Where Is the Southern Ocean?

The Southern Ocean surrounds the Antarctic continent, around the South Pole. It is a circumpolar ocean, which means it covers all 360° longitude around the globe. Since this is not true for other oceans, it may partially explain why people have debated for centuries about what the Southern Ocean should be called—and whether it is an ocean at all!

Some credit James Cook with starting this debate, when he called it the “Southern Ocean” in his 1772–1775 voyage journals. Since then, the name “Southern Ocean” has gone into and out of existence. In 2000, nations voted on the name of this ocean and “Southern Ocean” narrowly won over “Antarctic Ocean” (18 votes to 10). Nations also struggled to agree on the location of the Southern Ocean’s northern boundary. In 2021, the National Geographic Society officially recognized the Southern Ocean on its maps by no longer extending the boundaries of the Pacific, Atlantic, and Indian Oceans to Antarctica. Oceanographers prefer to define the Southern Ocean by the position of its currents rather than by a specific latitude or land feature (Figure 1). Despite debates about its name and location, we know the Southern Ocean is vital to life on Earth.

Currents of the Southern Ocean

The Southern Ocean is home to the mightiest current in the world, called the Antarctic Circumpolar Current (ACC). The ACC flows eastward around the globe, carrying about 170 million cubic meters of water per second past the southern end of South America. The ACC is made up of a bunch of smaller currents, like thin branches of quickly flowing water, that together can be up to 2,000 km wide and reach down more than 4,000 m to the seafloor. On its northern boundary, the ACC connects with the ocean gyres to its north. Gyres are large, almost circular systems of currents. The ACC also connects the Indian, Pacific, and Atlantic Oceans together. Toward Antarctica, the Southern Ocean also has big gyre circulations in the Ross and Weddell Seas, and other important currents around the Antarctic continent. All of these currents are important for transporting water around the Southern Ocean. You can watch a beautiful computer simulation of Southern Ocean currents here.

Flipping the Southern Ocean Upside Down

In addition to the mighty eastward-flowing current system of the ACC, the Southern Ocean is also home to another, much slower, vertical circulation (Figure 2). In this up-and-down circulation, waters that cooled and sank in the North Atlantic hundreds to thousands of years ago finally make their way back to the sea surface in the Southern Ocean, by climbing upward as they follow the ACC around Antarctica [1]. These old, deep waters that rise back to the surface are replaced by downward flows of Southern Ocean surface waters, in two ways. On the northern edge of the Southern Ocean, the winds make surface waters subduct, or sink, into the mid-depths of the ocean. On the southern edge, waters near the Antarctic continent lose their heat to the atmosphere and gain salt when sea ice forms, becoming both colder and saltier. These dense waters sink to the bottom of the ocean, forming the deepest water mass on Earth, called Antarctic Bottom Water. This water fills up 30–40% of the world’s oceans [2].

The Southern Ocean Protects Us From Climate Change

The strong eastward-flowing currents of the ACC are like a wall, blocking the waters from crossing north to south or south to north. This blocks the extra heat that the Earth receives from the Sun at the equator from crossing the ACC and making its way directly to Antarctica. The barrier that the ACC makes, blocking heat traveling southward, is crucial for us because it keeps Antarctica cold and fully covered in ice. If all of the ice covering Antarctica were to melt, the average global sea level would increase by about 60 meters [3]! Therefore, without the ACC, there would be a lot less land available for us to live on, and most of today’s cities would be completely under water.

When surface waters plunge into the deep ocean, they take lots of heat and carbon from the atmosphere with them. By flipping the ocean waters upside down, the Southern Ocean controls how hot it feels to us on land. About 90% of the excess heat produced by human activities is being absorbed by the global ocean (Figure 3), with 75% of it going into the Southern Ocean [4]. The Southern Ocean also absorbs more of the greenhouse gas carbon dioxide (CO₂) emitted by human activities than any other ocean (for more information see here). So, the Southern Ocean can slow down climate change on land by storing heat and carbon in the deep ocean, but this ocean storage can have long-lasting climate impacts and have negative effects on those living in the ocean. For example, as the ocean warms and absorbs more carbon, it becomes more acidic. The acidity of the global ocean has increased by about 30% since the industrial revolution. This is harmful to some small ocean critters—especially those that have shells—and bigger animals rely on these smaller organisms for food.

The vertical motion in the Southern Ocean brings deep waters back up to the surface—waters that have not been near the ocean surface since they sank in the North Atlantic hundreds to thousands of years ago. During that long journey southward, this water mass builds up nutrients, as sinking organic particles (sometimes called marine snow) dissolve back into the ocean. So, when the deep water finally returns to the surface in the Southern Ocean, it is packed full of nutrients that provide food for phytoplankton—the base of the Southern Ocean food web. All life in the Southern Ocean, from tiny, shrimp-like krill to enormous humpback whales, depends on the nutrients that help phytoplankton to grow. Not only do phytoplankton make up the base of the food web, but they are also effectively massive ocean forests, inhaling CO₂ from the atmosphere and exhaling oxygen back out. In half of all the breaths we take, we are breathing in oxygen from phytoplankton—that is more oxygen than all of the tropical rainforests on land make, combined.

What About the Future?

We still have lots to learn about how carbon and heat move through the Southern Ocean. This ocean is a place of mystery and wonder, with many big unanswered questions. For example, how much natural CO₂ will the Southern Ocean eventually release back into the atmosphere, and how will this affect us? We are certain that the Southern Ocean has warmed and gotten less salty in recent decades due to greenhouse gas emissions from human activities [5]. We are also certain that changes in the circulation of the Southern Ocean will play a big role in Earth’s changing climate—by influencing how quickly the Antarctic Ice Sheets melt, how much CO₂ remains in the atmosphere, and the health of ocean ecosystems. Our actions play a key role in the wellbeing of the Southern Ocean. By reducing our greenhouse gas emissions, we will help the Southern Ocean adapt to our changing climate.

Glossary

Circumpolar: ↑ Existing across all longitudes; going all the way around the pole.

Antarctic Circumpolar Current: ↑ A series of strong currents that flow continuously across all longitudes of the Southern Ocean.

Gyre: ↑ A large system of ocean currents that create a circular pattern of horizontal motion, contained and guided by the coasts of nearby continents.

Subduct: ↑ To sink; in our context, the sinking of water down into the lower layers of the ocean as it becomes more dense.

Sea Ice: ↑ Ice that floats on the ocean’s surface, formed by the freezing of ocean water. Importantly, when sea ice forms, only the freshwater freezes and the salt remains in the ocean water.

Water Mass: ↑ A layer of water in the ocean that has different properties (like temperature, saltiness, and oxygen concentration) than the layers above or below it.

Acidity: ↑ A measure of how basic (like milk) or acidic (like orange juice or vinegar) something is.

Phytoplankton: ↑ Microscopic plants that live in the surface waters of the ocean and make up the base of the Southern Ocean food web.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.