Vertical & Horizontal Temperature Distribution of Oceans | UPSC

Vertical & Horizontal Temperature Distribution of Oceans

• More than 80% of the solar radiation that falls on the earth gets absorbed by the oceans. Water can hold a large amount of heat because of its high specific heat capacity.
• Just the top 10% of the ocean stores more heat energy than the whole atmosphere put together.
• Ocean temperature is not same everywhere. It changes with latitude and it also changes from the surface down to the bottom.

The main factors that decide ocean temperature are:

• Latitude: At the equator, sun rays hit the water surface straight (vertically). As you go towards the poles, these rays become slanted. Slanting rays spread over a bigger area and deliver less heat per unit area. So the surface temperature of oceans keeps dropping from the equator to the poles.
• Prevailing Winds: Winds like the Trade Winds and Westerlies blow in fixed patterns and they push surface water along with them. On the eastern side of oceans in the trade wind zone, warm surface water gets pushed away from the coast. Cold water from below comes up to fill the gap — this is called upwelling. That is why these eastern edges have cooler surface temperatures.
• Unequal distribution of Land and Water: There is more land in the Northern Hemisphere compared to the Southern Hemisphere. Land heats up quicker than water and also radiates heat faster. Because of more land in the north, the Northern Hemisphere oceans end up being warmer than the Southern Hemisphere oceans at similar latitudes.
• Evaporation Rate: Every year, roughly 350,000 cubic kilometres of ocean water turns into water vapour. This rate is not the same everywhere — tropical waters with higher temperatures lose more water to evaporation than the cooler waters of temperate regions. Since evaporation takes heat away from the surface, areas with high evaporation rates end up slightly cooler at the surface.
• Density of water: How dense ocean water is depends mostly on its temperature and salinity. Where salinity is high, the water tends to be relatively warmer. Where salinity is low, the water is generally cooler. Denser water sinks down and lighter water sits on top — this creates layers in the ocean.
• Ocean Currents: Warm currents bring heated water from lower latitudes and raise the temperature of the region they flow through. This also increases evaporation and leads to more rainfall in those areas. On the other hand, cold currents bring down the temperature. Coasts where a cold current flows tend to have more fog but get very little rain. For example, the Gulf Stream keeps western Europe much warmer than it should be at that latitude. The Labrador Current makes eastern Canada’s coast much colder.
• Local Factors: Things like submarine ridges, local storms, cyclones, fog, cloud cover, and even local rainfall patterns play a role in how warm or cold the ocean water is at a particular spot. These factors work at a smaller scale but they can create noticeable differences.

Why is the study of ocean temperature important? It helps us understand:

• How large volumes of water move around (ocean currents — both horizontal and vertical)
• What kind of marine life exists at different depths — different organisms survive in different temperature ranges
• The climate and weather of coastal areas — ocean temperature directly affects how much moisture and heat the nearby land receives

Source of Heat in Oceans

• The Sun is the main source of heat for the oceans. The energy that the sun sends is called Insolation.
• Apart from the sun, the ocean also gets some heat from inside the earth. The earth’s interior is very hot, and the oceanic crust at the sea surface is only about 5 to 30 km thick. Some of this internal heat seeps out through the ocean floor — you can see this at hydrothermal vents on the sea bed. But overall, this heat is very small compared to what the sun gives.

Vertical Distribution of Temperature:

• As you go deeper into the ocean, both sunlight and energy keep decreasing.
• Out of all the light energy hitting the ocean surface, only about 45% makes it to a depth of 1 metre. At 10 metres depth, just 16% is left. Go beyond 200 metres and practically no sunlight reaches there at all.

Based on temperature, the ocean can be divided into three vertical zones:

Surface Zone or Mixed Zone

• Also called Photic zone or Euphotic zone.
• This is the topmost layer. Winds and waves constantly stir this layer, so the temperature and salinity stay fairly uniform throughout.
• It has only about 2% of the total ocean water.
• It goes down to roughly 100 metres depth.
• Since sunlight is available here, photosynthesis happens. Phytoplankton grow in this zone, which is the base of the entire ocean food chain. So most of the ocean’s marine life is concentrated here.

