An orchestra without a conductor: the instruments set their own rhythm. That is how one might describe the climate system of the Indo-Pacific region, where sea surface temperatures oscillate across many time scales, from seasonal to multi-decadal. Previously, to hear a single instrument, you had to pre-tune filters, cutting off everything else. The new method, nonlinear Laplacian spectral analysis (NLSA), works differently: it listens to the entire orchestra at once and separates the sound into parts without requiring any pre-tuning.
Applying this approach to thousand-year control runs of the climate models CCSM4 (1300 years) and CM3 (800 years), as well as to observational data from HadISST (140 years), the authors obtained a hierarchy of modes. The annual cycle and its harmonics explain up to 70% of the raw variance in the southern mid-latitudes. El Niño–Southern Oscillation (ENSO) is represented by a pair of quadrature modes with a carrier frequency of about 4 years and a decadal envelope. But the most interesting are the combination modes of ENSO with the annual cycle (ENSO-A1 and ENSO-A2), which explain up to 35% of the variance in the eastern Indian Ocean, a region used to define the Indian Ocean Dipole.
This means that a significant portion of the variability traditionally attributed to the Indian Ocean Dipole may be a deterministic modulation of the ENSO annual cycle. In other words, what was thought to be an independent instrument turns out to be an overtone generated by the interaction of El Niño with the annual rhythm. The tropospheric biennial oscillation (TBO) emerges as a separate pair of modes with a period of about 2 years, but with an amplitude an order of magnitude weaker than ENSO.
WPMM is so slow that individual events can last longer than the entire period of satellite observations of the ocean. For example, in the CCSM4 model, one positive WPMM event lasted about 65 years. It is as if the orchestra had an instrument that plays a note once every few decades, and you can only hear it through very long observation.
So the orchestra without a conductor turned out to be not chaos but a complex polyphony, where each voice has its own part. The new method not only confirmed the existence of known modes but also discovered new ones and reinterpreted the nature of the Indian Ocean Dipole. This changes our understanding of climate predictability in a region on which monsoons and droughts in Asia and Australia depend.
The western Pacific multi-decadal mode (WPMM) is so slow that individual events can last longer than the entire satellite record of the ocean. For example, in the CCSM4 model, one positive WPMM event lasted about 65 years.
The idea of hidden periodicities governing climate echoes Isaac Asimov's novel 'The End of Eternity', where mathematicians compute cyclic changes in reality to manage human history.