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Eddies are spreading salt in the ocean

Image of the Month - March 2021

Sea surface salinity plays an important role in ocean dynamics. It also reflects the evaporation/precipitation cycle in the ocean and is thus a major variable utilized in climate studies. Prior to satellite-based observations, neither salinity, at the mesoscale, nor the eddy-induced transport of salt were well known. However, global systematic monitoring of sea surface salinity from space has now revolutionized our capabilities starting with the SMOS mission (ESA, 2009), Aquarius (NASA, 2011-2015) and SMAP (NASA, 2015).  Errors of this new technology are significant, though, hindering observations of mesoscale salinity features in individual measurements. 

Altimetry, likewise, was not primarily designed with mesoscale circulation in mind. The quality of the measurements plus the fact that several altimeters have been flying at the same time since late 1992 enabled one to overcome the spatial resolution issue of the nadir measurement. Now, datasets merging as many altimeters as there are flying are produced daily, and derived "eddy trajectory" datasets are generated from them.

Weekly-averaged SMOS or SMAP data, combined with altimetry sea level anomalies, can help to identify eddies in sea surface salinity. However, there are still relatively large discrepancies between the two salinity datasets at the mesoscale. To remove the noise, averaging, using an altimetry-generated eddy atlas as reference to define a moving frame, can be used to build composites. From the resulting maps, the salt fluxes, either meridional (north-south or south-north) or zonal (east-west or west-east) can be estimated, and the differences between the tropics, near-polar and mid-latitudes, as well as the differences between the oceans are highlighted. 

Combining several techniques enables one to reach improved resolutions, higher accuracies or even data not (yet) measured, or not globally available. Altimetry participates and benefits from these improvements, with its long, stable, time series of unprecedented accuracy. 

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Reference:

  • Melnichenko, O.; Hacker, P.; Müller, V. Observations of Mesoscale Eddies in Satellite SSS and Inferred Eddy Salt Transport. Remote Sens. 2021, 13, 315. https://doi.org/ 10.3390/rs13020315
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