Climate, 16(9), 1391-1403. doi:10.1175/1520-0442-16.9.1391

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