Thermocline

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Perhaps, like me, you’ve tried diving down towards the bottom of a lake and noticed how after the first few metres, the water quickly drops in temperature. It has been a personal joy to test this in mountain lakes. Even just a few metres down, the water can be much colder than at the surface. This temperature gradient is a thermocline.

In the ocean, the thermocline is the zone below the surface mixed layer where water temperature decreases rapidly with depth. Because cooler water is denser than warmer water, the deeper water is typically denser than the warmer surface water.

The thermocline tends to have a greater temperature difference in tropical and temperate latitudes, and in the summer, when the surface layer is heated more. In the polar regions, thermoclines are much weaker.

Thermocline comes from thermo-, for temperature or heat, and -cline for slope, lean or bend—along with incline (bend towards), decline (turn aside), recline and others.

Thermoclines and Ocean Layers

Changes in temperature, salinity, and density lead to layers in the oceans. Ocean layering has important and far-reaching effects. Deep water may be doing something quite different from water at the surface. And areas where density causes water to rise or sink help drive ocean currents. In some cases, cold water at the bottom may be travelling in one direction, while surface water driven by winds may be travelling in the other direction.

NASA Goddard created a beautiful visualisation of ocean currents at different layers that’s well worth a 5-minute watch.

The three main ocean layers are the surface layer, sometimes known as the mixed layer, the thermocline, and the deep ocean.

The surface layer gets heated by the sun. Mixing by winds and waves distributes the heat.
Underneath, the thermocline is where temperature changes significantly with depth. The thermocline usually begins tens of metres below the surface and can extend to around 1000 m.
In the deep ocean, at the bottom, temperature and salinity tend to change little. Most of the oceans’ water is down here.

And sea ice, being less dense than water, floats on the surface.

Haloclines and Pycnoclines

Also affecting ocean layering is salinity—how salty the water is. Salty water, as well as colder water, is denser than fresh water and so will tend to sink beneath it.

Ever since the incredible “underwater lake” in BBC’s Planet Earth , I’ve been meaning to sketch a halocline, the change in density when fresh and salt water meet.

Fresh water entering the North Atlantic from Greenland’s ice sheets reduces the density of surface waters. This may, in turn, weaken the sinking that helps drive major ocean circulation patterns.

A pycnocline is a density gradient, often caused by changes in both temperature (thermocline) and salinity (halocline).

Related Ideas to Thermocline

Also see:

Know your poles — polar bears or penguins?
Why ice doesn’t sink
Topography and bathymetry
Mirage — light bends as it passes through air at different densities
Moon pool
Latitude or longitude
Buoyancy — how mega ships float
Parallax — different layers moving at different speeds

Once, when swimming across the stunning Lake Sils in the Engadin Valley in Switzerland, an upwelling of ice-cold water from the bottom of the lake took my breath away. Just when I had decided that I couldn’t swim further, I arrived out the other side of the cold water back into the, relatively, warmer surface water. I thanked my lucky stars.

Thermocline can refer both to a temperature gradient in water and to the specific ocean layer where that gradient is strongest. I’ve tried to discuss both here, not just the layer.

I created this sketch for a series on oceanography with the brilliant team at the American Meteorological Society .

