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

Between July and October, more than half the Amazon plume water is carried eastward by the North Brazil Current (NBC) which separates from the South American coastline at 6°N–8°N and curves back on itself (retroflects) to feed the eastward North Equatorial Countercurrent (NECC). Large eddies, or rings, of plume water often detach from the retroflection and move northwestward toward the Caribbean.  These warm core rings can exceed 400 km in diameter, mix little with the surrounding saltier ocean, and can move a substantial mass of fresh water far north from the mouth of the Amazon.

 Strong spatial gradients in the sea surface salinity are therefore expected in this region and we have unambiguously detected Amazon plume water in the measured SMOS brightness temperatures. The distinguished surface signatures correspond to very low salinity, below 32 psu, and associated brackish water from the river.  This is also revealed by water clarity data obtained from satellite ocean color imagery.

Figure 2:Left panels: 7-day averaged SMOS salinity maps at 0.25° resolution. Right: 7-day averaged merged GSM Meris/Modis CDOM optical measurements at 0.25° resolution (http://www.globcolour.info/). Top mid-July. Bottom: mid-August 2010. As detected, both Amazon and Orinoco Plume evolution are well captured for these two weeks.

As illustrated in Figure 2, 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. A few weeks later the Amazon freshwater plume curves back on itself to extend eastwardly. At the same time, the Orinoco Plume is also visible as a freshwater tongue near 60°W,12°N.

A dramatic step forward gained with SMOS is the ability to track the time evolution and location of the low salinity surface layers over synoptic scales (see movie), particularly for large plumes such as the Amazon.

Movie: click on the image above to see an animation of the 7-day running mean SMOS salinity (left) and of the corresponding Colored Dissolved Organic Matter product which is a composite deduced from both Envisat/MERIS  and Aqua/MODIS data.