Thermocline

• Lies between 100 metres and 1000 metres depth.
• Holds about 18% of the total ocean water.
• Temperature falls sharply in this zone — you lose several degrees for every few hundred metres you go down.
• Water density keeps rising with depth here because cold water is heavier.
• This zone works as a wall between the warm surface and the cold deep ocean. It blocks the mixing of these two layers.

Deep Zone

• Everything below 1000 metres in the mid-latitudes falls in this zone.
• This is the bulk of the ocean — it has about 80% of the total ocean water.
• Temperature here does not change. It stays constant at just 1-2 degrees above the freezing point, no matter what is happening at the surface.
• No sunlight reaches this depth. It is permanent darkness. The organisms that live here either feed on dead matter sinking from above or survive near hydrothermal vents through chemosynthesis.

Thermocline

• If you look at a temperature profile of the ocean (temperature vs depth graph), you will see a clear boundary region sitting between the warm surface water and the cold deep water.
• This boundary starts at around 100 to 400 metres below the surface and goes down several hundred metres.
• In this region, temperature drops rapidly — that is why it is called the thermocline.
• About 90% of the total ocean water sits below the thermocline in the deep ocean. Down there, temperatures go close to 0°C.
• The thermocline is most well-defined in the tropics and subtropics. There is a huge temperature gap between the hot surface and the cold deep water in these regions. In polar areas, the thermocline is very weak or does not exist at all — because the surface water itself is already cold, so there is not much of a temperature gap.
• The thermocline stops the surface water from mixing with the deep water. This means nutrients and dissolved oxygen from the deep ocean cannot easily reach the surface (except in upwelling zones). This affects where marine life can survive.
• Alongside the thermocline, there are two similar boundaries. The pycnocline is where density changes rapidly with depth. The halocline is where salinity changes rapidly. All three — thermocline, pycnocline, halocline — often overlap in the same depth range.
• In some parts of the ocean, a seasonal thermocline forms in summer when the sun heats the surface layer a lot. In winter, this seasonal thermocline disappears because storms and cooling mix the upper layers again. The permanent thermocline sits deeper and stays throughout the year.

Horizontal Temperature Distribution of Oceans

• Surface water temperature near the equator (low latitudes) is around 26°C. As you move towards the poles, this keeps decreasing.
• The Northern Hemisphere oceans have an average surface temperature of 19.4°C. But this varies a lot by latitude — at 20° latitude, the average is about 22°C. At 40° latitude, it drops to about 14°C.
• At the poles, the temperature goes down to 0°C.
• In the Northern Hemisphere, ocean water is at its warmest in August and at its coldest in February. There is a time lag of about a month compared to land — the ocean takes longer to heat up and longer to cool down.
• The average annual temperature range (difference between warmest and coldest months) for ocean water is about 12°C.
• The biggest annual temperature range is found in the North Atlantic Ocean. This is because warm currents like the Gulf Stream and cold currents like the Labrador Current both operate in this ocean, creating large seasonal swings.
• Inland seas show a higher annual temperature range than the open ocean. They are smaller and surrounded by land on most sides, so they heat up and cool down faster with the seasons — much like how a small pond changes temperature quicker than a large lake.
• Isotherms are lines drawn on a map connecting places with equal temperature. On ocean maps, isotherms generally run parallel to the latitudes. But near the coasts, they bend because of the influence of warm and cold ocean currents. Where a warm current flows, isotherms bend poleward. Where a cold current flows, isotherms bend equatorward.
• Compared to the Northern Hemisphere, the Southern Hemisphere shows more uniform temperature distribution. There are fewer landmasses in the south to block or redirect ocean circulation, so the heat gets spread more evenly.

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