What drives ocean currents?

Welcome to the second installment of the Science on Main ocean-related blog posts! This week's topic comes from a part of the field called physical oceanography. This piece of the broader field of oceanography focuses on understanding the large-scale forces that drive ocean events, including currents, the topic of this week's post.

For most, especially anyone who has seen the movie Finding Nemo (Disney-Pixar, 2003), the fact that the ocean has currents is old news. Ocean currents are powerful features of their environments, continually pushing large volumes of water from one location to another, potentially thousands of miles away. One of the most famous ocean currents is the Gulf Stream, which is part of a larger circulation current called a gyre. There are five major gyres in the oceans: the north Atlantic, south Atlantic, north Pacific, south Pacific, and Indian.

Figure 1: The deflection of air currents due to the
Coriolis Effect

In the northern hemisphere, the gyres circulate in a clockwise direction. In the southern hemisphere, they circulate in a counter-clockwise direction. This is due to the Coriolis Effect. The Coriolis Effect is the name for the deflection of air currents in the atmosphere due to the rotation of the earth underneath it. If Earth didn't rotate, the air would naturally flow in a straight line from the high-pressure areas around the poles to the low-pressure equatorial area. Air deflects in opposite directions in the northern and southern hemispheres because of the direction of earth's rotation (Figure 1). So what does this have to do with ocean currents?

You may be surprised to learn that it is actually the wind that drives surface ocean currents! Each of the oceanic gyres has one eastern and one western boundary current, for a total of 10 major currents in the modern day oceans. The eastern boundary current is usually wider and travels slower, while the western boundary current is narrower and travels quickly. This is because the winds that drive surface currents are deflected out, the start to lose energy. Since they spiral west to east, the air currents are stronger on the western sides. The air pushes and shears the surface of the ocean, which causes the water to follow along. This is part of why hurricanes with high winds can produce huge waves and strong ocean currents.

Figure 2: The Gulf Stream. Warm colors represent warm water,
carried by the current to England and northern Europe.

One of the most famous currents is the Gulf Stream, which carries warm, nutrient-rich water from the Caribbean, up the east coast of the United States, and then up and over to England, Iceland, and the Norwegian Sea. Despite England's high latitude, it has a relatively mild climate, and it is primarily due to the Gulf Stream, and the water it carries with it.

The Gulf Stream is only a surface current, however. There are also currents that bring nutrients from deep in the ocean to the surface, which are used to feed phytoplankton and other small animals. As phytoplankton may be responsible for up to 80% of all organic matter on Earth, it's important to keep them fed! These deep currents also bring oxygen with them, which every animal needs to survive. Water that has no oxygen in it are called hypoxic, and cannot support life. Without these deep currents, life in the oceans could not survive.

One example of phytoplankton, a diatom called
Amphiprora. From Dawn Moran, WHOI.

Deep ocean currents carry water along the bottom of the ocean, collecting nutrients and oxygen, before they are brought to the surface. This changes some of the water's properties within the current, including its density. When the density of the water on the bottom of the ocean is less than the density of the water above it, the ocean begins to overturn. There are specific locations in the oceans where upwelling occurs, when less dense water rises to the surface, and locations where the opposite, downwelling, happens. This density-driven circulation is necessary to keep the surface water healthy for all the plants and animals that rely on it.

It is important to study the oceans because they are responsible for moderating climate, driving weather, and maintaining the global food-web. Currents are an important piece of the puzzle of how the oceans work, as they transport materials from one place to another, but there are still many questions that remain. If you studied the ocean, what question would you want to work on? What do you think would happen if all ocean currents suddenly disappeared?

Sources:
https://oceanservice.noaa.gov/education/kits/currents/05currents1.html
https://oceanexplorer.noaa.gov/facts/currents.html
https://www.nationalgeographic.org/encyclopedia/coriolis-effect/
http://www.edc.uri.edu/restoration/html/gallery/plants/phyto.htm
https://www.whoi.edu/main/topic/phytoplankton