Around Antarctica flows a vast ocean river. It has no banks, but it does have boundaries: where warm water meets cold, water properties change sharply. These boundaries are called fronts. Previously, they were located as lines on maps of sea surface height, and it seemed that the fronts were slowly creeping south. A new analysis of 21 years of satellite data shows that fronts do not creep; they split and merge, like the arms of a river in a delta.

Satellites measure sea surface height every day. If the water is warm, the level is slightly higher; if cold, slightly lower. At the boundary, you get a step. The new method looks for such steps and notes where they appear most often. It turns out that the number of steps varies from place to place: after underwater mountains, one front splits into several, and over flat bottom they merge again. Over two decades, the fronts have not moved south; they have only meandered slightly in place.

This perspective is important for understanding climate. If the boundaries between warm and cold water do not shift but reconfigure, then the ocean transports heat and carbon dioxide differently than previously thought. The new method can be applied to other oceans to test whether fronts everywhere behave like a branching river.