Ocean and atmosphere trade heat constantly, and this animation shows just how much that exchange follows the sun. It's the monthly net heat flux at the ocean surface (ORAS5 reanalysis, 1979-2025), in W/m². Red means the ocean is gaining heat from the atmosphere; blue means it's giving heat back.

Watch the whole map breathe with the seasons. When it's winter in one hemisphere, that ocean is releasing heat (blue), while the other hemisphere, deep in its own summer, is absorbing it (red). Six months later, it flips entirely. A handful of regions never join that back-and-forth: the deep tropics and warm equatorial currents stay red almost year-round, they're net heat absorbers regardless of season, which is exactly why they're the reservoirs that fuel storms and drive circulation in the first place.

Look closer and you'll notice smaller patches that don't follow the seasonal background at all, some warmer, some cooler than what surrounds them. Those are mesoscale eddies, and this is actually a clever way to spot them: because the large-scale seasonal signal is so strong and so predictable, anything breaking that pattern locally stands out as a real, physical feature rather than noise.

Data: ORAS5 (ECMWF ocean reanalysis)