Understanding Thermoclines In Ocean Waters

Earth’s oceans are home to a wide variety of marine flora and fauna. There are interesting and intriguing phenomena that do not yet have plausible explanations. But one common phenomenon that is well known and has even had its role in filmography, is ocean thermoclines.

A thermocline, briefly put, is a region of severe temperature change that demarcates 2 different pelagic zones in the ocean. The ocean consists of several layers, that are all separated and kept from mixing by this temperature gradient.

While it may not be visible, it can be felt by divers (due to the abrupt cold as one goes deeper) and recorded by instruments such as refractometers (to record refraction in physical media) and sophisticated temperature gauges.

Thermoclines were a relatively unknown phenomenon amongst people outside the scientific community until a series of Hollywood fictional thrillers used it as a barrier separating dangerous undersea monsters from the surface (most notably, The Meg which was released in 2018).

While scientists are confident that prehistoric creatures still live below the deeper thermoclines, they are mostly harmless, and cannot survive the temperature and sunlight change if they were to come to the surface.

But what is the pelagic zone? What are thermal gradients? Why do scientists bother with this phenomenon?

In this article, we will look at thermoclines, the role they play in the marine ecosystem, the various ocean layers, the impact on marine life, and a unique aspect of thermocline impact on the field of naval design and engineering.

For anything related to thermoclines, this is your go-to article!

Table of Contents

Understanding The Ocean Zones Or Layers

Earth’s oceans, covering over 75% of our planet’s surface area, include a wide variety of natural phenomena. These different water bodies have unique phenomena, ranging from the extreme salinity in the Dead Sea to the killer jacuzzi pools in the Gulf of Mexico. Another interesting feature of these oceans is the pelagic zones.

Pelagic zones are ocean regions extending vertically downwards from the surface, classified by their average temperature (day and night).

In general, these zones are also viewed as ocean layers, with a distinctive barrier between each region. This interface between the layers is generally invisible to the naked eye, but it can be observed using specialized IR cameras.

The pelagic zones form the basis behind thermoclines in the ocean and are integral to understanding the interactions between organisms in each zone.

They also enable scientists to study the physical and chemical impacts of these temperature gradients on the surrounding region.

The pelagic zones are as follows:

1. Epipelagic (up to 200 meters) – The uppermost layer where sunlight can penetrate. Photosynthesis is possible in this zone. The majority of marine flora and fauna are within this region. This region has ambient temperatures ranging from 20⁰C to 30⁰C depending on the geographic location.

2. Mesopelagic (from 200 meters to 1,000 meters) – The next region, which receives lesser sunlight. The organisms are usually bioluminescent and often travel between the upper reaches of this region and the epipelagic zone. This region has temperatures of around 10⁰C and below.

3. Bathypelagic (from 1,000 meters to 4,000 meters) – This region is characterized by having no available sunlight. There is no marine flora here, although certain bioluminescent species of fish are here (such as the angler fish). A characteristic creature that lives in this region is the giant squid. There is no visibility here, and submarines find it difficult to dive down to this depth. This region has temperatures of around 4⁰C and below.

4. Abyssopelagic (from 4,000 meters to the ocean bed) – Stemming from the term “abyss”, this region is nearly uninhabited and has harsh conditions. The main characteristics of this region are the complete lack of sunlight, cold temperatures, and extremely high pressures. Most creatures here have no eyes since they cannot use visible light. The majority of marine fauna here are echinoderms. This region has near freezing point temperatures (~ 0⁰C).

5. Hadopelagic (Hadal or Trench zone) – These zones are usually located within ocean bed trenches and gorges. They have harsh conditions, with very few living creatures. Worms, mussels, and crabs are the common marine inhabitants of the hadopelagic region. This region is often below freezing point temperatures (< 0⁰C). For this reason, frozen trench openings are often mistaken for the ocean bed.

Now that we have classified the ocean into the various temperature zones, let us look at thermoclines and their impact.

What Are Thermoclines?

A thermocline is a region of severe temperature changes, that create a division between the pelagic zones. It can be found in both the atmosphere and in Earth’s waterbodies.

However, the air is much rarer as compared to water, and so, this effect is not very prominent and is even ignored in the aircraft industry. The primary area of concern for thermoclines is the effects of these zones in the ocean.