Published 3 May 2026

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The 3 Phases of the El Niño-Southern Oscillation I expect you have heard of El Niño. Huge parts of the world, if not all of us in some way, are affected by El Niño, or, more completely, the El Niño-Southern Oscillation (ENSO). ENSO is a weather pattern that varies over a period of years. From typical conditions, to El Niño (the boy), the warm phase, to La Niña (the girl), the cold phase. ENSO includes changes in both the atmospheric and the tropical Pacific Ocean. It includes variations in sea surface temperatures, rainfall, air pressure and atmospheric circulation. El Niño refers mainly to the ocean warming pattern, while the Southern Oscillation describes accompanying shifts in atmospheric pressure and circulation. Now, El Niño always seemed complicated to me, and I never really understood it. But with El Niño on the horizon, and wonderful content from the American Meteorological Society, I wanted to try to explain the basics. El Niño is complex and varied, and the effects are widespread. I don’t profess to cover everything here. ENSO does not follow a perfectly regular cycle. Events vary in timing, strength and duration, and not every El Niño is followed by a La Niña. As George Box said: All models are wrong, but some are useful. Because El Niño is a change in the normal conditions, to understand it, I think you need to understand how it’s different from normal. So here, and in the sketch, I focused on the three phases: Neutral, El Niño and La Niña. The Neutral Phase - typical conditions In typical patterns, between El Niños, trade winds push warm surface waters in the tropics of the Pacific Ocean to the west. All this movement of warm water leads to strong evaporation, low pressure and high rainfall in the western Pacific towards Indonesia and Australia. It also raises the sea level in the western Pacific compared to the east. These patterns of winds, rainfall and rising and sinking air across the Pacific are part of what is called the Walker Circulation. At the same time, the western movement of surface waters creates an upwelling of deep water by South America, particularly along the coast of Peru. This upwelling provides nutrients that feed phytoplankton, which feed vast numbers of fish and, in turn, support major fisheries. The colder water is associated with higher pressure and drier conditions along the coast of Peru. El Niño - The Warm Phase El Niño is known as the warm phase because the warm surface waters spread east towards South America. This happens because the trade winds blowing westward weaken or even reverse. As the trade winds weaken, the slope in sea level across the Pacific relaxes, and warm water spreads east and suppresses the upwelling that supported the marine life near Peru. Rainfall decreases in the western Pacific causing dry spells or drought. The fish stocks decline along the coast of Peru. And with the reduction of Peru’s typical high-pressure system, rainfall there often increases causing flooding. Because the Pacific Ocean is so large, some El Niños cause weather changes in monsoon rains, the poles, Europe, Africa, and as far north as Alaska. La Niña - The Cold Phase Sometimes, after warm water spreads east during an El Niño event, conditions swing back strongly the other way—with stronger trade winds and increased cold upwelling. With the difference in sea level across the Pacific, I think of it a bit like a giant slosh in a bathtub. In La Niña conditions, the trade winds strengthen, pushing warm waters further west. High pressure often develops along South America, and the cold upwelling increases. Warmer water around Indonesia and Southeast Asia intensifies the low pressure, resulting in high rainfall and often flooding. Because the El Niño phase spreads warmer sea surface temperatures across a larger area of the Pacific, it often coincides with warmer-than-normal global average temperatures. Strong El Niño events can increase the risk of drought, flooding, heatwaves and wildfires in different parts of the world. Does this give you a better idea of it? More about El Niño The differences in low and high pressure for the Southern Oscillation are sometimes measured as the difference from normal between Darwin (Northern Australia) and Tahiti (in the central south Pacific). Why is it called El Niño? El Niño, Spanish for the boy, actually corresponds to Christ, as it was first noticed by South American fishermen around Christmas. The Walker Circulation The trade winds and rainfall patterns behind ENSO are part of a larger atmospheric circulation called the Walker Circulation. I sketched that separately, too, as it varies during Neutral, El Niño and La Niña phases. You can see it in the newsletter for this sketch. Related Ideas to El Niño Also see: Seasons Know Your Clouds Latitude and longitude: which is which? Weather and Climate, an analogy Thermocline Thunder Clouds Time hierarchy - fast and slow layers for robust, adaptable systems Temperature scales: Celsius and Fahrenheit All models are wrong, but some are useful Read more I learned about ENSO, El Niño, La Niña, the Walker Circulation and others from some oceanography textbooks. Find it in Chapter 7 of Introduction to Ocean Sciences 5th edition by Douglas Segar, the AMS Ocean Studies textbook, and the soon-to-be-released open-access Introduction to Ocean Sciences 