Why the water in the Mariana Trench is warmer (34-39 F) than the water the victims of the Titanic fell in (28 F)?


Why the water in the Mariana Trench is warmer (34-39 F) than the water the victims of the Titanic fell in (28 F)?

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Water near the surface changes temperature with the weather, after some lag (water is slow to heat and slow to cool down). The Titanic sank in the North Atlantic in April…it’s *cold* up there. The surface water was still below freezing…hence ice bergs hanging around, among other things.

Water deep in the oceans is insulated from the weather by, literally, miles of water above it. That’s a *very* good insulator. Water down deep with limited current, like the Mariana Trench, basically has no idea what’s going on with the weather or climate. It doesn’t care about air temperature, or seasons, or really anything. It’s at thermal equilibrium with the sea floor, which is (very very very very) slowly conducting heat up from the hot mantle rock below.

Edit: clarified that the mantle is hot but not molten.

Water is always 4°C at the bottom because at this temperature it has the highest density. That is also why fish don’t die in lakes during winter

0-3 °C/34-39 °F right at the ocean floor is a fairly constant temperature at the ocean floor; barring any volcanic vents etc.

The closer to the surface you get, water interacts with currents and flows in the water and with the atmosphere, where it can gain or lose energy and heat.

There is a cold, North Atlantic current, a return current from the interaction of the Gulf Steam and Artic Waters, traveling S/SW that passes almost exactly the same area where the Titanic sank. Because it’s cold water, it’s more dense and runs deeper.

Water is densest at about 4 degrees C (upper 30s F). Thus, that is the temperature that water at the very bottom of any water body will tend toward. Salinity does have some effect, but if salinity remains constant, the densest water is about there, at a temperature a few degrees about freezing.

Water is a weird substance. Its solid version (ice) is less dense than its liquid version. Most substances constantly increase in density as temperature drops. Water does not. However, the good news is that this is why lakes do not freeze solid. The ice floats. It is also why most lakes have warm water at surface but a cool deep layer.

So, the question here is really “why is the water so cold in the region of the Titanic?” It should not be if what I just said is true. The reason for this is that the deep water around the Titanic is coming from up north, in and near the Arctic Circle, and is kept at ice-temperatures (very near freezing, even a little lower because it is salt water and salt water freezes below 0C/32 F). It would “like” to go straight south along the surface, but the waters of the Gulf Stream are also trying to head north along that same route, more or less. The cold water from the polar region is denser than the warmer water from the equatorial region, so the colder water sinks under the warmer water (has to go somewhere and down is its best choice).

There is also a bit of a salinity enrichment thing happening, where the freezing out of pure ice makes the cold ocean get saltier, and thus more dense than average sea water. The salinity enrichment factor is more important during the cold season, of course.

The end result is that the deep water of the North Atlantic is very cold, unusually cold. This sinking of cold saline water is actually part of the driving of ocean current motion, and climate scientists worry that warming will release too much fresh water (from melted ice) and shut down that deep current, which would also shut down the Gulf Stream, and the north Atlantic would become a source of cold for western Europe, rather than a source of warmth like it is now.

At depth water is under a lot of pressure, which causes its temperature to rise. That warm water in the Mariana trench would be close to freezing if it were brought to the surface in a thermally insulated balloon.

Oceanographers try to get rid of the pressure effect by computing the “potential temperature” which is the temperature the water would have at a common depth (like the surface or 1000 m). Potential temperature always decreases with depth, unless salinity makes the deeper water more dense.

We are used to this idea with air. Warm air rises but it is cold in the mountains because that warm air expands under lower pressure and becomes cold. If you took the air on Mount Everest and brought it to sea level it would be like 130 F. The “adiabatic lapse rate” is about 10 C per 1 km elevation. Meaning the same air would be 10 C warmer if you were 1 km lower in elevation.

The same thing is going on with ocean water. The Mariana trench is deeper than the Titanic, so the water is slightly warmer, even if it would be colder if it were brought up to the depth of the Titanic.