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Why can differences between sea-surface maps and geoid maps help scientists study ocean currents?

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Diagram showing the deflection of the vertical from the geoid to the ellipsoid or spheroid, with labeled lines for geoid, spheroid, rock surface, ocean, and geoid height in a geology context. — geoid Deflection of the vertical from the geoid to the spheroid.

The geoid represents the theoretical, unperturbed surface of the ocean that would exist if it were influenced solely by gravity and Earth's rotation, with all dynamic effects like winds, tides, and currents removed. In contrast, the actual sea-surface map—measured by radar altimetry—includes these dynamic variations.12

Scientists study ocean currents by analyzing the differences between these two surfaces, often referred to as sea-surface topography. Because the ocean is in geostrophic balance, the tilt of the sea surface relative to the geoid creates horizontal pressure gradients. These gradients drive surface circulation, where "hills" in the sea-surface topography represent high-pressure areas and "valleys" represent low-pressure areas. By mapping the departure of the actual sea surface from the geoid, researchers can determine the direction and speed of these currents, as the water flow is constrained to move perpendicular to the pressure gradient due to the Coriolis force.234