T for THERMOCLINE

Definition

Intangible limit between two water levels of different temperatures, one index “it is good”, the other index “we curd our grindstones”… 

Synonymes

Pycnocline | halocline

Starter

Cenote Angelita - Yucatan - Mexico. At -30m, a layer of sulphurous, opaque water, below the translucent fresh water ...

In general, when you are a diver, you go into the water. In fresh water, when you are Swiss or Savoyard or in salt water, in the Channel, riding a Atlantic, riding a Mediterranean sea, riding a Red Sea or even in other even more exotic brines… Sometimes in both: typically swimming pool water in winter and that of the sea in summer. But not that: it would be too easy! Not in fresh water and then in salt water but in both at the same time! In some caves and marine resurgences, in the cenotes of Yucatan, it happens that we begin the dive in fresh water to continue it in salt water by crossing an immaterial border called halocline. Most of the time it is fresh water, less dense, which is above the salt water.

A discrimination based on salinity that we find in another similar phenomenon: the thermocline this time due to a temperature difference, well known to lake divers and which can reach several tens of degrees. This time, it is the hot water which overhangs the cold water. And nothing prevents to observe a mixture of the two phenomena or much more as I told it in a previous Article.

Main course

The halocline is often due to the differences in water temperature between summer and winter. The cold, denser and less agitated waters remain in depth. And so the normally constituted (and vaccinated) diver always expects to be colder on the way down. However, it is exactly the opposite that we encountered in a lake on the island of Palawan Asked Philippines : increasingly hot water, undoubtedly of volcanic origin, as it descends, as I wrote in the book Narcosis published by Glénat editions. Not to mention the Mescla cave Yes where, while exploring the cool karstic waters of siphon 3, I intersected underwater, far upstream, a pipe of boiling thermal water ...

Launching towards the Almyros de Melissi, in the Gulf of Corinth using a wooden boat rented from local fishermen.

But there are other situations where it is hot water rising from the bottom of the sea, as we discovered in the early 90s during an exploration campaign in the Peloponnese on behalf of IGME (the local BRGM). For irrigation, we were desperately looking for fresh water in the Gulf of Corinth. Oral tradition spoke of a geyser of water springing from the sea at the foot of Mount Parnassus. The fishermen who indicated the place to us were sorry not to catch any fish there, which should have put me on the alert: the underwater sources that make the water brackish around are always frequented by clouds of mullets and other mules may have come to be treated there.

Invent hot water

Here we are in the middle of the oil stain formed by the resurgence on the surface of the sea. Tilting backwards and rapidly descending into the vortex where the waters mix. At least 45 m a funnel of limestone scree covered with a thick layer of pulverulent greyish mud appears. There are four exits spread over approximately 30 m2. They are clearly visible thanks to a festoon of white filaments, colonies of bacteria that grow around them. The rock inside is black with yellow and orange deposits. In the salty darkness, it is an atmosphere of abysses, black smokers, mornings of the world ...

Almyros de Melissi, at less than 45 m, at the end of the Gulf of Corinth.

The current which leaves a narrow diaclase is very hot, violent, and causes optical aberrations. The presence ofH2S makes itself felt and attacks the skin, even through the mask. A sounding will allow to detect a depth of 52m for a temperature of 30 ° C. But freshwater, period. Subsequent chemical analyzes show, on the contrary, that it was water that was saltier than the surrounding sea, very rich in sulphates and whose low tritium content attested to its age. 

I stayed for a long time in front of the volutes of this venerable water troubled by heat and salinity, which in a way returned to the sea, after a long secret journey, the water lost in the surrounding abysses where the sea empties, the famous katavothres. But this is another story…

Dessert

The very notion of thermocline appeared briefly and late for the first time in 1942 in the book which was the reference in oceanography: Oceans, by Sverdrup, Johnson, and Fleming. It has since been known that in the oceans, 90% of the water that lies below the thermocline has a temperature between 0 and 3 ° C. In addition, fishermen are well aware that the area above the thermocline is called the epilimnion, and that below is called the hypolimnion. The latter is sometimes depleted in oxygen, even anoxic, which turns it into a dead zone. Here it is: you can leave with that under your arm ... 

La thermocline, just like the halocline, is also taken into account by military submarines because it considerably changes the characteristics of the environment from a sonic point of view. Submarines can then use it by maneuvering on the other side of it, in order to hide from enemy sonar. The scoundrels ...

See you soon for a new definition of Scuba Bécédaire. The irreverent lexicon of diving, but not only. Because sometimes ...

Francis Le Guen

Café

The different densities of water present in the oceans explained by oceanography.