A weak or a strong El Niño ahead? Hint: we live in a 3D world

If we take another look at the IRI ensemble forecast for El Niño some members show a clear and speedy rise in East Pacific tropical ocean temperatures. Before we conclude ‘El Niño ahead’ during northern hemisphere summer and autumn let’s not ignore that a handful* of the 23 climate models show only a weak increase in water temperatures, staying below the official El Niño threshold (+0.5C).

[*) We all know an average hand can hold around 7 climate models – which means some 70 percent of models say El Niño, whereas 30 percent says somewhat warmer (up to half a degree), but still neutral ENSO.]

Well then, is there any way of telling which of the climate models may be more inclined to predict actual climatic events for upcoming months? Yes, probably there is:

Kelvin wave El Niño 2014
Progressing Kelvin wave lays foundation for possibly strong El Niño 2014. Shown above is an animation of NOAA NCEP’s deep water temperature anomaly monitoring over the last 13 weeks – over a depth up to 450 meters below the tropical Pacific Ocean sea surface. Note that SST anomalies still show very little information about the extremely large temperature anomaly at a depth of around 100 meters. Each consecutive colour in the chart marks a one degree Celsius temperature difference, which means you are looking at a very strong Kelvin wave, with a core water body with a +7(!) degrees temperature anomaly. As it progresses towards the East and – with trade winds still blowing and East-Pacific upwelling still an active process – it is invited to reach closer to the surface. When a +1.5 degrees temperature anomaly reaches the surface, there is already a ‘strong El Niño’. Including this dynamical process at some depth in the ocean the NCEP forecasting model predicts a ‘strong El Niño ahead’.

Sometimes sea surface temperature (SST) anomalies [which define different states in the El Niño/La Niña Southern Oscillation (ENSO)] do a rather poor job in showing the actual oceanic process that is taking place. Statistical climate models, which base outcome on drawing historical parallels to SST anomalies – can be a bit lost in the dark, when it comes to direction.

Current SST charts for the Pacific for instance show relatively cold water off the coast of Peru, in what’s known as the Niño 1+2 regions. Closely to the west of this region (within Niño 3) there’s a clear positive temperature anomaly – and a bit confusingly there is another one much further to the West (in Niño 4) with a large neutral area in between.

What to make of that, if you were a statistical climate model?

Dynamical climate models try to understand physical processes that are taking place in the 3D atmosphere and oceans – a much more difficult job

With the strongly elevated water temperatures of that slowly progressing Kelvin wave, we are inclined to say the dynamical models [indeed the ones with the famous names] are much better equipped to predict the 2014 ENSO pattern, and therefore the relatively strong El Niño predictions for the northern hemisphere summer and autumn deserve special attention.

© Rolf Schuttenhelm | www.bitsofscience.org

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