Two of the mightiest rivers in the world, although a continent apart, have a lot in common. The Congo River in Africa and the Amazon River in South America both originate near the equator and experience a deluge of nearly constant rainfall. The Amazon is the world’s largest river in terms of discharge, and close behind it, the Congo sends more than 10 million gallons of water per second into the Atlantic Ocean.
Large amounts of freshwater that are discharged into the ocean near the equator such as these are known as near-equatorial river plumes (NERPs). They have a significant effect on water salinity, pollution, carbon and nutrient levels, sea surface temperature, and other physical properties in these regions of the ocean, and the variations they cause can, in turn, affect the well-being of marine ecosystems and climate.
Few in-depth studies have been done on NERPs, however. Palma and Matano look to the Congo River as an example of how NERPs worldwide might behave.
Using an ocean model, the researchers found that as NERPs spread from east to west, they leave westward moving eddies in their wake. They also transport most of their freshwater deep into the ocean, which is different from the behavior of river plumes located farther away from the equator.
When freshwater from a river enters a coastal ocean, a rotating mass of water called a bulge often forms near the mouth of the river. A bulge that forms in a NERP will spread westward, the researchers found, creating layers of water with different densities and allowing smaller, swifter, swirling eddies to form. At this point, the NERP may also generate cyclones, pumping saltier water to the surface and creating small anomalies in sea surface salinity.
NERPs are sensitive to changes in the shape of the river mouth and seafloor, the researchers found, as well as changes in freshwater flux, the amount of low-salinity water outflowing from the river mouth. For example, a large freshwater flux will generate proportionally larger eddies.
By modeling the Congo, the study sheds light on the behavior of river plumes in near-equatorial regions. The authors expect that an improved understanding will lead to more answers about how these plumes affect the ocean regions into which they flow and their potential effects on aquatic plants, animals, and climate. (Journal of Geophysical Research: Oceans, https://doi.org/10.1002/2016JC012554, 2017)
—Sarah Witman, Freelance Writer