Recent El Nino Southern Oscillation

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When the Christ child comes a year late

March 22, 2016/ IMAU/
Although the name El Niño (`little boy/Christ child’) actually refers to the fact that most ENSO (El Niño Southern Oscillation) events peak around Christmas, it also nicely captures a troublesome property of the phenomenon: It behaves as quirkily and unpredictably as a three-year-old child. Next winter’s El Niño can be reasonably well predicted in summer (which basically amounts to diagnosing a developing event), but for longer lead times predictability drops quickly: the `spring predictability barrier’.

The basic physics of El Niño are well understood (see figure): under normal (non-ENSO) conditions, the easterly trade winds over the Pacific push warm water to Indonesia. Near Peru this leads to upwelling of cold deep water. If the easterlies weaken, the warm water `sloshes back’ eastwards as an equatorial Kelvin wave. As a result, the water near coastal Peru gets warmer and heats the overlying atmosphere. This makes the air rise locally and the westerly wind anomaly over the Pacific is further amplified: a positive feedback (Bjerknes feedback) that eventually leads to a fully developed El Niño. But how does this process start?
 
IMAU director and oceanographer Will de Ruijter discovered in observational data that El Niño events tend to be preceded by cool anomalies in the Indian Ocean, more specifically over the Seychelles Dome (SD) region northeast of Madagascar. After the cool SD event in summer 2013 he predicted an El Niño for 2014/15. Meanwhile I spent the first year of my PhD on data analysis to prove that the relation between the SD and El Niño is indeed statistically significant. Observations support two ways in which the West Indian Ocean might influence El Niño (see figures a-d):
Figure 1: Illustration of the mechanisms by which a cool Seychelles Dome (SD) / SouthWest Indian Ocean in summer of year 0 supports the formation of an El Niño after 1.5 years (winter year 1/2). Explanations see text.

Figure: Illustration of the mechanisms by which a cool Seychelles Dome (SD) / SouthWest Indian Ocean in summer of year 0 supports the formation of an El Niño after 1.5 years (winter year 1/2).

1) The cool SD cools the overlying air, which then sinks. To compensate, the air over Indonesia rises (a). As Indonesia is warm and humid, this rising might be nonlinearly amplified so strongly so as to attract an (easterly) inflow over the West Pacific. This will lead to a greater-than-normal warm water reservoir, ready to be pushed over to the East Pacific (b).

2) A cool SD in summer tends to be followed by a stronger intraseasonal (time scales of several weeks) wind variability  over the West Pacific in the next winter-spring (c). This can trigger a first warm Kelvin wave, especially since there is a large warm water reservoir in the west Pacific. Now the Bjerknes feedback kicks in and a fully-grown El Niño develops (d).

This is a nice theory, and if it is correct, it may help to circumnavigate the spring predictability barrier. But unfortunately there was no El Niño 2014/15. Does this suggest that we are wrong? Not necessarily. In spring 2014, the warm water volume was about as large as in the period preceding the very strong El Niño event in 1997/98. The wind variability was high, too. So one could argue that the Indian Ocean has done its job, but that the Bjerknes feedback was not activated – i.e. the atmosphere did not respond to the initial East Pacific warming after the first Kelvin waves. At least not immediately. Instead, the warm water lingered through early 2015 and a fresh spell of intra-seasonal wind variability triggered Kelvin waves that led to one of the strongest El Niño events on record – the Christ child came a year late
.
Why did the Pacific give such a strong response in 2015 while remaining passive after a similarly strong forcing in 2014? We do not know – probably the whole oceanography community was puzzled by the failure of El Niño to develop in 2014/15. Maybe Qingyi Feng and Henk Dijkstra can shed some light on this question. They are working on detecting the stability of the El Niño system, using correlation-based networks.

Claudia Wieners, PhD student in the Ocean and Climate research group at IMAU, Utrecht University


http://news.imau.nl/?p=2641

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