Towards a better estimation of surface currents using satellite Sea Surface Salinity data

 

Sea Surface Salinity from SMOS (color) in the Gulf Stream region superimposed with surface currents deduced from altimeters (Reul et al., 2014).

 

Sea surface Height (SSH)  data from altimeters are key observations to reveal the ocean variability at large spatial scales (>100-200 kms). Currently orbiting altimeters do not provide the ocean variability at the smaller so-called meso-scales (from 50 to 200 km) and sub-mesoscales  (< 20 km). Recent theoretical, numerical and experimental studies (e.g. Earth Simulator simulations at very high resolution) have however revealed  that  the ocean surface dynamic in these ranges of horizontal scales is no more characteristics of bi-dimensional turbulence, so that the dynamic at smaller scale cannot be directly inferred from the large scale ones, e.g., through direct energy cascade.

 

SSH can allow a better constraint of the estimation of the surface density field to help assess in more detail the characteristics of correlation between these fields. As proposed in several recent papers (Lapeyre and Klein, 2006; Isern-Fontanet et al., 2008; LaCasce, J. H., 2012) subsurface velocity and density fields can be extrapolated from sea surface density and SSH via the Surface Quasi-Geostrophic (SQG) formalism. Such method was shown to be particularly successful in energetic regions like the Gulf Stream extension (Wang et al., 2013).

Lacking regular sea surface salinity (SSS) field mapping capability at sufficient spatial resolution, these previous works all assumed that temperature dominates the surface density variations and generally used a constant SSS value. SQG methods implicitly assume the surface density and interior potential vorticity (PV) are correlated. In fact, the two fields are often quite different. Were they perfectly correlated (or anti-correlated), the sea surface height

would resemble the surface density. But satellite SSH and SST anomalies are found to be not always coincident (e.g. Wang, 2013). While there are regions where SST and SSH anomalies are correlated, there are others where they are not. Surface heating/cooling can thus modify the surface signature of near-surface density anomalies (their strength and lateral structures) making them different from surface pressure gradient that is reflected in SSH variability. Moreover, interior PV can be non-zero due to baroclinic instabilities that affect surface pressure without changing the surface density. Finally, in presence of strong horizontal gradients of salinity, surface density is not always dominated by temperature.

            The combination of the new SMOS and Aquarius satellite SSS with SST and SSH data provide a first opportunity to estimate the synoptic surface density fields on mesoscales and to further assess the level of correlation between the latter and the SSH fields. The post-doc candidate will analyse such correlation in several key dynamical areas of the world ocean.

 

Key words: surface currents, sea surface salinity, SMOS, density, altimetry, temperature

 

 

Ifremer offers post-doctoral positions to young French or foreign scientists who have completed their PhD. and are motivated by development and innovation in various fields of Marine Sciences : technology and ecotechnology, aquaculture, fisheries, environment, risks analysis, physics of oceans, etc.

Postdoctoral positions are contracted for a duration of 12 months, possibly renewable for a non-renewable 6-month period.

Interested applicants should send :

1. A handwritten letter including the specific skills and competence for the particular postdoctoral position you are interested in.
2. A detailed curriculum vitae.
3. A summary of work previously done, mentioning the date of submission of the thesis,
4. A list of publications and communications / symposia.
5. Two letters of recommendation.

Complementary information can be obtained through exchanges with the Scientist Contact at ifremer.

 The criteria for selecting candidates are the following :

• The Curriculum Vitae of the nominee must be consistent with the proposal regarding the post-doctoral position.
• The candidate must be under 35 years old at December 31, 2014.
• The candidate has not previously performed post-doctoral research at Ifremer.
• The candidate has spent a majority of his (her) doctoral position out of Ifremer.

The list of subjects of the call for post-doctoral candidates can be downloaded here :

Postdoctoral positions will begin from the 1st of November, 2015.

The candidate will work in the Ifremer Mediterannean Center located in La Seyne-sur-Mer