Ocean surface topography

The ocean surface has highs and lows, similar to the hills and valleys of Earth's land surface depicted on a topographic map. These variations, called ocean surface topography (or sea surface topography), also dynamic topography, are mapped using direct (usually satellite-based) or indirect measurements of sea surface height relative to Earth's geoid. Earth's geoid is a calculated surface of equal gravitational potential energy and represents the shape the sea surface would be if the ocean were not in motion. The main purpose of measuring ocean surface topography is to understand the large-scale circulation of the ocean. The height variations of ocean surface topography over these scales can be as much as two meters and are influenced by ocean circulation (currents), temperature, salinity, tides, waves, and the loading of atmospheric pressure.

Ocean surface topography is used to map ocean currents, which move around the ocean's "hills" and "valleys" in predictable ways. A clockwise sense of rotation is found around "hills" in the northern hemisphere and "valleys" in the southern hemisphere. This is because of the Coriolis effect. Conversely, a counterclockwise sense of rotation is found around "valleys" in the northern hemisphere and "hills" in the southern hemisphere.

Ocean surface topography is also used to understand how the ocean moves heat around the globe, a critical component of Earth's climate, and for monitoring changes in global sea level. The sea surface height (SSH) is calculated through altimetry satellites, which determine the distance from the satellite to a target surface by measuring the satellite-to-surface round-trip time of a radar pulse. The satellites then measure the distance between their orbit altitude and the surface of the water. Due to the differing depths of the ocean, an approximation is made. This is called the Arbitrary Reference Surface. Arbitrary Reference Surface is an estimated surface that is calculated to factor in the shape of the Earth. The general shape of the earth is spherical, but flattened out at the North and South Pole. This approximated surface is called the reference ellipsoid. This enables data to be taken precisely due to the uniform surface level. The satellite’s altitude then has to be calculated with respect to the reference ellipsoid. It is calculated using the orbital parameters of the satellite and various positioning instruments. The sea surface height is then the difference between the satellite’s altitude relative to the reference ellipsoid and the altimeter range. The satellite sends microwave pulses to the ocean surface. The travel time of the pulses ascending to the oceans surface and back provides data of the sea surface height. In the image below you can see the measurement system using by the satellite Jason-1.

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