Geostrophic Balance Explained

In the last lecture you met the forces that move the ocean. Now Gael Alory shows what happens when two of them – the pressure gradient force and the Coriolis force – come into balance. This is geostrophic balance, and it governs most of the large-scale currents in the ocean.

The lecture builds the idea step by step:

• How a slope in the sea surface creates a pressure difference, and how a water parcel responding to it is turned by the Coriolis force until the two forces balance
• The simple rule that results: near the surface, the geostrophic current flows along the sea-level slope, not down it – to the right of the downhill direction in the northern hemisphere, to the left in the southern
• Why this means currents flow clockwise around a sea-level high in the northern hemisphere, and the reverse in the south
• How maps of sea level – dynamic topography – can therefore be read directly as maps of surface current, revealing features like the subtropical gyres and the Gulf Stream
• What happens at depth, where density differences matter, and why some currents fade with depth while others do not

This is one of the most powerful ideas in ocean dynamics: it means that a satellite measuring the height of the sea surface is, in effect, measuring the currents below. Keep that link – between sea level and current – in mind as you watch.