Last month, forecasters were predicting with 90 per cent certainty we'd see an El Nino by the end of the year, driving severe weather patterns worldwide. But with little sign so far of the ocean and atmospheric changes scientists expected, those odds have dropped off quite a bit.
We'll probably still see an El Nino before the year's out but it's unlikely to be a strong one, scientists are saying.
What is an El Nino?
Every five years or so, a change in the winds causes a shift to warmer than normal ocean temperatures in the equatorial Pacific Ocean – a phenomena known as El Nino.
Together, El Nino and its cooler counterpart La Nina are known as the El Nino Southern Oscillation. Between them, they're responsible for most of the fluctuations in global weather we see from one year to the next.
Figure 1: Sea surface temperature during El Nino (left) and La Nina (right). Red and blue show warmer and cooler temperatures than the long term average.
Source: Steve Albers, NOAA
What happens when an El Nino hits?
Each time a switch occurs, changes in the ocean and atmosphere above it affect global temperature and rainfall patterns worldwide.
In El Nino years, Indonesia and Australia see below average rainfall, while South America and parts of the United States see more than usual.
The ocean releases heat into the atmosphere in El Nino years. Added to the warming we're seeing from greenhouse gases, this puts El Nino years among the warmest on record.
You can see this in the graph below showing global temperature between 1950 and 2013. Five of the top 10 warmest years are El Nino years (orange bars).
Figure 2: Global temperature anomalies for 1950-2013 showing the phase of the El Nino-La Nina cycle (
Source: NASA/GSFC/Earth Observatory, NASA/GISS
When will the next El Nino be?
Since the last El Nino in 2009-2010 the Pacific has been in either a neutral or La Nina phase. Most of 2012 and all of 2013 saw neutral conditions – and we're still in neutral phase now.
Earlier this year, the ocean looked to be primed for an El Nino, with above average temperatures in the eastern Pacific lasting throughout March and May.
But the atmosphere has "largely failed to respond" to sea surface temperatures and scientists' confidence in an El Nino developing in 2014 has eased a bit, says the Australian Bureau of Meteorology, whose climate models now put the chances of this happening at about 50 per cent.
The US National Oceanic and Atmospheric Administration (NOAA) suggests higher odds. Itslatest predictions suggest a 70 per cent chance of an El Nino in Summer and 80 per cent during autumn or winter.
You can see in the graph below how climate models forecast sea surface temperatures in the Pacific will evolve in the next few months. Each colour is a different model.
Figure 3: Forecasts for sea surface temperature anomaly, as of 15th July 2014.
Source: the International Research Institute (IRI) for Climate and Society at Columbia University.
How strong will the next El Nino be?
Back in April, professor Mat Collins from Exeter University told us that although conditions in the Pacific at the time looked similar to those before the large 1997/8 event, the ocean can change very quickly.
In other words, the signs were there but it was still too soon to say if the next event would be a strong, moderate or weak one. We asked Collins what he thinks now, and he said:
"I think this year's El Nino has been a real enigma. Many of us were thinking that it would be a big event this year, but now the signal is pretty weak and a strong event seems quite unlikely."
"If an El Nino were to occur, it is increasingly unlikely to be a strong event."
Will we see the hottest year on record?
Back in April, scientists suggested the next El Nino, on top of continuing greenhouse gas warming, could see surface temperatures topping the charts for the warmest year on record.
At the time, Collins told us:
"[T]he influence of El Nino on global mean temperatures is well known so my guess is the chance of seeing a record warm year is pretty high, especially if it is a large event".
But any temperature-boosting potential of the next El Nino looks fairly limited at the moment, Collins says:
"If you compare the situation now with the state of the Pacific in summer 1997 (i.e. before the big 1997/98 event), it is very different … [Right now] we just have some weak and disorganised sea surface temperature anomalies, pretty average trade winds and even the subsurface ocean doesn't have much of a signal."
We could still see chart-topping temperatures even without an El Nino. 2013 and 2002 were the 4th and 6th hottest years on record, respectively, despite ENSO-neutral conditions.
So far, 2014 equals 2002 as the third warmest January to June on record, with global surface temperatures 0.67 degrees above the 20th century average. The map below shows regions with June temperatures four degrees higher than the 1981-2010 average.
Figure 4: Land and surface temperatures in June 2014, compared to the 1981-2010 average.
Are El Ninos getting stronger?
Recent research suggests ENSO events have been unusually strong in the later part of 20th century compared to the past 7000 years – 42 per cent stronger, on average.
Professor Kim Cobb from Georgia Tech university told us recently how human-caused warming could increase the strength of ENSO:
"[C]limate change is much more than 'global warming' […]. Because ENSO involves feedbacks between the wind strength, ocean temperature, and circulation, a change in any related climate parameter would arguably have some effect on ENSO strength".
A new paper in Nature Climate Change suggests El Ninos and La Ninas are likely to intensify with rising greenhouse gases, peaking at 13 per cent stronger than the late 20th century average by about 2040.
But after 2040, the authors expect to see ENSO events weaken again as different rates of warming across the Pacific Ocean bring about a change in the winds, the paper explains.
Given the destructive consequences of El Nino and La Nina, it's not hard to see why research into the potential links with greenhouse gas warming is so important.
As for what this year's El Nino has in store, the next official update from the Australian government is expected on August 12.
Originally published by Carbon Brief. Reproduced with permission.