Satellite altimetric measurements are the basic product from which sea surface height and geostrophic currents may be derived. A radar altimeter on the satellite is able to measure the range from the sea surface to the satellite itself, and since its orbit is known with respect to the reference ellipsoid, the sea surface height above that reference is also known. However, to estimate meaningful magnitudes the sea surface height above the geoid (the equipotential surface with which the sea surface would coincide if the ocean were at rest) must be known. In altimetry this magnitude is called absolute dynamic topography. Since the geoid is not known accurately, an estimation of the absolute dynamic topography is estimated as follows. Instead of direct measurements, an average of several year satellite readings are subtracted from the sea surface height measurement, removing the effect of the geoid but also the mean sea level of the averaging period, called mean dynamic topography. The resulting product is the sea level anomaly. The mean dynamic topography is estimated using available data and models for the averaging period.
Once obtained the absolute dynamic topography, currents can be calculated by applying a geostrophic approach:where Ug and Vg are the zone and meridional geostrophic velocity components, f is the Coriolis parameter, g is the gravity acceleration and x and y are the distances in length and latitude.
The base altimetric data is an L4 product by Ssalto/Duacs and distributed by Aviso (Archiving, Validation and Interpretation of Satellite Oceanographic data), with support from Cnes (Centre National d'Etudes Spatiales) (http://www.aviso.oceanobs.com/duacs/). It consists of a daily near real time gridded dataset of sea level anomalies with a resolution of 1/8 degrees over the Mediterranean region, based on multiple satellite readings.
More information in: http://www.aviso.oceanobs.com/
Rio, M-H, P-M Poulain, A. Pascual, E. Mauri, G. Larnicol, 2007: A mean dynamic topography of the Mediterranean Sea computed from altimetric data and in-situ measurements. Journal of Marine Systems, 65, 484-508, doi:10.1016/j.jmarsys.2005.02.006
Users should keep in mind that the current fields provided here are an approximation of the geostrophic component of sea water velocity derived from the available altimetric measurements as above mentioned. As such, their accuracy and completeness is not guaranteed by any means. Therefore, this data should not be used for navigation, emergency planning or decisions involving the safety of human life and property at sea, and no responsibility can be accepted by those involved in its compilation or publication for any consequential loss or damage arising from its use.
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