🌊 Where the tide is, and where it isn't. This globe shows the tidal range of M2, the lunar semidiurnal constituent and the largest contributor to the tide almost everywhere. Not the tide at any particular moment, but how far the sea can move at each point: the difference between high and low water. Where the range is large the surface bulges outward; where the sea barely moves, it stays flat.

🕳 The holes. Scattered across every basin there are points where the range falls to almost nothing. These are amphidromic points, and they are the most counterintuitive feature of the global tide. At these points the M2 tide vanishes, while a few hundred kilometres away the same wave lifts the sea by a metre or more.

🌀 Why they exist. The textbook picture of two tidal bulges following the Moon describes an ocean with no continents on a planet that doesn't spin. The real ocean is partitioned into basins, and the Earth's rotation deflects every moving parcel of water. What results is a set of waves rotating around fixed nodes, and each node is one of these holes.

🏔 The ridges. The largest ranges are not in the open ocean but on continental shelves, where a shoaling, narrowing sea floor squeezes the wave and forces it upward. The same tide that lifts the mid-Pacific by half a metre lifts parts of the Patagonian shelf, the northwest Australian shelf and the European shelf seas by several metres.

🔵 And the vast flat blue. Most of the open ocean has an M2 range well under a metre. The dramatic tides people know about are a coastal phenomenon, produced by local geometry rather than by a stronger pull from the Moon.

⬇️ Worth keeping in mind: M2 is one constituent among dozens. The real tide at any coast is their sum, which is why spring and neap tides alternate over a fortnight as M2 beats against the solar constituent S2.

The relief is a visual device to make the pattern readable, not real topography. Grey is land and areas without data.

Data: FES2022 (AVISO/CNES/LEGOS)