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Ocean Surface Topography
Ocean surface topography is the height of the ocean surface relative to a level of no motion defined by the geoid, a surface of constant geopotential, and provides information on tides, circulation, and the distribution of heat and mass in the Earth’s global ocean.
Ocean surface topography is measured using a satellite altimeter. Satellite altimetry combines precise orbit determination with accurate ranging by a microwave altimeter of the distance of ocean surface to the satellite to determine the ocean surface height relative to known reference surface or the center of the Earth. Precision orbit determination uses satellite tracking information and models of the forces (e.g., gravity, aerodynamic drag) that govern the satellite motion to produce accurate estimates of the satellite's orbital height. The range from the satellite to the ocean surface is measured using an onboard altimeter that bounces microwave pulses off the ocean surface and measures the time it takes the pulses to return to the spacecraft. The highly accurate altimeter range measurements are subtracted from the satellite orbital height, resulting in ocean height measurements relative to well-defined reference system. Ocean surface topography is estimated by referencing the sea surface height to the geoid. Other ancillary information on the ocean surface is also obtained from satellite altimetry including information on surface waves and roughness.
The primary contribution of satellite altimetry to satellite oceanography has been to:
- Improve the knowledge of ocean tides and develop global tide models.
- Monitor the variation of global mean sea level and its relationship to changes in ocean mass and heat content.
- Map the general circulation variability of the ocean, including the ocean mesoscale, over decades and in near real-time using multi-satellite altimetric sampling.
Altimetry data are often combined with data from other ocean-observing satellites and from in situ measurements taken from ships, profiling floats and satellite-tracked drifting buoys. For example, these combined data sets are used to:
- Calculate the transport of mass, momentum, heat, nutrients and salt by the oceans.
- Increase understanding of ocean circulation and seasonal changes and how the general ocean circulation changes through time.
- Test how ocean circulation is caused by winds.
- Improve forecasting of climatic events like El Niño and La Niña as well as global climate change.
- Describe the nature of ocean dynamics and develop a global view of Earth's ocean.
- Monitor the regional variation of mean sea level and its relation to global climate change.
- Provide estimates of significant wave height and wind speeds over the ocean.