🌀 The ocean’s weather, three and a half years of it

Every dot is a mesoscale eddy: a rotating body of water tens to hundreds of kilometres across, tracked by satellite altimetry from the dip or bulge it makes in the sea surface. Only eddies that survived at least 90 days are shown, and even so there are more than two thousand alive on any given day.

🟠🔵 Orange eddies rotate anticyclonically, blue ones cyclonically. The distinction is not cosmetic. Anticyclones trap warm water and depress the thermocline; cyclones lift cold, nutrient-rich water towards the surface and can turn a patch of open ocean into a feeding ground.

⬅️ Watch the direction. Almost everything drifts west. This is one of the most robust results in ocean dynamics: eddies propagate westward because the Coriolis parameter changes with latitude, and that gradient supports waves that travel in one direction only. A few metres per second of current, but the eddies themselves crawl at two or three kilometres a day.

🔥 Where they crowd. The Agulhas region south of Africa, the Gulf Stream, the Kuroshio, the Brazil-Malvinas confluence: wherever a strong current becomes unstable, it sheds eddies. The Southern Ocean is a continuous belt of them.

💤 And where they don’t. The centres of the subtropical gyres stay comparatively empty, as does the equatorial band, where the Coriolis force vanishes and coherent rotation cannot organise.

⬇️ Two caveats. The map is a Spilhaus projection, which shows the ocean as the single connected body it is, but it compresses distances near the centre of the disc, where Antarctica sits, so Southern Ocean eddies look slower than they are. And dot size is a marker, not eddy size: at this scale a 100 km eddy is smaller than a pixel.

Data: META 4.0 mesoscale eddy atlas (AVISO/CLS/CNES), DUACS altimetry