Not far below the sunny sea surface, there's a vast abyss of absolute darkness and frigid cold. Cold blackness fills the entire deep ocean, from the poles to the Equator and everywhere between. It's always down there, just a few hundred meters below the ship, even in the deepest tropics.
If you've ever been scuba diving, you know that the colorful fish and corals at snorkel depth become dim blue-gray shadows if you descend to 100 feet (30 meters), which is the deepest that recreational divers can go. Half the sunlight is absorbed by the first 10 meters of water. At 100 meters depth, there's only 1% as much sunlight as at the surface, and only the bluest wavelengths can penetrate that far. But this is still warm surface water.The ocean is an average of 4000 meters deep, about 13,000 feet. Most of the water column is completely lightless. It's extremely cold too — around 3 °C or 37 °F — all the time, even in the deepest tropics. This is Davy Jones Locker!
The density of seawater depends on temperature. Cold water is dense and sinks to the bottom whereas seawater that's warmed by the sun floats like a raft on the frigid water beneath. Think about trying to dive to the bottom of a swimming pool while holding a basketball. It's impossible. That's how warm tropical water behaves relative to Davy Jones Locker.
The only place in the oceans that gets cold enough for surface water to sink into the depths is in the polar winter dark. Near Greenland in January or Antarctica in July, the air temperature is frequently -40° or colder so even saltwater starts to freeze. Sea ice is pure H2O (without any salt in it), so fresh ice gets a coating of super-salty brine that's WAY more dense than even the freezing cold seawater around it. The super cold brine sinks like a rock. Often, little blobs of brine from millions of little icebergs combine to form a huge dense plume of extremely cold water that sinks all the way to the bottom of the ocean.
About 1/1000 of the volume of the global ocean undergoes this polar plunge each year. The very cold water slowly fills the deepest ocean from both poles and creeps ever so slowly along the bottom toward the Equator. It takes around 1000 years for Davy Jones Locker to fill with near-freezing seawater. Eventually, it displaces the warm tropical surface water where it warms in the sunlight and spreads slowly toward each pole, completing the millennial global overturning cycle.
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| Warm surface and cold deep conveyor belts of the global Thermohaline Circulation |
The extremely slow overturning of the oceans is driven by the cold polar night and the salty water that is left behind when sea ice forms. Oceanographers call this the thermohaline circulation (thermo=heat; haline=salty). The thermohaline circulation is completely different from the currents on the ocean surface in three important ways:
- It involves the entire body of the ocean, not just the surface. All. the. way. DOWN!
- It's driven by heat and salt, whereas surface currents are driven by wind
- It takes many centuries to complete a loop, unlike surface currents which take mere decades
Davy Jones Locker is physically sealed off from the air because the water is so dense that it sinks and so can't mix with the warm surface above. Seawater down there last saw the surface in the polar night 1000 years ago, during Europe's High Middle Ages or China's Song Dynasty.
As we sail through the sweltering Equatorial heat of the Indian Ocean, remember that the freezing black abyss is never more than a few hundred meters below our feet!