It takes eight years for a wave to circle Antarctica. It carries warmth and cold in turn, and sea ice advances and retreats with its crests. Previously, researchers caught this wave using different methods and saw either a propagating signal or a frozen dipole. Now it's clear: the dipole is just a snapshot of the wave when it stretches between two oceans.

The new method works like an orchestra without a conductor. Each ocean oscillation is an instrument with its own part. In the past, researchers decided which notes to keep, inadvertently mixing different melodies. Now the algorithm listens to everything at once and separates it into clean voices, discarding nothing.

But the orchestra has other parts too. When the annual cycle, the changing seasons, superimposes on the four-year wave, combined oscillations emerge. They are weaker, but in the Indian Ocean their voice is louder than the main wave. These secondary waves can travel both eastward and westward, like echoes bouncing off shores.

The connection to the tropics turns out to be selective. The two-crest wave almost perfectly matches the rhythm of El Niño, as if the tropical Pacific pulls a string stretched across the planet. In contrast, the three-crest wave lives on its own, born from local winds and currents.

Now that the score is read, we can learn to predict the next bars. If the tropics set the rhythm and local winds sing along, then seasonal forecasting for the Southern Ocean is no longer a guessing game. The wave continues its eight-year circuit, and at last we hear its music in full.