Congo River: The Congo River releases 40,000 cubic meters of fresh water into the Atlantic Ocean every second. Scientists track its whereabouts
The Congo River releases approximately 40,000 cubic meters of fresh water into the Atlantic Ocean every second. This makes it the second largest river in the world. But once a lot of water flows into the sea Where did it go?A new study answers this question by tracking the fresh water in rivers after they leave the coast of Africa. Using high-resolution computer models, satellite observations and measurements collected at sea, researchers found that giant rotating ocean currents can capture fresh water from the Congo River and carry it hundreds of kilometers out into the Atlantic Ocean.The findings suggest that this movement is not steady or gradual. Instead, brief but powerful ocean events play most of the role, helping move freshwater, nutrients and other materials far away from the estuary. Researchers say this has important implications for ocean circulation, climate, marine ecosystems and fisheries in the tropical Atlantic.
800 kilometers of freshwater plume
The Congo River is one of the world’s largest sources of fresh water flowing into the ocean. On average, about 40,000 cubic meters of water are released per second. At the estuary near the west coast of Central Africa, fresh water spreads to the ocean surface, forming huge plumes, or large areas of less salty water, that extend up to 800 kilometers offshore.This plume changes with the seasons.Around December, the river reaches its highest flow, while around August, the water volume drops to its lowest level. Because of this seasonal cycle, freshwater plumes grow and reach their greatest offshore extent around March before shrinking again in July and August.During the wetter times of the year, January through April, the plume moves southwestward under the influence of winds, ocean currents, and the shape of the coastline. It is during this period that large rotating ocean currents called mesoscale eddies become particularly important.Mesoscale eddies are huge annular flows that can be hundreds of kilometers in diameter and last for weeks or even months. They behave like giant spinning pools in the ocean, trapping water inside as they move.
Follow the current into the Atlantic Ocean
To understand how these eddies affect the Congo River plume, scientists from the Laboratory for Geophysical and Oceanographic Studies in Space (LEGOS) and collaborating institutions focused on 2016. They chose this year because it provided an unusually rich collection of observational data from satellites, ships and ocean monitoring instruments.The researchers used NEMO (short for Nucleus European Ocean Modeling), a complex computer model that can simulate ocean circulation at a resolution of about three kilometers. The model covers the Gulf of Guinea and surrounding waters and includes daily measurements of Congo River discharge.To check whether the simulation reflected real conditions, the team compared it with several independent sources of information. These include sea surface salinity data from NASA’s Soil Moisture Active Passive satellite, sea surface height measurements from satellite altimeters, current measurements from the Tropical Atlantic Forecasting and Research Mooring Array (known as PIRATA), and surface current estimates using Automatic Ship Identification System data processed by eOdyn.The comparison showed that the model accurately reproduced the size, position and seasonal movement of the Congo River plume, giving researchers the confidence to study individual events in more detail.
Huge ocean whirlpool changes scene
One incident in particular stands out.During March and April 2016, a large anticyclonic eddy formed near the freshwater plume. In the Southern Hemisphere, anticyclonic vortices rotate counterclockwise. The rotating current lasted for 49 days, eventually reaching a radius of about 150 kilometers.As it spins, the eddy traps low-salinity water from the Congo River plume at its center. It then carried fresh water about 200 kilometers from the coast, where it eventually broke.
Track over 5,000 virtual particles
To pinpoint the source of the trapped water, the researchers conducted particle tracking experiments. They released more than 5,000 virtual particles within a computer model and traced them through time.The results show that water found in the center of the eddy in April can be traced to the southern part of the Congo River plume in early March.This discovery revealed something important. Freshwater diffuses slowly into the Atlantic Ocean not through constant mixing, but in occasional but violent events as large eddies capture river water and carry it offshore.
Fresh water flows in bursts, rather than in a steady flow
The scientists also studied how fresh water moved in and out of the study area over the course of the year.They found that most of the net freshwater export flows westward into the Atlantic Ocean. The strongest changes in salinity, or the amount of salt dissolved in seawater, come from water entering the study area from the eastern boundary and vertical mixing between surface and deeper layers.The role of mesoscale eddies is different. Their effects are strong but intermittent. During periods when freshwater plumes extend furthest offshore, these swirling currents may dominate the movement of river water into the open ocean.The team also studied smaller rotating features called submesoscale eddies. They are only a few kilometers wide and exist for a much shorter period of time. Although they generally play a smaller role during the year, during brief events they occasionally account for more than 30% of salt transport.Despite these bursts of activity, the researchers found that overall seasonal cycles remain the main driver of freshwater flow throughout the year.
Why these vortices are important
The importance of the Congo River plume extends beyond ocean salinity.The river carries large amounts of dissolved organic carbon, nutrients and other materials that help sustain marine life. Previous research has shown that the Congo is one of the world’s largest rivers exporting particulate organic carbon to the ocean. These nutrients increase biological productivity and support the plankton that form the basis of ocean food webs.As freshwater moves across the Atlantic Ocean, it also affects the distribution of heat, salt and other materials in the upper ocean. These changes affect regional ocean circulation, which in turn affects climate patterns.By showing how giant eddies move fresh water away from the coast, the study provides a clearer picture of how these materials are redistributed across the tropical Atlantic Ocean.
What will researchers do next?
The researchers say their work demonstrates the importance of mesoscale ocean activity in removing freshwater from the Congo River during brief but significant events. At the same time, they cautioned that their analysis focused on a single year.Future studies over many years are needed to understand how these processes change from one year to the next, as well as using new high-resolution satellite missions to provide more detailed views of ocean currents.The findings were published in a peer-reviewed journal Journal of Geophysical Research:ocean June 10, 2026. The study is titled Dynamics along the Congo River at different time scales and their impact on coastal and open water exchangesled by C. Cardot and co-authored by I. Dadou, DC Napolitano, HMA Aguedjou, R. Ngakala, Y. Morel, G. Morvan, G. Alory, C. Le Goff, G. Jan and J. Jouanno. Researchers used the high-resolution NEMO ocean model along with satellite observations and field measurements to study how freshwater from the Congo River is transported to the Atlantic Ocean.
