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Same Warm Ocean, Different Outcomes: Why Some El Niños Drench and Others Parched

Original: "Differential imprints of distinct ENSO flavors in global patterns of very low and high seasonal precipitation"
· Marc Wiedermann, Jonatan F. Siegmund, Jonathan F. Donges, Reik V. Donner
arXiv:1702.00218 · 2017-02-01 · CC BY 4.0 · 2 min
Atmospheric and Oceanic Physics
El Niño and La Niña each have two distinct flavors, and that determines droughts and floods in different parts of the world.

The same orchestra sounds different when two different conductors take the podium. One swings the baton wide, and the whole hall feels his accents; the other barely nods, and only certain instruments respond. So it is in the Pacific: the warm phase El Niño and the cold phase La Niña are two conductors of global rainfall, but each has two distinct personalities.

The classic type warms the waters off the coast of South America, while the other, called the central Pacific type, shifts the warm pool to the middle of the ocean. Scientists set out to test how each type relates to very wet and very dry seasons on land. To do this, they took rainfall data spanning 65 years and calculated how often extreme seasons coincided with a given phase of El Niño or La Niña.

The method is like checking two alarm clocks: if one always rings when the sun rises, and the other rings only occasionally and on random days, then the first is linked to dawn, and the second is not. Here, instead of alarm clocks, we have rare events: seasons with precipitation above the 80th or below the 20th percentile.

It turned out that the classic type produces broad and coherent anomalies: droughts in Indonesia, the Philippines, and the northern Amazon, and wet conditions in western North America. The central Pacific type acts more locally: it brings droughts to the east coast of Australia and wet conditions to southern Chile and Central Asia. For the first time, it has been shown that the central Pacific El Niño is accompanied by increased rainfall in Central Asia, while the central Pacific La Niña is accompanied by a deficit there.

If the type of a developing El Niño can be identified early enough, it would be possible to provide advance warnings of likely droughts or floods in specific regions. This would help farmers plan their plantings and cities prepare for water shortages. Then the two conductors would no longer be a mystery: their gestures could be read in advance, before the first chords.

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The term "Modoki" for Central Pacific El Niño comes from a Japanese word meaning "similar but different," which accurately reflects its character: it looks like classic El Niño on the surface but behaves differently.

P^{\pm}_{s,i}(t) = \Theta(\pm P_{s,i}(t) \mp p^{\pm}_{s,i})
The Heaviside function equals 1 if the seasonal precipitation total exceeds the 80th percentile (for wet) or falls below the 20th percentile (for dry), and 0 otherwise.
ECR^{\pm}_{s,i,\bullet} = \frac{\sum_t X_{\bullet}(t) P^{\pm}_{s,i}(t-\tau)}{\sum_t X_{\bullet}(t)}
The numerator is the number of years when an ENSO phase of a given type coincided with an extreme season; the denominator is the total number of such phases. The lag τ is 0 for SON and DJF, and 1 for MAM.
Concepts
ENSO El Niño La Niña spring predictability barrier Multivariate ENSO Index El Niño index Southern Oscillation Indian Ocean capacitor effect ERSST NOAA CPC Jin-Timmermann model TAO/TRITON moorings interannual variability Pacific decadal oscillation relative ONI recharge oscillator model equatorial wave propagation Westerly wind burst Warm water volume natural variability