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 by flow. But once that huge amount of water reaches the ocean, where does it go?A new study has answered that question by following the freshwater of a river after it leaves the African coast. Using high-resolution computer models, satellite observations and measurements collected at sea, researchers found that giant swirling ocean currents can trap freshwater from the Congo River and carry it hundreds of kilometers away into the open Atlantic.The findings show that this movement is not constant or gradual. Instead, short-lived but powerful oceanic events do much of the work, helping to transport fresh water, nutrients and other materials away from river mouths. Researchers say this has significant impacts on ocean circulation, climate, marine ecosystems and fisheries in the tropical Atlantic.
Pile of fresh water extending for 800 kilometers
The Congo River is one of the world’s largest sources of freshwater flowing into the sea. On average, it releases about 40,000 cubic meters of water every second. At its mouth, near the west coast of Central Africa, that fresh water spills out onto the ocean surface to form a giant plume, or broad area of low-saline water, that extends up to 800 kilometers from the coast.This plumage changes with the seasons.The river reaches its highest discharge around December, while water volume drops to its lowest level around August. Due to this seasonal cycle, the freshwater reef grows and reaches its greatest offshore extent around March before shrinking again during July and August.During the wet part of the year, from January to April, the plume shifts towards the south-west under the influence of winds, ocean currents and the shape of the coastline. It is during this period that large rotating ocean currents known as mesoscale eddies become particularly important.Mesoscale eddies are huge circular currents that can measure hundreds of kilometers and last for weeks or months. They behave like giant moving ponds in the ocean, trapping water inside them as they move.
following the water in the atlantic
To understand how these eddies affect the flow of the Congo River, scientists from the Laboratory for Space Geophysical and Oceanographic Studies (LEGOS) and partner institutions focused on the conditions during 2016. They chose that year because it offered an unusually rich collection of observations from satellites, ships, and ocean monitoring instruments.The researchers used NEMO, short for NUCLEUS, for European modeling of the ocean, a sophisticated computer model that simulates ocean circulation at a resolution of about three kilometers. The model covered the Gulf of Guinea and surrounding waters and included daily measurements of Congo River discharge.To check that the simulation reflected real conditions, the team compared it to 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 Prediction and Research Moored Array in the Tropical Atlantic, known as PIRATA, and surface currents estimated using Automatic Identification System data from ships processed by eOdyn.Comparisons showed that the model accurately reproduced the size, position, and seasonal movements of the Congo River’s flow, giving researchers confidence to investigate individual events in more detail.
A giant sea whirlpool changes the picture
One incident in particular stood out.During March and April 2016, a large anticyclonic vortex formed close to the Freshwater Pile. In the Southern Hemisphere, an anticyclonic vortex rotates counterclockwise. The swirling current survived for 49 days and eventually reached a radius of about 150 kilometres.As it rotated, the whirlpool trapped low-salinity water from the Congo River basin into its center. It carried that fresh water about 200 kilometers from the coast before finally breaking up.
Tracking over 5,000 virtual particles
To find out where the trapped water came from, the researchers conducted particle-tracking experiments. They released more than 5,000 virtual particles inside the computer model and tracked them over time.The results showed that the water found inside the center of the eddy during April could be found in the southern part of the Congo River as early as March.Something important came to light from that discovery. Rather than slowly spreading into the Atlantic through constant mixing of freshwater, most of the transport occurs during occasional but intense events when large eddies capture river water and carry it offshore.
Fresh water flows fast, not stagnant
The scientists also examined how fresh water moved in and out of the study area over the course of the year.They found that most of the net export of freshwater went westward into the Atlantic. The strongest changes in salinity, or the amount of salt dissolved in seawater, came from water entering the study area from the eastern boundary and vertical mixing between the surface and deeper layers.The role of mesoscale eddies was different. Their influence was intense but intermittent. During periods when the mass of freshwater was widespread, these swirling currents may have dominated the movement of river water in the open ocean.The team also observed small rotating features called submesoscale eddies. These are only a few kilometers wide and exist for very short periods of time. While they generally played a minor role throughout the year, they were sometimes responsible for more than 30 percent of salinity transport during short-term events.Despite these bursts of activity, the researchers found that the overall seasonal cycle remained the main driver of freshwater movement throughout the year.
Why do these swirling currents matter?
The water level of the Congo River is more important than sea salinity.The river carries large amounts of dissolved organic carbon, nutrients and other materials that help support marine life. Previous studies have shown that the Congo is one of the largest rivers in the world to export particulate organic carbon to the ocean. Those nutrients can boost biological productivity, supporting the plankton that form the basis of the marine food web.As fresh water spreads into the Atlantic, it also affects how heat, salt and other substances are distributed in the upper ocean. Those changes can affect regional ocean circulation and, in turn, climate patterns.By showing how giant eddies carry freshwater away from the coast, the study offers a clearer picture of how these materials are redistributed in the tropical Atlantic.
What comes next for researchers
The researchers say their work shows the importance of mesoscale marine activity in moving freshwater away from the Congo River during small but significant episodes. Also, they caution that their analysis focuses on one year.Understanding how these processes change from one year to the next will require future research covering multiple years and using new high-resolution satellite missions that will provide an even more detailed view of ocean currents.The findings were published in peer-reviewed Geophysical Research Journal: ocean On 10 June 2026. study, title Congo River dynamics and its impact on water exchange between coastal and open ocean at different time scales.It was led by C. Cardot and co-authored by I. Dadou, DC Napolitano, HMA Aguedjou, R. Nagakala, Y. Morel, G. Morvan, G. Alori, C. Le Goff, G. Jaan and J. It was Juanno. The researchers used a high-resolution NEMO ocean model combined with satellite observations and in situ measurements to investigate how freshwater from the Congo River is transported to the Atlantic Ocean.