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Understanding ocean currents: the giant conveyor belts of Earth

Going with the flow

Much happens in the waters of this world that affects our lives on land. Beyond natural disasters like tsunamis and cyclones, there is some pretty regular phenomenon happening in the world’s oceans that continuously alter and influence our everyday lives, regardless of where we are on the planet. One such phenomenon is ocean currents.

Broadly speaking, an ocean current is any regularly occurring mass movement of water, primarily horizontal, in all of the world’s major water bodies. But they’re much more than just good old H2O ambling about the globe. Much of the world’s climate and biodiversity has been affected and determined by ocean currents. How exactly? Let’s find out.

What creates ocean currents?

To understand why ocean currents are what they are, we need to grasp how they come to be. There is a multitude of factors that influence the formation and propagation of ocean currents. But before we get into them, our understanding of ocean currents needs to be further subdivided into its two major types – surface currents and deep water currents.

Surface currents form about 10% of the water in the oceans. The rest of the 90% of the oceans are comprised of the thermohaline currents in the deep water. Here “thermo” means temperature and “haline” means salinity, which gives us the first two factors that affect current formation. Essentially, different temperatures and salinity give rise to different densities by depth. As colder, more saline water sinks deeper, warmer water seeks to replace it which gives rise to massive movements of water. This sinking happens mostly towards the poles where the water cools faster and ice formation leaves behind more salt. But this creates a different picture of ocean currents than what happens in reality – which means that there are more factors affecting it.

A diagram of the various movements of water in the ocean described

Solar heating, especially around the equators, leads to faster evaporation and a higher density of salt in the water. It also causes expansion in water volume, leading to a water level higher by 8cm near the equator as compared to mid-latitudes. Along with that, persistent surface winds lead to friction between the layers of water, with the upper layers having a greater tendency to flow in the direction the wind is blowing as compared to the lower layers.

On top of these factors, there’s the Coriolis effect to account for. Without going into much detail, let us tell you that the Coriolis force is the force that makes objects maintain the rotational inertia that Earth’s rotation grants them. So in the currents moving towards the poles, it acts towards the right or left depending on which hemisphere you’re in. All these factors combine to create the complex network of ocean currents that are spread across the globe today.

Ocean current map

Why are they important?

Back when travelling on the sea was not as advanced as today, and was the only option to travel across the world in (comparatively) shorter amounts of time, ocean currents played a huge role in speeding up travel around certain routes (and hindering on others). Circumnavigational expeditions harness ocean currents to this day to save time and fuel.

Most of the heat received by our planet is absorbed by the oceans. As a result, ocean currents, of days gone by and today, have persistently affected the climate of our planet. The absorbed heat is transported by currents across the globe, affecting climate everywhere. Take the Gulf Stream for example. It makes the climate of northwestern Europe much more temperate than any other place in the same latitude. That is because the Gulf Stream is a warm, swift Atlantic ocean current, and its northern offshoot, the North Atlantic Drift is what keeps Europe warmer than what it would be without it. The Gulf Stream is typically 100 kilometres (62 mi) wide and 800 metres (2,600 ft) to 1,200 metres (3,900 ft) deep, with the maximum speed typically about 2.5 metres per second (5.6 mph). It has caused quite a few cyclones in the area.

Tip: Ocean currents are measured in sverdrup (sv), where 1 sv is equivalent to a volume flow rate of 1,000,000 m3 (35,000,000 cu ft) per second.

These properties have a larger fallout. For instance, colder currents flowing in from the pole bring plankton-rich waters with them, which are crucial for the survival of larger creatures in the sea. Additionally, points, where cold and warm currents meet, are also rich in nutrients and hence usually filled with marine life. One of the most famous fishing centres in the world is Newfoundland in North America, where the Labrador current and the Gulf Stream meet.

Another new area where ocean currents are showing a lot of promise is as an alternative source of energy. As opposed to wind and solar power, ocean currents are far more predictable and offer consistent, constant and clean energy. While the technology to harness this isn’t as mature as other sources, it is still being explored around the world, with ideas such as submerged turbines.

Where are they headed?

One would imagine that a phenomenon as disconnected from humans as earthly possible would be free from the clutches of climate change’s vice. The rate at which sea ice and ice around the poles is melting due to global warming also affects the formation of ocean currents. With a lower quantity of ice freezing, there’s lower salt in the water at the poles. This leads to less water sinking to the depths, and less warm water rushing in to replace it – creating fewer, or less powerful ocean currents. Even in an indirect way, lesser ice leads to fewer winds, which is also a factor that adversely affects ocean currents.

Due to the lack of circulation that ocean currents provide, there could be less oxygen in the water leading to a disruption of oceanic life cycles, and possibly the extinction of species that depend on oxygen in the water. The same would also lead to less CO2 absorption by the ocean, leading to higher levels of CO2 in the atmosphere. We all know what that can do.

Deep Argo deployment

Projects like Deep Argo are trying to further our understanding of deep ocean currents and how their gradual demise can be stopped. A submerged network of floats that look like a robotic, glass orb with an antenna to transmit signals to satellites and a two-meter-long tail that detects when it has reached the seafloor is going down there to scan the unknown. Unlike previous Argo floats that worked on the surface, these can withstand the immense pressure of the deep for our bid to understand how the oceans have stored 90% of the heat received by Earth. This entire project will take some time to conclude. But, as with most issues that aid the cause of global warming on our planet, we need to act now before our oceans turn stagnant.

Arnab Mukherjee

Arnab Mukherjee

A former tech-support desk jockey, you can find this individual delving deep into all things tech, fiction and food. Calling his sense of humour merely terrible would be a much better joke than what he usually makes.