6th Edition.The 3 Phases of the El Niño-Southern Oscillation I expect you have heard of El Niño. Huge parts of the world, if not all of us in some way, are affected by El Niño, or, more completely, the El Niño-Southern Oscillation (ENSO). ENSO is a weather pattern that varies over a period of years. From typical conditions, to El Niño (the boy), the warm phase, to La Niña (the girl), the cold phase. ENSO includes changes in both the atmospheric and the tropical Pacific Ocean. It includes variations in sea surface temperatures, rainfall, air pressure and atmospheric circulation. El Niño refers mainly to the ocean warming pattern, while the Southern Oscillation describes accompanying shifts in atmospheric pressure and circulation. Now, El Niño always seemed complicated to me, and I never really understood it. But with El Niño on the horizon, and wonderful content from the American Meteorological Society, I wanted to try to explain the basics. El Niño is complex and varied, and the effects are widespread. I don’t profess to cover everything here. ENSO does not follow a perfectly regular cycle. Events vary in timing, strength and duration, and not every El Niño is followed by a La Niña. As George Box said: All models are wrong, but some are useful. Because El Niño is a change in the normal conditions, to understand it, I think you need to understand how it’s different from normal. So here, and in the sketch, I focused on the three phases: Neutral, El Niño and La Niña. The Neutral Phase - typical conditions In typical patterns, between El Niños, trade winds push warm surface waters in the tropics of the Pacific Ocean to the west. All this movement of warm water leads to strong evaporation, low pressure and high rainfall in the western Pacific towards Indonesia and Australia. It also raises the sea level in the western Pacific compared to the east. These patterns of winds, rainfall and rising and sinking air across the Pacific are part of what is called the Walker Circulation. At the same time, the western movement of surface waters creates an upwelling of deep water by South America, particularly along the coast of Peru. This upwelling provides nutrients that feed phytoplankton, which feed vast numbers of fish and, in turn, support major fisheries. The colder water is associated with higher pressure and drier conditions along the coast of Peru. El Niño - The Warm Phase El Niño is known as the warm phase because the warm surface waters spread east towards South America. This happens because the trade winds blowing westward weaken or even reverse. As the trade winds weaken, the slope in sea level across the Pacific relaxes, and warm water spreads east and suppresses the upwelling that supported the marine life near Peru. Rainfall decreases in the western Pacific causing dry spells or drought. The fish stocks decline along the coast of Peru. And with the reduction of Peru’s typical high-pressure system, rainfall there often increases causing flooding. Because the Pacific Ocean is so large, some El Niños cause weather changes in monsoon rains, the poles, Europe, Africa, and as far north as Alaska. La Niña - The Cold Phase Sometimes, after warm water spreads east during an El Niño event, conditions swing back strongly the other way—with stronger trade winds and increased cold upwelling. With the difference in sea level across the Pacific, I think of it a bit like a giant slosh in a bathtub. In La Niña conditions, the trade winds strengthen, pushing warm waters further west. High pressure often develops along South America, and the cold upwelling increases. Warmer water around Indonesia and Southeast Asia intensifies the low pressure, resulting in high rainfall and often flooding. Because the El Niño phase spreads warmer sea surface temperatures across a larger area of the Pacific, it often coincides with warmer-than-normal global average temperatures. Strong El Niño events can increase the risk of drought, flooding, heatwaves and wildfires in different parts of the world. Does this give you a better idea of it? More about El Niño The differences in low and high pressure for the Southern Oscillation are sometimes measured as the difference from normal between Darwin (Northern Australia) and Tahiti (in the central south Pacific). Why is it called El Niño? El Niño, Spanish for the boy, actually corresponds to Christ, as it was first noticed by South American fishermen around Christmas. The Walker Circulation The trade winds and rainfall patterns behind ENSO are part of a larger atmospheric circulation called the Walker Circulation. I sketched that separately, too, as it varies during Neutral, El Niño and La Niña phases. You can see it in the newsletter for this sketch. Related Ideas to El Niño Also see: Seasons Know Your Clouds Latitude and longitude: which is which? Weather and Climate, an analogy Thermocline Thunder Clouds Time hierarchy - fast and slow layers for robust, adaptable systems Temperature scales: Celsius and Fahrenheit All models are wrong, but some are useful Read more I learned about ENSO, El Niño, La Niña, the Walker Circulation and others from some oceanography textbooks. Find it in Chapter 7 of Introduction to Ocean Sciences 5th edition by Douglas Segar, the AMS Ocean Studies textbook, and the soon-to-be-released open-access Introduction to Ocean Sciences 6th Edition.

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