Topography Ocean

[Amilcar Labrosse]

Onlyfrom space can we observe the height of our vast ocean on a global scale and monitor critical changes in ocean currents and heat storage. Continuous data from satellites like TOPEX/Poseidon, Jason-1, OSTM/Jason-2, and Jason-3 help us understand and foresee the effects of the changing oceans on our climate and on catastrophic climate events such as El Nio and La Nia.

In an environment of constrained resources, U.S. and international partnerships are necessary to gain as much understanding of our planet as possible. Not only do they do they reduce costs for NASA, they also engage a larger and more diverse group of scientists.

The mean dynamic ocean topography (DOT) is the difference between the time-averaged sea surface and the geoid (the equipotential surface of the Earth's gravity field that best fits the mean sea surface). All geoid slopes are 'horizontal'. A tilt of the the sea surface relative to the horizontal measures the strength of surface 'geostrophic' currents. The mean DOT (MDOT) measures the long-term-averaged strength of ocean currents, the 'steady-state' circulation. One example is the Gulf Stream, whose position averaged over any one year now is about the same as in 1786, when Benjamin Franklin and Timothy Folger charted it (Richardson, 1980). The North-South (meridional) gradient of the DOT is proportional to the East-West (zonal) geostrophic component of ocean surface current velocities (the rest is the wind-driven Ekman current); the zonal gradient of the DOT is proportional to the meridional velocity.

The DOT can also be constructed by combining in-situ oceanographic data (temperature and salinity of seawater, direct measurements of current velocity, etc) (Niiler et al, 2003). A third way is by combining the geodetic estimate (altimetry minus geoid) with the traditional oceanographic estimate (Maximenko et al, 2009; Rio et al, 2011).

The most up-to-date and accurate MDOTs are computed using data from ESA's GOCE satellite (e.g., Bingham et al, 2011), sometimes in combination with GRACE data. An MDOT computed with GOCE data can be constructed using the very useful GOCE User Toolbox (GUT).

Maximenko, Niiler et al (2009) Mean dynamic topography of the ocean derived from satellite and drifting buoy data using three different techniques. J. Atmospheric and Oceanic Technology 26, pp 1910-1919.

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.

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]

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.[6]

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.

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]

Currently there are nine different satellites calculating the earth ocean topography, Cryosat-2, SARAL, Jason-3, Sentinel-3A and Sentinel-3B, CFOSat, HY-2B and HY-2C, and Sentinel-6 Michael Freilich (also called Jason-CS A). Jason-3 and Sentinel-6 Michael Freilich are currently both in space orbiting Earth in a tandem rotation. They are approximately 330 kilometers apart.

Ocean surface topography can be derived from ship-going measurements of temperature and salinity at depth. However, since 1992, a series of satellite altimetry missions, beginning with TOPEX/Poseidon and continued with Jason-1, Ocean Surface Topography Mission on the Jason-2 satellite, Jason-3 and now Sentinel-6 Michael Freilich have measured sea surface height directly. By combining these measurements with gravity measurements from NASA's Grace and ESA's GOCE missions, scientists can determine sea surface topography to within a few centimeters.

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 ocean circulation around the globe in order to better understand its interaction with the time varying components and the involved mechanisms for initializing ocean models. To monitor the low frequency ocean variability and observe the season cycles and inter-annual variations like El Nio and La Nia, the North Atlantic oscillation, the pacific decadal oscillation, and planetary waves crossing the oceans over a period of months, then they will be modeled over a long period of time due to the precise altimetric observations.[11] It aims to contribute to observations of the mesoscale ocean variability, affecting the whole oceans. This activity is especially intense near western boundary currents. Also monitor the average sea level because it is a large indicator of global warming through the sea level data. Improvement of tide modeling by observing more long period components such as coastal interactions, internal waves, and tidal energy dissipation. Finally the satellite data will supply knowledge to support other types of marine meteorology which is the scientific study of the atmosphere.

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.[12] This includes the study of short-term climatic changes such as El Nino, La Nina. The satellites detect global sea level mean and record the fluctuations. Also detecting the slow change of upper ocean circulation on decadal time scales, every ten years. Studying the transportation of heat and carbon in the ocean and examining the main components that fuel deep water tides. The satellites data collection also helps improve wind speed and height measurements in current time and for long-term studies. Lastly improving our knowledge about the marine geoid.[12] The first seven months Jason-2 was put into use it was flown in extreme close proximity to Jason-1. Only being one minute apart from each other the satellites observed the same area of the ocean. The reason for the close proximity in observation was for cross-calibration. This was meant to calculate any bias in the two altimeters. This multiple month observation proved that there was no bias in the data and both collections of data were consistent.[12]

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