Ocean currents are continuous, directed movements of seawater that govern the distribution of heat and nutrients across the globe, shaping marine environments and global weather patterns. Among the planetary system of ocean circulation, the longest current in the world is the Antarctic Circumpolar Current (ACC).
The Antarctic Circumpolar Current (ACC)
The Antarctic Circumpolar Current (ACC) flows eastward in an uninterrupted loop around the entire continent of Antarctica. This immense stream links the Atlantic, Pacific, and Indian Oceans, making it the only current to connect all three major global ocean basins. It is often referred to as the West Wind Drift due to its primary driving force and direction.
This current is the largest by volume, transporting an estimated 135 to 147 Sverdrups (Sv) of water. By comparison, this volume is roughly 135 times the combined flow of all the world’s rivers. While the speed of the water flow itself is relatively slow, the sheer scale contributes to its enormous transport volume. The ACC is not a single, narrow stream but a broad system that can span over 2000 kilometers in width and extend to depths between 2000 and 4000 meters, reaching the seafloor in many places.
The current is composed of several distinct jets, or fronts, which meander across the Southern Ocean. Its movement is generally confined between the latitudes of 40°S and 60°S. This colossal, unblocked flow of water is a dominant feature of the Southern Ocean’s circulation.
What Powers the ACC’s Unbroken Path
The exceptional length and velocity of the Antarctic Circumpolar Current are primarily dictated by the powerful, persistent Westerly winds in the Southern Hemisphere. These winds transfer momentum to the surface water, pushing the current along its eastward path. The lack of any major landmasses to block this wind-driven flow allows the energy transfer to result in continuous water movement.
A key geographical feature enabling this uninterrupted circulation is the Drake Passage, the narrow strait between the southern tip of South America and the Antarctic Peninsula. This deep, open gateway is the final choke point that ensures the ACC can flow completely around the globe without being diverted. Without this open passage, the current would be blocked by the continents, behaving more like the boundary currents found in the northern oceans.
The constant wind stress on the ocean surface is balanced by an interaction with the seafloor topography, particularly within the Drake Passage. Submarine mountain ranges and ridges exert a drag that helps prevent the current from accelerating indefinitely. This encounter with the seafloor topography is a significant factor in steering the ACC.
The ACC’s Role in Global Climate Regulation
The Antarctic Circumpolar Current performs a major function as a thermal barrier, isolating Antarctica from the warmer waters to the north. By continuously circulating cold water, the ACC prevents heat from being transported southward, helping to maintain Antarctica’s extreme cold and its vast continental ice sheet.
The current acts as a primary mixer of the global ocean basins, facilitating the exchange of water properties like temperature and salinity between the Atlantic, Pacific, and Indian Oceans. This process is integral to the larger system of global deep-ocean circulation. The ACC also plays an important part in the upwelling of deep water, bringing cold, nutrient-rich water toward the surface.
This upwelling brings essential nutrients to the surface layer, supporting a highly productive marine ecosystem that forms the base of the Antarctic food web. The ACC is also a substantial component in the global distribution of heat and dissolved gases, including carbon dioxide. It influences the ocean’s overall capacity to absorb atmospheric heat, making it a central feature in regulating the Earth’s climate system.
Distinguishing Surface Flow from Deep Ocean Circulation
The Antarctic Circumpolar Current is the longest single, continuous, wind-driven current on Earth. However, the entire system of global ocean circulation, known as the Thermohaline Circulation or the Global Conveyor Belt, covers a much longer total distance. This deep-ocean circulation is driven by differences in water density, which are controlled by temperature and salinity.
The ACC, while technically a surface current because it is wind-driven, extends to great depths and is intimately linked to the deeper system. It acts as a major conduit, transporting and transforming water masses that feed into the vast, slow-moving deep-ocean conveyor. The distinction is that the ACC is a relatively fast, unbroken zonal flow that circles the planet, while the Thermohaline Circulation is a global network of interconnected currents that travel across all ocean basins over thousands of years.