Ocean surface topography

Ocean surface topography is the height and shape of the sea surface relative to a reference level. It varies with gravity, currents, tides, winds, and the ocean’s temperature and salinity.

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Geoid: An equipotential surface of Earth’s gravity field that approximates global mean sea level. It provides the gravity-based reference surface against which ocean height is interpreted.
Mean sea level: The average height of the sea surface over a specified period and location. It is a common baseline for describing sea-surface height, though it varies geographically.
Ocean gyre: A large system of circulating ocean currents, typically organized around a basin-scale center. The height gradients across gyres reveal their circulation and central sea-level patterns.
Geoid undulation: The height difference between the geoid and a chosen reference ellipsoid. It describes gravity-driven reference-surface shape, unlike ocean topography above that surface.
Ocean circulation: The large-scale movement of seawater driven by winds, density differences, tides, and Earth’s rotation. Currents redistribute water and create broad differences in sea-surface height.
Sea-level rise: A long-term increase in the average height of the global or regional sea surface. It describes persistent changes in the baseline beneath shorter-lived topographic variations.
Ocean eddy: A rotating body of water that transports heat, salt, and momentum through the ocean. Eddies appear as localized sea-surface highs or lows in altimetry maps.
Mean dynamic topography: The long-term average sea-surface height above the geoid. It is the time-averaged circulation signal, contrasted with the full changing surface.
Ekman transport: The net movement of water caused by wind stress and the Coriolis effect across the ocean’s surface layer. Wind-driven transport piles water up or draws it away, changing local surface height.
Gravity anomaly: The difference between measured gravity and a reference gravity field at a location. Gravity anomalies reveal mass variations that influence the shape of the sea surface.