Southern Ocean overturning circulation

Big ocean water moves in a loop.

This loop happens in the cold Southern Ocean.

Water moves from the top to the bottom. It also moves from the bottom to the top. This moves heat and food around the world.

Wind helps push the water at the top. In the deep parts, cold and salty water sinks. This helps the loop keep moving.

This water carries things like iron. Tiny sea life uses these bits to grow.

The ocean loop helps keep our Earth healthy.

The Southern Ocean has a giant loop of moving water.

This loop is called the overturning circulation. It helps move heat and nutrients around our planet.

The loop has two main parts. The first part is the upper cell. Strong winds help push this water. This wind-driven movement brings deep water up to the surface. This is called upwelling.

At the surface, melting ice makes the water lighter. This lighter water stays near the top.

The second part is the lower cell. This part is much larger. It is driven by how salty and cold the water is. In winter, sea ice forms. This process leaves salt behind in the water. The extra salt makes the water very heavy. This heavy water sinks deep down. We call this water Antarctic Bottom Water.

This loop is changing. The upper cell has grown faster since the 1970s. But the lower cell has become weaker. This may happen because of climate change. Melting ice adds fresh water to the sea. This makes the water less salty and less heavy. If the water is not heavy, it cannot sink as well.

The Southern Ocean has a giant system of moving water. This is called the Southern Ocean overturning circulation.

It is a huge part of a global loop of water. This loop connects different parts of the world's oceans. It moves heat, oxygen, and nutrients across the planet. This movement helps keep the Earth's climate steady.

This circulation works in two main parts called cells. The upper cell is smaller and closer to the surface. Strong winds called the Westerlies push this water around. These winds cause upwelling, which brings deep water up to the top.

In this upper part, melting ice makes the water lighter. This lighter water is called Subantarctic Mode Water or Antarctic Intermediate Water. The larger lower cell is driven by how cold and salty the water is.

In the lower cell, water sinks deep down near Antarctica. When sea ice forms in the winter, it leaves salt behind. This makes the water very salty and heavy. This heavy water is called Antarctic Bottom Water.

It starts in places like the Ross Sea and the Weddell Sea. About 5 Sv of this water forms in the lower cell. This is about one third of all the heavy bottom water in the world. This movement is very slow and can take over a century.

Scientists have noticed big changes in these water loops recently. Since the 1970s, the upper cell has grown by 50 to 60 percent. However, the lower cell has weakened by 10 to 20 percent.

This happens because climate change adds fresh meltwater to the ocean. This fresh water makes the salty water less heavy. If the water is not heavy, it cannot sink as well. One study says the circulation could lose half its strength by 2050.

This moving water is like a giant pump for the planet. It helps the ocean soak up carbon dioxide from the air. The Southern Ocean is responsible for about 40 percent of this uptake.

It also brings minerals like iron up to tiny plants called phytoplankton. These plants use the nutrients to grow and help clean the air. Without this circulation, the Earth would stay much warmer. It is a vital part of how our world stays balanced.

The Southern Ocean overturning circulation, also known as the Southern Meridional overturning circulation (SMOC), is a massive system of moving water.

It serves as the southern half of a global thermohaline circulation. This system connects different ocean basins across the entire planet. By moving water, the circulation also transports heat, dissolved oxygen, dissolved organic carbon, and nutrients like iron. Because it moves these vital elements, it plays an essential role in the Earth's climate system and its carbon cycle.

This circulation works through two distinct parts called cells: the upper cell and the lower cell. The upper cell is smaller and sits closer to the ocean surface. It is driven primarily by wind-generated flow from the Westerlies. These strong winds cause a process called Ekman transport. This process induces upwelling, which is the upward movement of water, near the pole.

In this upper cell, the water gains buoyancy. This happens because precipitation and melting sea ice add fresh water to the surface. This makes the water lighter, creating Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). About 22 ± 4 Sverdrup (Sv) of upwelled water is transformed into these lighter waters.

The larger lower cell is defined by the temperature and salinity of the water. This cell is driven by buoyancy loss rather than wind. Near the Antarctic coast, sea ice forms during the winter. When ice forms, it leaves salt behind in the liquid water, a process called brine rejection. This makes the water very salty and dense. This cold, heavy water is known as Dense Shelf Water (DSW). Much of this DSW eventually transforms into Antarctic Bottom Water (AABW). This AABW originates from the Ross Sea, the Weddell Sea, and the eastern coast of Antarctica. About 5 ± 5 Sv of AABW is formed in this lower cell, which is roughly one third of all AABW formation.

Scientists have observed significant changes in these cells over recent decades. Since the 1970s, the flow of the upper cell has increased by 50–60%. In contrast, the lower cell has weakened by 10–20%. These changes are partly due to the natural Interdecadal Pacific Oscillation. However, climate change has played a substantial role. As the ocean absorbs more heat, Antarctic ice sheets melt. This adds massive amounts of fresh meltwater to the ocean. This fresh water dilutes the salty Antarctic bottom water. Because the water is less salty, it is less dense and less likely to sink. This leads to increased ocean stratification, where layers of water do not mix easily.

The Southern Ocean is a critical component of the global carbon cycle. It acts as a massive carbon sink, absorbing about 40% of the ocean's carbon uptake. The circulation helps this by bringing deep water to the surface. This deep water has not been in contact with modern human-caused emissions for centuries. Because its carbon levels are low, it can absorb a lot of new carbon dioxide. Additionally, upwelling brings mineral nutrients like iron to the surface. These nutrients allow phytoplankton to grow through photosynthesis. This process is known as the biological pump. When phytoplankton die, the circulation moves their organic matter to the depths before it can release carbon back into the atmosphere.

Looking toward the future, the stability of this system is a major concern. One study suggests the circulation could lose half its strength by 2050 under the worst climate change scenarios. If greenhouse gas emissions are not strongly lowered, global warming will continue. The status of the circulation will determine how much warming occurs. Paleoclimate evidence shows that this circulation has collapsed or weakened strongly in the past. Some research suggests a collapse might be likely when global warming reaches specific levels. Even if a collapse begins soon, it may not be complete until close to the year 2300.

The impacts of a slowing circulation would be felt for many centuries. A change in this system could reduce precipitation in the Southern Hemisphere and increase it in the North. It could also cause a decline in fisheries and a collapse of certain marine ecosystems in the Southern Ocean.

Because the Southern Ocean connects the interior ocean to the surface, it is the key to closing the Atlantic meridional overturning circulation. It does this by upwelling North Atlantic Deep Water. This slow process means it can take 60 to 90 years for just half of that water to reach the surface. Some water may take more than a century to complete its journey.