Spatio-Temporal Coherence between Spaceborne Measurements of Salinity and light Absorption in the Amazone Plume Region
Fournier, Severine; Reul, Nicolas; Chapron, Bertrand
Two of the major low salinity pools in the western part of the Atlantic basin are the Amazon and Orinoco river plumes spreading offshore from the South America North-eastern coasts, and influencing a large fraction of the western Tropical North Atlantic. The Amazon river discharges into the Atlantic Ocean at the equator with an approximate contribution of 16% of the global freshwater run-off. This results in large-scale low salinity 'lenses' at the Tropical Atlantic surface possibly traced over distances ranging from several hundreds up to thousands of kilometers at the ocean surface. These lenses are characterized by relatively small thicknesses as well as by distinct and strongly seasonal spatial extents. Tracking the temporal variability of their locations over synoptic scales is essential with regard to ecology, biogeochemistry and bio-optics. Primary production is indeed first stimulated by the nutrient inputs carried by the run-off of the rains over the land and by the flood of the larger rivers, and sinking fluxes will then results in carbon sequestration. As freshwater input by river decreases ocean surface salinity, it indeed reduces concentration of total inorganic CO2 and CO2 fugacity (fCO2). Undersaturations have been reported in the western Tropical Atlantic due to the influence of Amazon waters causing strong biological uptake. Several authors already identified a significant negative linear relationship between SSS and bio-optical parameters such as the diffuse attenuation coefficient at 490 nm (K490) and the absorption coefficient for dissolved organic and detritus material (CDOM) originating from river discharge in the Tropical Atlantic. High correlations generally found in the equatorial region between biogeochemical and bio-optical parameters (fCO2, Chl, K490, CDOM) and SSS thus further stresses the need for high spatial and temporal resolution SSS data. Analyzing SMOS data, we can unambiguously detect the Amazon plume water in the measured SMOS brightness temperatures. Distinguished surface signatures corresponding to very low salinity, below 32 psu, are generally associated brackish water from the river. SMOS data can then be used in conjunction with Altimetry, SST and ocean color data to better characterize the Amazon and Orinoco plumes seasonal cycle evolution. As demonstrated, this helps first, to document the spatial coherence between Amazon discharge, SSS, CDOM, and the nature of the salinity-CDOM relationship, second, to obtain the temporal and spatial variability of CDOM along the trajectory of the low-salinity Amazon plume, and third, to explore the departure from conservative mixing behavior along the plume trajectory into the open ocean.