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  • AntoineR

Thermohaline Circulation: Uncovering the Chilling Secrets of the Deep Ocean

Updated: May 18, 2023

Ever wondered why the deep ocean is cold? 🌊🥶

The sun does not penetrate to these depths, that’s simple! Well.., not quite.


A picture of a glacier


Sunlight not penetrating the ocean depths only explains why the deep ocean isn't warming up. It doesn't explain why it is cold in the first place.


The natural state of water is not to be cold, it’s to stay at the temperature it is at. Water is a conductor of heat. Given that heat conduction takes more time as the depth increases, for the heat at the surface of the ocean to propagate all the way down the bottom, it’d take thousands of years. Since, oceans have been around for millions of years, conduction alone should warm them up, especially in regions like the Tropical Pacific!

So, why are ocean depths so cold?


Oceans are stratified with waters of different densities 🧅

Two parameters affect seawater density:


1. 🌡️ Temperature. Cold water is denser than warm water.

2. 🧂 Salinity. Increasing salt-content makes seawater denser.


If you want to increase seawater density: make it cooler and/or make it more saline.

This is actually what happens in regions of ice formation, where surface waters are near freezing. In these regions, water gets colder and more salty. It becomes denser, and sinks down to the ocean depths. Once cold water reaches the ocean depths, there is nothing to warm it up (no sunlight etc.).


So, ocean waters are stratified, inhibiting mixing in the vertical, and, this stratification is fuelled by "sinking waters" in the poles. This forms what is called the Great Ocean Conveyor (aka thermohaline circulation). Places like the Arctic act as a sort of “pump” for the oceans. Cold waters sink down the depths of the ocean in the poles, then circulates in the depths of the ocean for a while when it gradually rises before emerging back to the surface elsewhere around the globe.


What's the relevance to climate?

Studying the climate comes with studying energy flows. Energy comes in (as visible light from the 🌞) & comes out (as infrared). GHGs are a one way door. They don’t interact with visible light, but they do with infrared. When they do, they re-emit energy in all directions, some of which goes out to space and some gets reflected towards Earth’s surface. This creates a (big!) surplus of energy in the climate system, that needs to go somewhere: energy is conserved. It doesn’t magically disappear.

So, where does this energy go?


Studies show that a very big part of this extra energy is accumulated in oceans, which warms their upper water layers.


So, the Q is: How will climate perturbations affect ice forming regions (Labrador Sea etc), and how will this affect the penetration of cold water in deep oceans? If we mess enough with the climate, can we impact the ocean circulation? i.e. can we “stop” the "ocean pump"?


For more info on the roles of the oceans in the climate system, check M. Allen’s Gresham College lecture on the topic: https://www.gresham.ac.uk/watch-now/ocean-zero

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