The thermocline divides warmer water above it and the colder water below. Also, the water above the cline is often turbulent and in a mixed state, while the thermocline keeps the water below it calm.

Thermoclines are variable and can be found during different seasons. The location of this cline is dependent on its geographical position, seasonal weather patterns, turbulent water conditions, state mixing, and surface wind shear.

The depth of the ocean bed also influences the thermocline formation. The thickness or width of the cline is dependent on other parameters such as latitude, ambient weather conditions, tides, currents, wave heights, and seasonal lag.

Thermocline formation is prolonged in tropic conditions, variable in temperate conditions, and very rare in cold conditions (such as those at the poles).

It is also visible in smaller bodies of water, such as lakes and deep inland seas.

Due to the weather conditions, a warmer region known as the epilimnion exists above the cline, which is above the colder region known as the hypolimnion.

Intra-region mixing occurs, but the cline prevents the water from mixing between the 2 regions.

One main impact of thermoclines is that due to the lack of inter-region mixing, oxygen content in the hypolimnion rapidly depletes (since organisms utilize it, and there is no source below the cline).

During cold winters, the surface epilimnion may become colder (and hence denser) than the hypolimnion. This causes a reversal or mixing between the 2 layers, with the now colder surface water sinking to the bottom.

With this, the oxygen content is replenished, and algal blooms are common. Since the condition for this is that the surface epilimnion is colder than the hypolimnion, it is more commonly experienced in the Arctic and Antarctic regions.

Small bodies of water have regular over-turning of water between these 2 regions, which sustains organisms at the bottom with periodic replenishment of oxygen.

Underwater Thermocline known as the thermal layer

Physical And Chemical Features Of A Thermocline

Density is dependent on temperature, and thermoclines indicate a difference in density. As temperature increases, the medium becomes rarer (with increased spaces between molecules), which decreases the density. On the other hand, colder temperatures indicate an increase in density.

Since the thermocline has a sharp temperature gradient, it also has variable density along with its thickness. This mainly impacts the passage of fish across the thermocline, since they cannot withstand the sudden change in temperature and density.

Moreover, this difference in density is the primary factor that affects the physical and chemical properties of the thermocline.

One of the most important impacts of the density gradient is the negative sound speed gradient across the thermocline. The sound speed gradient is defined as the rate at which the speed of sound changes across the clines thickness.

A change in speed leads to the refraction of sound waves, which can distort audio signals. The speed of sound is directly proportional to the temperature, i.e., waves travel slower at lower temperatures.

On the other hand, the radius of curvature of the sound wave (due to refraction) is inversely proportional to the rate at which the speed changes.

This density gradient results in a velocity gradient, which creates variable acoustic propagation across the ocean’s depth. The main use of this phenomenon is in naval underwater warfare and stealth mechanisms.

In regions outside the thermocline, the speed of sound is nearly constant, and instead of being variable, it is known as sound iso speeds, where the same speed of sound exists at any point on a vertical plane within this constant density zone.

Cline oscillation is a phenomenon where waves are created on the thermocline due to the density gradient. Here, the refraction of light makes the cline appear as if it is oscillating along with the waves. This phenomenon is commonly visible at those depths where the gradient is severe. Such a thermocline oscillation is in the form of standing waves (known as a seiche).

While we have looked at the physical features of thermoclines, they also serve a very useful purpose in meteorological monitoring – hurricane detection.

Potential hurricanes can be identified and detected by the ocean surface temperature and the height of water above the first thermocline.

Any water above this thermocline can evaporate, fueling hurricane formations and increasing the wind speeds. Lower thermocline depths mean that there is a lesser chance of a hurricane formation in the region.

Scientists can identify the thermocline zone based on refractometry and studying the temperature gradient with the ocean’s depth. Then, based on the location of the thermocline and the surface temperature, it is possible to determine the possibility of a hurricane forming.

Impact Of Thermoclines On Marine Flora And Fauna

Creatures that live in the ocean are finely tuned to surviving at specific depths, temperatures, and salinity conditions. This is because these parameters vary widely over the ocean’s depth, and require different evolutionary systems to survive.

