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Predicting El Nino: a tale of two authorities

Whither the weather: the Bureau of Meteorology’s dynamic climate modelling is not the only forecasting method. AAP/Paul Miller

Over the last two summers, Eastern Australia has experienced two of the hardest hitting La Niña events since 1974. Widespread floods resulted across great swaths of the country. As expected, the La Niña conditions faded to result in the current neutral conditions as measured by eastern equatorial Pacific Ocean sea surface temperatures and the Southern Oscillation Index (SOI), a measure of regional atmospheric behaviour in the western Pacific.

The question many will be asking is “What comes next”? Will the next summer season be a repeat of the previous flooding? Or will the El Niño/La Niña Southern Oscillation (ENSO) climate pattern settle down into a neutral state? Or perhaps even a return to El Niño conditions with the prospect of deficient rainfalls?

The leading provider of seasonal forecasts in Australia is the Bureau of Meteorology, who issue a bi-weekly ENSO update. The most recent BoM ENSO update came on 5th June 2012, and states:

Tropical Pacific climate indicators remain at neutral values for this time of the year. This includes the Southern Oscillation Index (SOI), trade winds, cloudiness, and sea surface temperatures. Ocean temperatures below the surface are currently warmer than average in the central and western Pacific on a monthly scale, with the eastern subsurface Pacific closer to normal, but slowly warming.

Climate models surveyed by the Bureau of Meteorology show that the tropical Pacific Ocean is likely to warm further over the coming months. All seven models surveyed indicate conditions are likely to approach, or possibly exceed, El Niño thresholds during the late winter to early spring period. Large parts of eastern Australia are typically drier and warmer than normal in winter/spring as El Niño events develop. No climate models favour a return to La Niña.

This most recent update first acknowledges the current neutral conditions but then goes on to state that further warming is expected. Of note, all seven models surveyed indicate El Niño or near-El Niño conditions to develop. Perhaps of even more relevance is that simple observation that no climate models indicate a return to La Niña. So on the basis of this synopsis of climate models, El Niño and reduced rainfalls seems highly likely, whereas La Niña and potentially another series of summer floods events seems highly unlikely.

Indeed, the BoM’s manager of climate monitoring, Dr Karl Braganza, seemed very certain of a coming El Niño this week when he spoke to ABC’s Radio National. While the strength of the coming El Niño was as yet unknown, Dr Braganza said, the modelling left little doubt it is on its way:

Look, it’s looking reasonably straight forward now in terms of the way the models are going. When you get a whole lot of models going in one direction that increases your confidence that they’re doing something real.

Rain or shine? Flickr/PNNL Pacific Northwest National Laboratory

Dr Braganza’s confidence is misplaced as it is based on only one approach to ENSO prediction - dynamical climate models.

In contrast, whilst the US National Oceanic and Atmospheric Administration (NOAA) is similarly engaged in forecasting ENSO, it also surveys statistical models of ENSO behaviour.

The difference between the two approaches is that BOM’s attempts to simulate everything in the ocean-atmosphere system, whilst the other attempts to compare current ENSO behaviour to the statistical historical occurrences of ENSO. Dynamical climate models are based on the simulation of the physics of ocean-atmosphere interactions requiring substantial data inputs and computational resources.

In contrast, statistical models are much simpler as they are based on the recent observations of the leading indicators of ENSO initiation and growth. The likely ENSO behaviour is then based on the previously observed ENSO behaviour.

The US authority NOAA therefore provides a more comprehensive survey of the alternative approaches to predicting ENSO when compared to the climate models used by the BoM. NOAA have recently released (7th June 2012) their ENSO diagnostic discussion.

On the basis of a greater diversity of models and most importantly, modelling approaches, NOAA have declared their ENSO Alert System Status to be “El Niño Watch”. That is, they say that El Niño conditions may be developing and that we should be on guard for that possibility. Importantly, NOAA is not prepared to definitively call an El Niño at this early stage of the ENSO cycle. Unlike the Bureau, their surveyed models display a range of possible outcomes. They state:

… most of the dynamical models predict El Niño to develop during JAS (July, August, September), while the statistical models tend to favor the continuation of ENSO-neutral. Thus, there remains uncertainty as to whether ENSO-neutral or El Niño will prevail during the second half of the year. The evolving conditions, combined with model forecasts, suggest that ENSO-neutral and El Niño are roughly equally likely during the late northern summer and fall. The CPC/IRI forecast calls for ENSO-neutral conditions through JAS, followed by an approximately 50% likelihood for El Niño during the remainder of the year.

NOAA offer a note of caution that the Bureau should take into account – it is still very early in the typical annual ENSO cycle to make strong predictions, and the different approaches to ENSO prediction disagree. Undeterred by this reality, the Bureau have put all of its faith in the complex dynamical climate models, ignoring the other approaches to prediction.

The dynamical climate models predicting early El Niño development require large increases in sea-surface temperatures over the next two months. Indeed, closer inspection of the individual model predictions reveals that they expect a 0.1-0.2 degree Celsius warming for the month of June, followed by a whopping 0.6-0.7 degree rise over the month of July alone.

In contrast, NOAA’s survey of statistical models of ENSO are largely based on behaviour of the observations alone. These appear to be far more conservative in that they are indicating neutral conditions to persist.

I would argue that it is certainly too early to be definitive about the expectations of ENSO for the next summer period. The Bureau’s apparent confidence is almost certainly premature.

El Niño does appear a possibility at present, but what we do know for certain is that it is only over the next few months that we will be able to gauge where ENSO goes and whether the climate models or statistical models were right. It is only then that we might prepare for what ENSO might throw at us next.

If an El Niño event does emerge over the next few months, then it would be reasonable to expect a drier, warmer summer than we’ve had over the last two years. One can also argue that if an El Niño does emerge, we enter it in the best position possible as eastern Australia is currently primed with water.

Perhaps the most important insight gained from the Australian and US authorities is not so much the chances of El Niño forming - rather that La Niña conditions look highly unlikely.

This itself should serve to provide some form of comfort to flood-ravaged Australians: south-east Queensland and northern New South Wales are particularly hit by ENSO as we have witnessed with the two summer seasons of La Niña-driven floods. The prediction that La Niña is highly unlikely should mean a much reduced risk of seeing the catastrophic flooding of recent summers.

Above all, whether El Niño or neutral conditions persist, in either case it should give low-lying floodplain dwellers some welcome relief.

Comments welcome below.

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