Geophysical Waves: planetary scale wave motion
The waves in the last lecture broke on beaches. The waves in this one can span an ocean basin and take months to cross it. Serena Illig now zooms out to the geophysical waves – the huge, slow motions governed by the two pillars of ocean dynamics: the Earth’s rotation and the ocean’s stratification.
The lecture builds them up one at a time, each answering a question the previous one raises:
- Adding rotation – what happens to a simple gravity wave when the Coriolis force enters? At large scales, the wave stops travelling and collapses into local inertial oscillations, water parcels circling in place
- Kelvin waves – so can large waves travel on a rotating planet? Yes – if there is a coast. Kelvin waves hug the coastline, are trapped against it, and travel in one direction only (with the coast on their right in the northern hemisphere)
- Rossby waves – and in the open ocean, with no coast? Yes again. Because the Coriolis force varies with latitude, displaced water parcels spin and swing back, creating vast planetary waves that always drift westward
- Internal waves – finally, add layering. Waves can travel inside the ocean, on the boundary between lighter and denser water, reaching huge amplitudes and stirring the deep sea
Two ideas are worth holding on to as you watch. First, each wave type arises from a different restoring force – gravity for surface waves, the variation of Coriolis with latitude for Rossby waves, buoyancy for internal waves. Second, these waves are the ocean’s long-distance messengers: they carry signals across entire basins, which is exactly why they matter for forecasting.
