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Why El Niño Comes in Different Flavors: The Ocean as a Guitar String

Original: "A spatiotemporal oscillator model for ENSO"
· Yaokun Li
arXiv:2306.04074 · 2023-06-07 · CC BY 4.0 · 1 min
Atmospheric and Oceanic Physics
The warm Pacific current oscillates in two ways at once, like a string producing two notes simultaneously.

Every few years, warm water reaches the coast of Peru, and fish dive deeper. This is El Niño, a warm current that disrupts weather across the planet: floods in some regions, droughts in others. Scientists have long noticed that El Niño comes in different forms: sometimes warm water sits near the coast of South America, and sometimes right in the middle of the Pacific. Why this happens has remained a puzzle for a long time.

The ocean can be compared to a guitar string. When you pluck a string, it produces not just one tone but several at once, low and high. Similarly, when the ocean is 'plucked' by wind, it starts oscillating in two ways simultaneously.

A new model explains this simply. The Pacific Ocean has two main 'tones.' The first, low tone oscillates slowly, over 4.3 years. In this mode, warm water accumulates in the east, near the coast of the Americas. The second, high tone oscillates faster, over 2.3 years, and warm water appears in the center of the ocean. When both tones sound together but one is louder than the other, different events result, from weak to giant ones like in 1997, when the ocean warmed by three degrees. So El Niño is not a single phenomenon but a mixture of two oscillations, like music from two notes.

Whole graph →

The period of the second mode is exactly half that of the first, reminiscent of overtones in music: the fundamental tone and its octave create a complex melody of climate oscillations.

\frac{\partial^2 T'}{\partial t^2} = c^2 \frac{\partial^2 T'}{\partial x^2} + \frac{\partial Q'}{\partial t}
The temperature anomaly propagates as a wave with speed c, and external heating Q' acts as a source.
\omega_n = c \lambda_n = \frac{n \pi c}{L}
The oscillation frequency of each mode is determined by the wave speed and the basin size L; for n=1 the period is about 4.3 years, for n=2 it is 2.3 years.
\omega^2 - 2 u k \omega - c^2 k^2 - i \alpha_k \omega = 0
Relates frequency and wavenumber; shows that waves decay or grow depending on the sign of the coefficient α_k, which is related to heating.
Concepts
ENSO El Niño-Southern Oscillation recharge oscillator model Jin-Timmermann model La Niña El Niño index Southern Oscillation ERSST Multivariate ENSO Index spring predictability barrier TAO/TRITON moorings Warm water volume equatorial wave propagation sea surface temperature relative ONI natural variability North Equatorial Countercurrent Westerly wind burst sea surface temperature anomaly Kelvin wave