Thermohaline Circulation

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In the last lesson, we mentioned there were two components to ocean circulation. We spoke extensively about the surface gyres, Ekman pumping, and upwelling. All of these are driven by wind “pushing” and “pulling” on the ocean surface. So what is the second component of the circulation?

An important additional mode of ocean circulation is the thermohaline circulation, which is sometimes referred to as the meridional overturning circulation or MOC for short. The term “thermohaline” combines two key components: “thermo” refers to temperature (how warm or cold the water is), and “haline” refers to salinity (how salty or fresh the water is). Therefore, “thermohaline” describes ocean currents and processes that are driven by differences in water temperature and salinity. The circulation pattern is shown below:

The therrmohaline circulation functions differently from the surface wind-driven currents we just learned about. Instead of horizontal movements across the ocean’s surface, thermohaline circulation involves a vertical flow pattern that spans the ocean’s depths.

Much like the Hadley circulation in the atmosphere, the thermohaline circulation can be thought of as a giant conveyor belt moving water up and down throughout the ocean. In the North Atlantic, near the pole, water becomes very cold and salty, making it denser and causing it to sink deep into the ocean. This sinking action creates a sort of vacuum, pulling water from other regions to replace it. To balance the water being pulled down, more water must flow in from elsewhere—otherwise, we’d end up with a giant divot in the center of the Atlantic!

In contrast, in warmer tropical and subtropical regions, like the Indian and Pacific Oceans, water tends to be warmer and less salty. This lighter, less dense water rises to the surface to fill the gaps left by the sinking cold water in the North Atlantic.

The main driving forces behind this circulation are differences in water density, primarily due to variations in temperature and salinity. Similar to the atmosphere, warmer water is less dense and tends to rise, while colder, saltier water is denser and sinks.

This balance of rising and sinking water plays a crucial role in regulating Earth’s climate by redistributing heat. It acts like a massive heat pump, moving warmth from the tropics toward the poles, helping to moderate temperatures in different regions.

Take a look at the schematic above. It’s worth mentioning that this is a somewhat simplified view of the actual vertical circulation patterns in the ocean—they are, in reality, much more complex. Still, you get the idea of how this circulation resembles a conveyor belt.

You might also notice, “Hey, I see the Gulf Stream in both sections!” You know, that current that is moving towards the north off of the eastern half of the United States. But hold on—that’s not quite right. The Gulf Stream is primarily driven by winds, not by the temperature or salinity of the water. When we talk about the thermohaline circulation, the northward extension in the North Atlantic is better known as the North Atlantic Drift. Like the Gulf Stream, this current also involves the net transport of warm surface waters to higher latitudes in the North Atlantic. Ultimately, the goal is to transfer that extra energy from the tropics to the poles!

Thermohaline shutdown?

Hopefully, I’ve convinced you by now that ocean currents, including the thermohaline circulation, play a crucial role in redistributing heat across the globe.

Within the thermohaline circulation, there are various components, much like the different currents we discussed in the context of wind-driven circulation. One of the most important of these is the Atlantic Meridional Overturning Circulation (AMOC). As we saw earlier, the North Atlantic Drift—a key part of AMOC—carries warm surface waters from the tropics to the North Atlantic, where they cool and sink, forming a deep ocean current that flows southward. This movement helps redistribute heat globally, influencing weather patterns and climate conditions, especially in regions like northern Europe.

Recent research suggests that AMOC may be approaching a “tipping point” where its behavior could change dramatically. Previous studies indicate that AMOC has two stable modes: a “strong” mode, which efficiently transports warm water into the North Atlantic, moderating the climate of northern and Western Europe, and a “weak” mode, where this transport is significantly reduced. Some scientists believe that current conditions may be pushing AMOC towards a transition from its strong state to a weaker one, leading to numerous—sometimes sensational—news articles warning of a potential climate crisis.

This possible shift to a weaker AMOC state is largely driven by the influx of cold, fresh water from the melting Greenland ice sheet (we’ll delve deeper into this topic in a few lectures). This freshwater disrupts the thermohaline circulation by reducing the salinity and density of the ocean water, which in turn weakens the sinking motion in the North Atlantic. If AMOC slows down, less heat will be transported to the North Atlantic, reducing its moderating effect on the climate of northern and Western Europe. As a result, these areas could experience colder conditions that are more typical of regions at similar latitudes, like Canada and Russia. Additionally, decreased heat transfer to the North Atlantic could lead to further warming in the southern Atlantic and surrounding areas. While the most significant impacts are expected in regions directly influenced by AMOC, broader changes in global temperature and precipitation patterns are also anticipated.

One thing worth noting is that it is best to think of these potential changes as a “tipping point” rather than a true collapse. For example, in the 2004 movie “The Day After Tomorrow,” the AMOC circulation plays a key role!