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My question centers around the temperature of the deep ocean at depths of <7000m. I know a few things already.

  1. The temperature of the ocean rapidly declines up to 1000m in depth, before falling more gradually. This is because sunlight does most of the heating of ocean water, where it is darker, there is less heating.

Ocean Thermocline Toy Model

  1. In the deepest parts of the ocean >7000m down, the ocean can get very close to 0°C, as explored in the answers to this previous question.

Ocean Temperature data

The thing I find puzzling is why the ocean temperature never seems to go below 0°C, despite getting quite close to it in some cases. I can't seem to figure out the source of this apparent limit. Thus far, I've heard several explanations that don't seem to make sense, for example:

  • A region of water below 0°C will freeze and float to the surface rewarming in the process.

    If this is true, why don't we ever hear of icebergs from the deep suddenly surfacing in otherwise tropical waters?

  • The freezing temperature is lower in the deep ocean due to pressure.

    This may be true, but then what is the source of the apparent limit at 0°C?

  • The temperature of the seafloor is about 0°C, the deep ocean adopts this temperature.

    In this case, why does the seafloor have that temperature? In general, an object (e.g. one in space) that gets no sunlight should have a temperature of near absolute zero. Of course, if we say the temperature is set by the ocean water itself, we have a chicken-and-egg problem.

I seem to be very confused and would appreciate if someone could set me straight!

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    $\begingroup$ deep water below 0 is not uncommon, but of course seawater does not freeze at 0. $\endgroup$
    – John
    Commented Jun 14 at 19:50

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Several of your assumptions are wrong

Water in the ocean does get below 0 Celsius, -2°C is about as cold as ocean water gets near the poles, but ocean water being saltwater does not freeze at 0°C.

The ocean floor is actually a source of heat, the earth is radiating heat to the ocean. The ocean floor tends to be slightly warmer than the ocean water.

Water reaches maximum density at 4 Degrees water does not just expand when it freezes it expands below 4°C. So below that it starts to rise not sink, which causes it to mix with warmer water. Its the same reason lakes freeze from the top down. Note where the thermocline lies. Because of this constant circulation deep ocean water is remarkably uniform in temperature.

enter image description here

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  • $\begingroup$ Saline water (>24.7‰ salt) doesn't show any density anomaly! $\endgroup$
    – klanomath
    Commented Jun 14 at 23:49
  • $\begingroup$ This clarifies quite a bit, it's good to see the "0°C limit" doesn't hold as well as I thought! As a follow-up, what sets the -2°C temperature at the poles - is it the cold temperature of shallow polar water? I.e., if the poles were cooler (like in an ice age) or warmer (like in the Cretaceous period) would the deep ocean respond by cooling and warming respectively? $\endgroup$
    – user2944352
    Commented Jun 15 at 23:49
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    $\begingroup$ Below -2 the ocean begins to freeze, thats where sea ice comes from. $\endgroup$
    – John
    Commented Jun 16 at 2:13
  • $\begingroup$ I see! To fill in the blanks, there is a temperature limit but it's at -2°C, set by the surface freezing temperature of saltwater. If water gets colder than this on the surface it forms ice which does not sink into the deep to transfer heat. If the climate were cooler nothing would change, if it warms you might get an increase in the temperature of the deep ocean since the sinking water is warmer. Ultimately, the minimum temperature of the deep is set by the freezing point of saline water! (up to any heating from geothermal sources) $\endgroup$
    – user2944352
    Commented Jun 16 at 9:27
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    $\begingroup$ @user2944352 well if it was colder tha area with sea ice and the amount of sea ice would increase. During snowball earth sea ice nearly reached the equator. But yes everything else is sound. $\endgroup$
    – John
    Commented Jun 16 at 12:40

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