What Is El Niño – and Why Does It Matter? Q&A with Duke Expert Shineng Hu
With El Niño now officially declared, what does it mean for weather and climate conditions?
It usually starts to develop in the northern hemisphere spring season and peaks in the winter season, around Christmas. That’s why it’s called El Niño, after baby Jesus. In the following spring it starts to die off.
The opposite phase of El Niño, when the central eastern tropical Pacific cools down, is called La Niña.
Is El Niño going to affect weather where we live?
El Niño does influence our local weather conditions, and in different forms depending on where you live.
In general, the Southern U.S. will experience relatively wet conditions. The Northwest and Northeast of the U.S. will experience relatively dry conditions.
How does El Niño impact extreme weather events like hurricanes?
When El Niño happens, the eastern tropical Pacific temperature warms up and facilitates the development of hurricanes over that region. But, on the Atlantic side, there is actually suppressed hurricane activity due to wind changes.
How does El Niño affect climate overall?
Even though El Niño itself is a regional ocean phenomenon in the Pacific, it does have far-reaching impacts worldwide.
When the tropical Pacific warms up, the warmer ocean heats the atmosphere. Then, the atmosphere carries the heat away from the tropical Pacific through winds and waves in the atmosphere, reaching the rest of the globe. The atmosphere keeps extracting the heat from the ocean, and that heat raises the global temperature.
In the meantime, the warmer ocean in the central eastern tropical Pacific also alters the atmospheric circulation and global hydrological cycle. That’s why some regions experience flooding conditions while others experience drought conditions during an El Niño.
Are we going to have a Super El Niño this year – and what does that mean?
Super El Niño is basically a very strong El Niño event. If the temperature of the eastern Pacific is over half a degree Celius above average, it qualifies as an El Niño event. Once it exceeds 1.5 or 2 degrees, people call it a very strong or Super El Niño event.
At this point, there is a very high chance of El Niño, likely a Super El Niño, by the end of this year based on model forecasts. However, it is still a bit early to declare a Super El Niño – if we wait one more month, I think we will have much higher confidence about whether this El Niño will be really strong or not. For example, the 2014 El Niño was forecasted to be strong when the forecast was made in spring, but it only developed into a weak event. My previous study suggested that the 2014 El Niño was stalled by an easterly wind event in mid-June 2014.
How does El Niño form?
El Niño is really something fluctuating upon a background state, so we need to understand the background state first.
The sun heats up the sea surface evenly across the equator, but persistent wind blowing from east to west drags more of the warm surface water to the western Pacific. These easterly prevailing winds over the tropical Pacific are called the trade winds, because back then the sailors use those winds to sail for trading.
These winds push warm surface water to the western Pacific and leave relatively cold water behind in the eastern Pacific, fed by the cold water beneath the surface ocean layer. This east-west ocean surface temperature difference further accelerates the easterly trade winds. That's the background state.
El Niño develops because of some positive feedback in this ocean-atmosphere system.
Let's do a thought experiment: If we somehow warm up the eastern tropical Pacific a little bit, the east-west temperature difference gets reduced, right? Then, the trade wind becomes weaker, and the east-west temperature difference gets further reduced. That positive feedback is what acts to amplify a very small initial perturbation in temperature and then make it develop into an El Niño event.
How has El Niño changed over time?
El Niño information in the past climates can be recorded in corals, similar to how tree rings record local climate information on land. Based on coral records, El Niño has been active for thousands of years.
Unfortunately, it’s hard to have corals from millions of years ago, so for those studies we often rely on climate models.
A couple of years ago, we published a paper modeling the different climates over the past 250 million years. On this geological timescale, climate states have changed quite a lot due to the changes in the CO2 level, the solar radiation, the land-sea distribution, etc. For example, the Pacific Ocean 100 million years ago was much wider than now, and the global mean temperature was about 14°C warmer. Interestingly, El Niño was active in all those wildly different climates, although its amplitude did vary, based on our simulations.
Back to instrumental approaches, oceanographers have been using ship-based measurements, buoys and remote sensing techniques to track the El Niño development for more than a century. There seems to be a trend of stronger El Niño in the past few decades, but we need to be cautious because of the relatively small sample size.
Dive deeper into El Niño research: Hu’s recent, NASA-supported investigation into the impact of salinity on El Niño.