Most creatures do not live on the thermocline, since this region has conditions that frequently change based on other conditions. While fishes can swim across thermoclines, this becomes increasingly difficult as depth increases.

In the epipelagic zone, most creatures and phytoplankton can survive, since there are adequate sunlight and sources of nutrition.

As one goes deeper, creatures begin attuning themselves to the changing parameters. They develop external coats of fat and scales to keep them warm and insulated.

Due to the reducing visibility, many of them are also bioluminescent. This helps them see and also acts as a lure for unwary fish (as used by the angler fish).

Beyond a certain point, there is no sunlight, and marine life has no use for eyes. They survive on other senses, do not swim in groups, and are sedentary.

Moreover, since there is no sunlight, they are almost transparent (lack of sunlight begins to bleach their skin until it becomes almost clear). These characteristics differentiate fish between the different zones.

In general, very few organisms can survive in the hadopelagic and abyssopelagic zones. The majority are worms and mussels that can live on minimal nutrition. Their systems are tuned to surviving alone, navigating by using sounds and vibrations and using phytoplankton and zooplankton for sustenance.

Upper reach thermoclines can be crossed easily since the gradient is lesser. However, as depth increases, the thermocline width increases and the gradient becomes harsher.

Beyond a certain region, fish cannot survive the different conditions and do not venture out of their pelagic zones. These thermoclines serve as control over fish crossing between different regions.

One creature that can swim down to great depths in the hunt of prey is the deep-water sperm whale. Although it lives in the mesopelagic and epipelagic zones, they often swim down to the bathypelagic region in search of giant squids.

They are natural predators of this species of squid and can survive the thermocline transition between the different zones. Note, not all sperm whales can swim into the bathypelagic zone.

We observe how thermoclines and the varying temperature conditions have forced organisms to evolve in a variety of ways. Some features allow them to survive without sunlight, while others help them counter the high pressure and low temperatures.

Impact Of Thermoclines On Submarines, Ships, And AUVs

Due to the temperature difference, the thermocline creates a density gradient between the various pelagic ocean zones. This density zone, while invisible to the naked eye, plays a major role in the design of underwater crafts.

In this section, we will look at why and how this density gradient must be accounted for in the naval design.

For a vessel to propel itself forward in the oceans, there are 2 main components involved- thrust forces to propel, and resistance forces acting against this thrust.

The top speed of the vessel is generally provided by the client, based on the ship type and purpose. The thrust force is then calculated by finding out the resistance forces.

Most of these drag forces are intuitive- flow separation, appendage drag, atmospheric drag (due to wind), etc. However, there is another drag known as wave-making resistance.

Wave-making resistance is a force required to overcome the pressure gradient across the interface of 2 density gradients. Consider a ship moving between the water-air media.

At this interface, the wave pressure must equal the atmospheric pressure. To ensure this, a wave of height “h” is generated at the fore of the vessel (near the bulbous bow ). This can be represented mathematically as:

1/2 ρV^2=ρgh
h=V^2/2g

This is the height of the wave that is formed at the fore, and the energy required for this acts as a resistance force for our vessel. This is known as wave-making resistance.

Now consider the thermocline, which also has a density gradient. Depending on how the densities are spread across the gradient, sub-surface waves will also be generated at the fore of the submarine.

As a result, the submarine will be required to expend more power to overcome this new resistance component.

As seen above, these underwater vehicles try to operate at depths between the epipelagic and deep-water zones, so that they can avoid these density gradients.

Another problem that arises with having an abrupt density and temperature gradient is that thermoclines create a negative sound speed gradient. This means that sound and acoustic waves reflect when they encounter a temperature gradient.

Submarines below the surface must stay away from such gradients, since their movements may be amplified to enemy crafts when sailing near a thermocline.

The physical phenomenon that creates the negative sound speed gradient is acoustic impedance. This factor must be taken into account during the design of naval communication technologies.

Thus, we have seen how a factor as simple as the change in temperature across the ocean’s depth can impact ships, submarines, and other underwater vessels.

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About Author

Ajay Menon is a graduate of the Indian Institute of Technology, Kharagpur, with an integrated major in Ocean Engineering and Naval Architecture. Besides writing, he balances chess and works out tunes on his keyboard during his free time.