Eli5 If putting something under pressure increases the temperature why is the bottom of the ocean so cold?
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The first thing is that water is an odd substance in its density response to pressure. The maximum density of water occurs at a temperature slightly above freezing (about 4 degrees C for pure water). Second is that water is liquid, a fluid, and moves fairly easily.
Water that is colder or warmer than the 4 degrees max density temperature will be less dense, so will rise. If you cool that water OR heat it up (change its density), it leaves (convects away). The heat migrates with the water.
So, what happens? Water near 4 degrees flows down and water not quite as near to 4 degrees moves up. The densest water is down at the bottom, and it is densest because of its temperature.
It isn’t a closes system where work done must increase total energy (raise temperature eventually). The system is open, if work is done that somehow changes temperature, the material leaves and takes the heat with it.
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