A tuning fork produces a pure tone, but if you strike it too hard, it starts to rattle, and a musician might mistake that rattle for complexity in the sound. A similar story unfolded with tropical Atlantic sea surface temperatures: for three decades, they were described as a multifractal system, as if the ocean behaved differently at different scales, like an orchestra where each section plays its own part. New research shows that the rattle was an artifact of how the instrument was tuned, not a property of the Atlantic itself.

The scientists took three time series: the temperature of the entire tropical belt, the temperature of the southern part, and the temperature difference between north and south. They measured how temperature fluctuations change when moving from short to long time intervals. Previously, it was thought that the pattern differed across scales, and this was called multifractality. But when they selected the correct range of scales (from 18 to 100 weeks), all three series behaved the same across all scales, which is called monofractality. The differences seen earlier arose because the analysis included overly long segments, where the pattern changes for a different reason.

The tuning fork sounds clean again if you hold it properly. Before declaring a system complex, it is worth checking whether the instrument itself is rattling. This lesson applies not only to the ocean but also to any measurements where hidden complexity is sought in data.