Davy Jones, in days gone, by was a kind of nautical antithesis to Neptune.
He was, according to the novelist Tobias Smollett (whose hero Commodore Trunnion has more than one encounter with the beast) "in the mythology of sailors the fiend that presides over all the evil spirits of the deep, and is often seen in various shapes perched among the rigging on the eve of hurricanes, shipwrecks and other disasters to which seafarers are from time to time exposed, warning the devoted wretches of the death and we to come".
Davy Jones lived, it was said, at the bottom of the sea, where his proverbial locker no doubt still exists.
It is cold down there. The topmost layers of the ocean vary in temperature from about 1 in the polar regions to 20 or even 30 Celsius in parts of the Tropics. But at the bottom of the ocean its temperature is remarkably uniform and low, regardless of the latitude: it is almost everywhere in range 0.5 to 1.25 Celsius.
This cold bottom water has its origins in surface cooling near the poles, where it sinks and spreads globally over the floor of the ocean to form a massive cold pool deep below the waves in Davy Jones's locker.
With relatively high sea-surface temperatures in the midlatitudes and in the Tropics, and a cold pool underneath, it is obvious there must be a decrease in water temperature with depth. But this decrease does not take place uniformly: there is normally a warm layer of water of near uniform temperature near the surface, and then a very sharp decrease in temperature across a zone perhaps one or 200 feet in thickness, below which the water temperature is almost uniform again the rest of the way down. The shallow transitional layer is called the thermocline.
The contrast in temperature across the thermocline is greatest at the summer's end, after the surface waters have absorbed large amounts of energy from the sun. But the distance below the surface of the thermocline also varies from place to place and from season to season.
The thickness of the relatively warm surface layer depends to a large extent on storminess: strong winds mix the waters near the surface, and deliver a uniform temperature throughout the depth affected - so the stormier the weather, the deeper the warm surface layer will be. Typically at these latitudes, the surface layer might be 500 feet in depth after a stormy winter, while at the end of a calm summer, the thermocline might be a mere 100 or 200 feet below the surface.