The river that circles Antarctica does not flow in a single channel: it sometimes splits into branches, sometimes merges into one stream. That is what the fronts of the Southern Ocean look like on maps built from satellite data on sea surface height. Previously, these fronts were sought along lines of equal water level, and they seemed to creep slowly southward. The new analysis shows that there is no movement; there is rearrangement.

The WHOSE method works like a careful cartographer who does not draw a line along a single height but looks for steps in the water's relief. To do this, it decomposes daily sea level maps into small waves and selects places where the signal behaves not like random noise but like a clear boundary. Over 21 years, thousands of marks accumulated where a front appeared most often.

On these maps, you can see that the number of river branches changes from place to place. After underwater mountains and plateaus, for example near Kerguelen, one front splits into several. Where the seabed flattens, the branches merge again. Meanwhile, each branch only meanders slightly in place: its position varies by about half a degree of latitude, and no noticeable southward shift was found over two decades.

This view changes our understanding of the dynamics of the Antarctic Circumpolar Current. If fronts do not migrate but rearrange, then both heat transport and mixing of water masses occur differently than some climate models assume. The WHOSE method can be applied to other oceans and to sea surface temperature data to test whether fronts everywhere behave like a branching river rather than a shifting line.