SMOS detects and monitors the Amazon and Orinoco freshwater plumes

Category : CATDS, L3, Ocean

SMOS instrument detects the evolution of the Amazon and Orinoco freshwater plumes

Nicolas Reul (Ifremer) and Joe Tenerelli (CLS)

CATDS

On an annual basis, the Amazon river discharges ~15% of the total freshwater input to the ocean.

Between July and October, more than half the Amazon plume water is carried eastward by the North Brazil current which separates from the South American coastline at 6°N–8°N and curves back on itself (retro-flects) to feed the eastward North Equatorial Counter-current. Large warm and fresh core rings that can exceed 400 km in diameter can then detach from the retroflection and move northwestward toward the Caribbean.

July 9 to August 21 2010

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(See film explanations below)

Anticipated strong gradient of salinity in this region are therefore expected and have been unambiguously detected in the measured SMOS brightness temperatures. These strong surface signatures are corresponding to very low salinity below 32 psu and associated brackish water, as also revealed by ocean color imagery.

SSS Amazon period 2

Figure: Left panels: 7-day averaged SMOS salinity maps. Right: 7-day averaged merged GSM Meris/Modis CDOM optical measurements. Top mid-July. Bottom: mid-August 2010. As detected, both Amazon and Orinoco Plume evolution are well captured for these two weeks.

In mid-july, SMOS clearly detects the entrainment of river-influenced shelf waters offshore as a North Brazilian Current ring flows across the mouth of the Amazon River. Few weeks later the Amazon freshwater plume curves back on itself to extend eastwardly. At the same time, the Orinoco Plume is also visible with a freshwater tongue near 60°W, 12°N.
The SMOS instrument temporal and spatial sampling characteristics further help to track the detailed temporal variability and the location of the low salinity surface layers over synoptic scales (see movie). As evidenced, the quantitative links between surface salinity and optical properties of the plumes can now be more robustly assessed to study bio-optical and bio-chemical properties of this key oceanic freshwater pool region.

Nicolas Reul & Joe Tenerelli

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