Séminaire

The seasonal variability of the surface horizontal thermohaline structure is investigated in the subtropical and tropical north Atlantic Surface Salinity Maximum (SSM) at length scales from 5-10 km to more than 500 km. The near surface temperature and salinity data from merchant ship thermosalinograph (TSG) transects across the Atlantic, from ISAS product interpolated from Argo profiles, and a combination of SST from Tropical Microwave Imager (TMI) and SSS from the recently launched Soil Moisture and Ocean Salinity (SMOS) satellite, were used as complementary tools to compute the horizontal temperature, salinity and density gradients and the density ratio in the northern subtropical-tropical Atlantic. During late winter, in north-eastern SSM, the thermohaline compensation is observed for wavelengths from 5-10 km to more than 500 km. In spite of large and sharp surface thermohaline fronts a very weak density surface horizontal gradient is observed. In contrast, in the southern SSM, at large scale (>100 km) the density ratio is controlled by the salinity gradient and the horizontal density gradient is sharpened by a constructive contribution of opposite salinity and temperature gradients. New SMOS products demonstrate promising capacity for investigation horizontal mesoscale SSS variability.

The Argo data set is then used to study interannual variability of the late winter condition in the horizontally compensated region of the north-eastern subtropical-tropical Atlantic during 2007-2012. During winter 2010, the mixed layer is abnormally thin and a strong negative anomaly of density compensated salinity ('spiciness') is generated in the permanent pycnocline. This is primary explained by unusual weak buoyancy flux during the late winter in the subtropical-tropical north Atlantic. These conditions contrast with the 5 other studied winters, that shows deeper mixed layer and positive spiciness anomalies in the permanent pycnocline. Particular, deep mixed layer and strong spiciness anomalies are observed during late winter 2012. The conditions during winter 2010 are likely explained by historically low North Atlantic Oscillation (NAO) and high Tropical North Atlantic index (TNA).