Ocean surface topography
Ocean surface topography or sea surface topography, also called ocean dynamic topography, are highs and lows on the ocean surface, similar to the hills and valleys of Earth's land surface depicted on a topographic map. These variations are expressed in terms of average sea surface height (SSH) relative to Earth's geoid.[1] The main purpose of measuring ocean surface topography is to understand the large-scale ocean circulation.
Time variations
Unaveraged or instantaneous sea surface height (SSH) is most obviously affected by the tidal forces of the Moon and by the seasonal cycle of the Sun acting on Earth. Over timescales longer than a year, the patterns in SSH can be influenced by ocean circulation. Typically, SSH anomalies resulting from these forces differ from the mean by less than ±1 m (3 ft) at the global scale.[2][3] Other influences include changing interannual patterns of temperature, salinity, waves, tides and winds. Ocean surface topography can be measured with high accuracy and precision at regional to global scale by satellite altimetry (e.g. TOPEX/Poseidon).
Slower and larger variations are due to changes in Earth's gravitational field (geoid) due to melting ice, rearrangement of continents, formation of sea mounts and other redistribution of rock. The combination of satellite gravimetry (e.g. GRACE and GRACE-FO) with altimetry can be used to determine sea level rise and properties such as ocean heat content.[4][5]
Applications
Ocean surface topography is used to map
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 collection of the data is useful for the long-term information about the ocean and its currents. According to NASA science this data can also be used to provide understanding of weather, climate, navigation, fisheries management, and offshore operations. Observations made about the data are used to study the oceans tides, circulation, and the amount of heat the ocean contains. These observations can help predict short and long term effects of the weather and the earth's climate over time.
Measurement
The sea surface height (SSH) is calculated through altimetry satellites using as a reference surface the ellipsoid,[7] which determine the distance from the satellite to a target surface by measuring the satellite-to-surface round-trip time of a radar pulse.[8][9] 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 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. However, the ellipsoid is not an equipotential surface of the Earth's gravity field, so the measurements must be referenced to a surface that represents the water flow, in this case the geoid. The transformations between geometric heights (ellipsoid) and orthometric heights (geoid) are performed from a geoidal model. 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.[10]
Satellite missions
This section needs to be updated.(November 2020) |
Currently there are nine different satellites calculating the earth ocean topography,
Ocean surface topography can be derived from
Jason-1 was launched by a Boeing Delta II rocket in California in 2001 and continued measurements initially collected by TOPEX/Poseidon satellite, which orbited from 1992 up until 2006.[11] NASA and CNES, the French space agency, are joint partners in this mission.
The main objectives of the Jason satellites is to collect data on the average
Jason-2 was launched on June 20, 2008, by a Delta-2 rocket out of the California site in Vandenberg and terminated its mission on October 10, 2019. Jason-3 was launched on January 16, 2016 by a Falcon-9 SpaceX rocket from Vandenberg, as well as Sentinel-6 Michael Freilich, launched on November 21, 2020.
The long-term objectives of the Jason satellite series are to provide global descriptions of the seasonal and yearly changes of the circulation and heat storage in the ocean.
A new satellite mission called the
See also
References
- ^ [In most absolute form, it may be expressed in terms of a geocentric radius.]
- ^ Stewart, R.H. (September 2008). Introduction To Physical Oceanography (PDF).
- ^ "Sea Surface Height Anomaly".
- .
- .
- ^ "TOPEX/Poseidon on-line tutorial. Part II". Ocean Surface Topography from Space. Jet Propulsion Laboratory. Archived from the original on 2008-09-16.
- S2CID 233532488.
- ISBN 9780080516585.
- . Retrieved 28 November 2018.
- ^ "Correcting to Improve Accuracy - CNES." Correcting to Improve Accuracy - CNES. Retrieved from http://www.cnes.fr/web/CNES-en/3773-about-cnes.php
- ^ S2CID 129436213.
- ^ S2CID 128627477.
- ^ "Following the Water with the Ocean Surface Topography Mission". Surface Topography From Space. Jet Propulsion Laboratory. September 2008. Archived from the original on 2009-06-20.