The Next El Nino
February 13, 2014
The amount of heat stored in the oceans is one of the most important diagnostics for global warming, because about 90% of the additional heat is stored there (you can read more about this in the last IPCC report from 2007). The atmosphere stores only about 2% because of its small heat capacity. The surface (including the continental ice masses) can only absorb heat slowly because it is a poor heat conductor. Thus, heat absorbed by the oceans accounts for almost all of the planet’s radiative imbalance.
If the oceans are warming up, this implies that the Earth must absorb more solar energy than it emits longwave radiation into space. This is the only possible heat source. That’s simply the first law of thermodynamics, conservation of energy. This conservation law is why physicists are so interested in looking at the energy balance of anything. Because we understand the energy balance of our Earth, we also know that global warming is caused by greenhouse gases – which have caused the largest imbalance in the radiative energy budget over the last century.
If the greenhouse effect (that checks the exit of longwave radiation from Earth into space) or the amount of absorbed sunlight diminished, one would see a slowing in the heat uptake of the oceans. The measurements show that this is not the case.
The increase in the amount of heat in the oceans amounts to 17 x 1022 Joules over the last 30 years. That is so much energy it is equivalent to exploding a Hiroshima bomb every second in the ocean for thirty years.
Completely independently of this oceanographic data, a simple correlation analysis (Foster and Rahmstorf ERL 2011) showed that the flatter warming trend of the last 10 years was mostly a result of natural variability, namely the recently more frequent appearance of cold La Niña events in the tropical Pacific and a small contribution from decreasing solar activity. The effect of La Niña can be seen directly in the following figure, without any statistical analysis. It shows the annual values of the global temperature with El Niño periods highlighted in red and La Niña periods in blue. (weekly Nino updates here)
One finds that both the red El Niño years and the blue La Niña years are getting warmer, but given that we have lately experienced a cluster of La Niña years the overall warming trend over the last ten years is slower. This can be thought of as the “noise” associated with natural variability, not a change in the “signal” of global warming (as discussed manytimes before here at RealClimate).
This is consistent with the finding that reduced warming is not mainly a result of a change in radiation balance but due to oceanic heat storage. During La Niña events (with cold ocean surface) the ocean absorbs additional heat that it releases during El Niño events (when the ocean surface is warm). The next El Niño event (whenever it comes – that is a stochastic process) is likely to produce a new global mean temperature record (as happened in 2010).
The reason for the change is a specific change in the winds, especially in the subtropical Pacific, where the trade winds have become noticeably stronger. That altered ocean currents, strengthening the subtropical sea water circulation thus providing a mechanism to transport heat into the deeper ocean. This is related to the decadal weather pattern in the Pacific associated with the La Niña phase of the El Niño phenomenon.
Compare Trenberth’s quote to yesterday’s post on the role of Trade winds in ocean heating.
There’s a betting pool forming on when the next el Nino, and the next record warm year, will begin.
Hold onto your ice lollies. Long-term weather forecasts are suggesting 2014 might be the hottest year since records began. That’s because climate bad-boy El Niño seems to be getting ready to spew heat into the atmosphere.
An El Niño occurs when warm water buried below the surface of the Pacific rises up and spreads along the equator towards America. For nine months or more it brings rain and flooding to areas around Peru and Ecuador, and drought and fires to Indonesia and Australia. It is part of a cycle called the El Niño-Southern Oscillation.
It is notoriously hard to make a prediction before the “spring barrier” as to whether there will be an El Niño in a given year. “The El Niño-Southern Oscillation cycle more or less reboots around April-May-June each calendar year,” says Scott Power from the Bureau of Meteorology in Melbourne, Australia.
The problem is that there is so much background variability in the atmosphere and ocean that it is hard to see any signal amidst the noise, says Wenju Cai from the CSIRO, Australia’s national research agency in Melbourne. “Even if there is a developing El Niño, it is hard to predict.”
Links in the air
But now a model aimed specifically at predicting El Niño seems to be able to sift through the noise by examining a previously-unexplored feature of Pacific weather.
Previous predictions have relied on full climate models. Rather than using this traditional approach, Armin Bunde of Justus Liebig University in Giessen, Germany, and his colleagues looked at the strength of the link between air temperature over the equator and air temperature in the rest of the Pacific. The records showed that, in the year before each El Niño, the two regions became more closely linked, meaning their temperatures became more similar than at other times.
The team also found that, once these atmospheric links reached a critical strength, around 75 per cent of the time an El Niño developed within a year (PNAS, doi.org/rdn). “There is certainly a correlation between the cooperative mode in the atmosphere that we measure and the onset of an El Niño event,” says Bunde. Nobody knows why.
Now they say the threshold was crossed in September 2013. “Therefore, the probability is 0.76 that El Niño will occur in 2014,” says Bunde. In other words, there is a 76 per cent chance of an El Niño this year.
The most important driver of climate variability is the El Niño Southern Oscillation, which can trigger disasters in various parts of the globe. Despite its importance, conventional forecasting is still limited to 6 mo ahead. Recently, we developed an approach based on network analysis, which allows projection of an El Niño event about 1 y ahead. Here we show that our method correctly predicted the absence of El Niño events in 2012 and 2013 and now announce that our approach indicated (in September 2013 already) the return of El Niño in late 2014 with a 3-in-4 likelihood. We also discuss the relevance of the next El Niño to the question of global warming and the present hiatus in the global mean surface temperature.
Wake up call ahead.