I know that there are a couple of questions covering latitude and longitude for water temperature, but overall I'm more interested in the rate of temperature change by depth.

Thought Process: Baseline geothermal gradient is defined as the rate of increasing temperature with respect to increasing depth in the Earth's interior, and approximated at about 25–30 °C/km (72-87 °F/mi) of depth near the surface in most of the world. That's maybe, what, approx. 1°F/75 ft, barring circumstance and variables unknown.

Question: Is there a similar approximate rate by which the temperatures of various depths of water can be estimated, allowing for the same acknowledgement that it's an approximation without context? Or are thermoclines so varied that such an estimated rate cannot be generalized?

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    $\begingroup$ In water it would be highly variable due currents, mixing, thermoclines, and seasonal cicles. A similar thing happens in the atmosphere, but air compresibility allows the adiabatic temperature gradient to dominate. No such thing exists on water. Although, there must be a trend, but maybe it is not widely known because it is too rough and inaccurate for most applications. If you want to calculate it yourself anyway, you can download worldwide temp vs depth data at nodc.noaa.gov/OC5/SELECT/dbsearch/dbsearch.html and crunch the numbers. I just check and all the records amount to 44Gb! $\endgroup$ – Camilo Rada Feb 5 '19 at 19:05
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    $\begingroup$ To be frank, I posted the question after downloading a similar dataset and balking at the sheer volume. I'll probably end up actually going through both what you linked and what I have, if I don't find a workaround or internet hero. In the meantime, deep ocean being fairly uniform at 0-3 °C, or 400°C near hydrothermal vents which only extends a few meters, will have to be a base low range for my estimates - allowing for marked differences of delta T in the transition zones :P $\endgroup$ – T BG Feb 5 '19 at 19:29
  • $\begingroup$ For completion, I'll be using the following until I've actually sifted through data: Epipelagic: 104-27 °F, Mesopelagic: 68-39 °F, Bathylpelagic: 39 °F, Abyssopelagic: 36-37 °F, Hadopelagic: 34-39 °F $\endgroup$ – T BG Feb 5 '19 at 20:34
  • $\begingroup$ Don't you technically need a negative sign (or "decreasing temperature" instead of "increasing temperature")? As posed it looks like you're saying it gets warmer with depth? I actually have no idea if it increases or decreases truly in the deeper ocean, as in such a region, you're between the top layer where I expect the atmosphere is a control (particularly above the thermocline) and the bottom, where I'd expect the Earth's internal temperature is the control? But naively I'd think temp decreases for quite a while in the free ocean due to all solar energy being absorbed towards the top? $\endgroup$ – JeopardyTempest Feb 6 '19 at 0:49
  • $\begingroup$ Most of the ocean is apparently pretty static as far as temperature goes. Each pelagic zone has a range, but it doesn't seem to decrease in any steady way, and stops around 0-3 °C/34-39 °F right at the ocean floor. That temperature is pretty steady over all terrestrial waters at that depth. As for me saying things increase, I was using an example of geothermal gradient: temperature does increase rather steadily the further down you go in rock and earth. I was just hoping there was some similar line or curve that someone smarter than I had already extrapolated for water temperature/depth. $\endgroup$ – T BG Feb 6 '19 at 1:41

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