Ocean Science in Action - Oceans of the future
4. Introduction to remote sensing
4.3 The Hills and Valleys of the Ocean: Sea Surface Height and Ocean Circulation from Satellite Altimetry
Video duration - 05:41
Satellite altimeters have been exploited to derive information on the ocean surface since the 1978 Seasat mission.
These are active sensors which transmit signals, in the microwave portion of the electromagnetic spectrum, to the sea surface and measure the return signal travel time. Such microwave measurements are not affected by clouds or night, which allows homogeneous and repeated global observations of Sea Surface Height (SSH), ocean surface currents, significant wave height and winds, among other environmental parameters.
A synoptic view of these parameters can be provided by mapping the data onto a regular grid, typically at 25 km spatial resolution and on a daily basis, from the along-track measurements from several radar satellite altimeters (TOPEX/Poseidon, Jason-1, Jason-2, Jason-3, ERS-1, ERS-2, GFO, Envisat, Sentinel 3).
The changing height of the ocean surface can be assessed from SSH anomalies. An “anomaly” is the difference between the long-term average and what is actually observed by the satellite. These anomalies help identify unusual patterns in the ocean. A key piece of information inferred from SSH anomalies is defining regions at risk of sea level rise. In some areas, the sea level is rising by more than 10 mm/ year as in the North Atlantic and in others it is decreasing to more than -10 mm/year as in the South Atlantic. The average global rise in sea level derived from 25 years of satellite measurements increased steadily by 3 mm/year between 1993 and 2015 and has accelerated to 5 mm/year in the last 5 years. That might not seem much, but a permanent global 3 mm /year rate becomes 3.7 cm in 10 years and 30 cm in 100 years’ time. By that time, some ocean regions could even face 2 m rise in sea level, as the rate of rise varies spatially.
By measuring SSH variations, it is possible to calculate ocean surface currents using the geostrophic approximation. Satellite altimetry processing techniques are also applied to study complex coastal areas and shallow-water regions, which present major economic and ecological issues. The measurements can also be used to study the Earth’s shape and size, gravity anomalies, bathymetry, sea ice thickness and land ice topography.
Dr F. Jebri, Prof M. Srokosz - NOC
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