Is the SOI noisy or is it signal?

Applying the analytical solution to Laplace’s tidal equations, we can isolate the parts of the Southern Oscillation Index signal that appear quite noisy (i.e. 1880-1885, 1900-1905, etc).

For this 3-month averaged SOI fit, it’s a sin(sin(f(t))) function in the ENSO model that generates the folded signal which appears as a rapidly fluctuating and noisy signal. Although my simplification of Laplace’s equation was originally applied to QBO, it is applicable to other equatorial standing wave phenomenon such as ENSO, of which the SOI is a measure. The SOI signal has always been considered noisy โ€” especially in contrast to other ENSO measures such as NINO34 โ€” but perhaps this needs to be rethought, as the higher frequency components may be real signal.

These results will be presented at next month’s AGU meeting:

3 thoughts on “Is the SOI noisy or is it signal?

  1. Another paper that claims to find the “heartbeat” of ENSO

    J. T. Bruun, J. Allen, and T. J. Smyth, โ€œHeartbeat of the Southern Oscillation explains ENSO climatic resonances,โ€ Journal of Geophysical Research: Oceans, 2017.

    And recently reported “reduced predictability” for ENSO

    “ENSO forecasts near the spring predictability barrier and possible
    reasons for the recently reduced predictability”


    • The ones I noted they found are {2.5, 3.8, 5, 12, 65} years. Yet, I would not consider these resonant frequencies, as the system is forced to begin with. The impulse response of the diffEq I am using has a weak resonance at 1 year and that is more from the 1-year delay differential than anything else.

      So you have the correct interpretation, in that this paper has completely failed to determine the real drivers. ENSO is no different than ocean tides when it comes to its fundamental behavior — all the periods observed are forced from external drivers, in both cases arising from the influence of the moon and the sun.


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