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Altimetry to check Argo quality

Image of the month - September 2008

RMS of the differences between Sea Level Anomalies (SLA) and the Dynamic Height Anomalies computed from Argo measurements, as percentage of the SLA variance. One point represents the value for one float time series at its mean position: left, all floats, right, floats showing anomalous values. The background field for left map corresponds to the reference. (Credits CLS/Coriolis)

The Argo array, consisting in 3,000 autonomous floats drifting in the ocean, has been fully deployed for nearly one year now . It is complementary of altimetry since it gives an in-depth view of the global ocean. The floats are, of course, calibrated and tested before being launched, but many things can happen that lead to malfunctions, drifts or bias in the measurements. Altimetry can be used to validate those.

A good part of altimetry's sea level anomalies variations are due to temperature and/or salinity changes. Which are what Argo floats measure. So height anomalies can be computed from Argo and compared to altimetry. They won't match exactly -- due to the variations not linked to either temperature or salinity, and to what's happening at great depths (below 900 m). But they should be close enough, and big deviations of one from the other may be due do problems in one of the two measurements. If we take altimetry as reference, this enable to check the quality of the Argo floats using a totally independent technique.

Examples of comparisons between SLA and DHA for specific floats. Up, an example where the float and altimetry correlate well, down another that shows a progressive drift of the DHA time series regarding the SLA time series as the float is traveling from east to west in the Tropical Atlantic Ocean.  (Credits CLS/Coriolis)

See also:

Websites on this subject:

References:

  • Guinehut, S., C. Coatanoan, A-L. Dhomps, P-Y. Le Traon, G. Larnicol, On the use of satellite altimeter data in Argo quality control, JAOT, 2008 (accepted).
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