Looking out over the Southern Ocean off Antarctica, I see whales and seabirds diving in and out of the water as they feed on marine life in the lower levels of the food web. At the base of this food web are tiny phytoplankton – algae that grow on the ocean surface and absorb carbon from the atmosphere through photosynthesis, just as plants do on land.
Because of their small size, phytoplankton are at the mercy of the ocean’s swirling motions. They are also so numerous that the green swirls are often visible from space.
Usually, phytoplankton stay near the surface of the ocean. Some can slowly sink to the depths due to gravity. But in the turbulent Drake Passage, an 850-mile-wide (850 km) bottleneck between Antarctica and South America, something unusual is happening, and it’s affecting the way the ocean releases carbon dioxide — the main driver of global warming — from the ocean. of the atmosphere.
The Drake Passage
The Drake Passage is notorious for its violent seas, with waves reaching 12 meters high and powerful converging currentssome flow as fast as 150 million cubic meters per second. Cold water from the Southern Ocean and warmer water from the north collide here and spin away powerful and energetic swirls.
New scientific research I am involved in as an oceanographer now shows how the Drake Passage and a few other specific parts of the Southern Ocean play an inordinate role in how the oceans trap carbon from the atmosphere.
That process is crucial to our understanding of climate. The global ocean is a huge reservoir of carbon that will endure 50 times the carbon like the atmosphere. However, it is only when water carries carbon reaches the deep ocean that carbon can be stored for long periods – up to centuries or millennia.
Photosynthetic phytoplankton are at the heart of that exchange. And in the Drake Passage, my colleagues and I have discovered that seamounts stir things up.
The role of ocean layers
The ocean can be visualized as layers. With constant surface waves and winds, the top layer is always stirring around, mixing water. It’s like mixing milk into your morning coffee. Stir this mixes with solar heat and gasessuch as carbon dioxide, absorbed from the atmosphere.
Water resistance generally increases as the water gets deeper, colder and saltier. That forms density layers that are typically flat. Because water prefers to keep its density constant, it mostly moves horizontally and does not move easily between the surface and the deep ocean.
But despite this physical barrier, water tests show that carbon dioxide produced by human activities is making its way into the deep ocean. One way is through chemistry: carbon dioxide dissolves in water, creating carbonic acid. Living creatures in the ocean are another.
A look into the Drake Passage
Oceanographers have long pointed to the North Atlantic and Southern Ocean as places where surface water is moved to the depths, taking large amounts of carbon with them. However, recent work has shown that this process is in fact dominated by only a few areas: including the Drake Passage.
Despite being one of the most famous parts of the ocean, scientists have only recently been able to observe this window in action.
The main current of the Drake Passage is created by the effect of strong westerly winds over the Southern Ocean. Scientists have found that westerly winds create a slope in water density, with dense waters shallower closer to Antarctica, where colder meltwater covers the surface, but run deeper into the ocean further north toward South America.
With progress in autonomous underwater robots and computer modeling, we’ve been able to show how the Southern Ocean current interacts with an underwater mountain in the Drake Passage. This underwater interaction mixes the oceanenhancing that coffee-like stirring process.
The stirring along the sloping density levels provides a path for water from the top layer of the ocean to move into the depths. And phytoplankton on the surface of the ocean are carried along by this stirring and go to the depths much faster than if they were to sink by gravity alone.
In a less energetic area, these phytoplankton would die and respirate their carbon back to the atmosphere or slowly sink. However, at the Drake Passage, phytoplankton can be swept to the depths before this happens, meaning that the carbon they absorbed from the atmosphere is stored in the deep ocean. Carbon dissolved and stored in the deep ocean can also escape from these locations.
Scientists have estimated that the deepest ocean waters interact directly with the atmosphere via only about 5% of the surface of the ocean. This is one of those special places.
Exploring the Drake Passage and other oceanographic windows will help science better understand climate change and the workings of our blue planet.