Heat & Salt Budgets | Geography Optional for UPSC (Notes) PDF Download

Source of Heat in Oceans

The primary source of heat for the world's oceans is the sun, which provides energy not only for the oceans but for all life on Earth. Additionally, heat from the Earth's interior also contributes to warming the ocean water.

There are three main mechanisms through which ocean water is heated:

• Absorption of solar radiation is highest in low latitude regions due to direct sunlight and longer daylight hours, and gradually decreases towards the poles. Even within the same latitude, the amount of solar energy absorbed by the ocean can vary due to factors such as ocean currents and cloud cover.
• Convection currents within the ocean also contribute to heating the water. As the Earth's temperature increases with depth, the water at the bottom of the ocean heats up more quickly than the water closer to the surface. This causes convection currents to form, transferring heat from the deeper layers to the upper layers of the ocean.
• Friction generated by surface winds and tidal currents produces kinetic energy, which in turn heats the ocean water.

On the other hand, the ocean loses heat through the following processes:

• Back radiation occurs when the solar energy absorbed by the ocean is re-emitted as long-wave radiation from the water's surface.
• Heat exchange between the ocean and the atmosphere takes place when there is a temperature difference between the water and the surrounding air.
• Evaporation cools the ocean water when the surface is cold, the water is warm, and the atmospheric conditions are unstable.

In summary, the sun is the main source of heat for the oceans, while internal heat from the Earth also contributes. Heat is transferred through absorption of solar radiation, convection currents, and friction from winds and currents. The ocean loses heat through back radiation, heat exchange with the atmosphere, and evaporation.

Heat Budget of the Oceans

• Ocean Heat Budget: The concept of heat budget suggests that the total energy supply is balanced by an equal amount of energy loss. It has been observed that the average annual surplus of solar radiation between the equator (0°) and 10°N latitude is about 0.170 gm cal/cm2/min, while it is about 0.040 gm cal/cm2/min between 60°N to 70°N. However, this difference in surplus solar radiation disappears when considering all latitudinal regions.
• Ocean Temperature Range: Oceans and seas heat up and cool down more slowly than land surfaces. As a result, even if the solar radiation is at its maximum at 12 noon, the ocean surface temperature reaches its highest point at 2 p.m. The average daily temperature range in oceans and seas is only about 1 degree. The highest temperature in surface water occurs at 2 p.m., while the lowest occurs at 5 a.m. The daily temperature range is highest in oceans when the sky is cloud-free and the atmosphere is calm.

The annual temperature range is influenced by the annual variation in solar radiation, the nature of ocean currents, and the prevailing winds. The maximum and minimum temperatures in oceans are slightly delayed compared to land areas, with the maximum occurring in August and the minimum in February. The northern Pacific and northern Atlantic oceans have a greater temperature range than their southern counterparts due to differences in the strength of prevailing winds from land and more extensive ocean currents in the southern parts of oceans.

In addition to annual and daily temperature ranges, there are also periodic fluctuations in sea temperature. For example, the 11-year sunspot cycle causes sea temperatures to rise after an 11-year gap.

• Sea Surface Temperature: The surface temperature of oceans is represented graphically by isotherms, which show a decrease in temperature from the equator to the poles. However, the highest sea surface temperature is observed not exactly at the equator but slightly to the north of the equator due to the presence of more land area north of 0° latitude.
  Water bodies in the southern hemisphere generally have higher average temperatures than those in the northern hemisphere because the greater proportion of land area in the northern hemisphere absorbs more solar energy than water. Moreover, due to the presence of continents in the northern hemisphere, the circulation of water and heat transport is less efficient than in the southern hemisphere.
• Horizontal Distribution of Temperature: The horizontal temperature distribution is shown by isothermal lines, i.e., lines joining places of equal temperature. In the Atlantic Ocean, for example, the isothermal lines in February reveal that the lines are closely spaced south of Newfoundland, near the west coast of Europe and the North Sea, and then widen out and bulge toward the north near the coast of Norway.

This phenomenon is caused by the cold Labrador Current flowing southward along the North American coast, which reduces the temperature of the region more sharply than in other places at the same latitude. At the same time, the warm Gulf Stream moves toward the western coast of Europe, raising the temperature of the west coast of Europe.

• In the southwestern part of the Atlantic, isotherms bulge southwestward due to the warm Brazil current, while in the eastern part of the South Atlantic, isotherms bend northwestward due to the cold Benguela current. Further south, isotherms are parallel due to the constant prevailing west wind drift.
• Temperature distribution in the North and South Atlantic is not symmetrical. For example, the 5°C isotherm touches 70°N latitude in the North Atlantic, whereas it never crosses 50°S latitude in the South Atlantic because the warm Gulf Stream is more powerful and reaches a higher latitude than the cold Brazil current. There is also a significant difference between the eastern and western parts of the Atlantic. In the western part near the Labrador coast, 0°C temperature is recorded, but 9° to 13°C temperature is found on the west coast of Europe.
• In marginal seas, temperature varies due to latitude and location. For example, the Mediterranean has higher temperatures than the neighboring Atlantic Ocean, but the Baltic and Hudson Bay are colder than the Atlantic.
• In the northern half of the Pacific, isotherms and latitudes are almost parallel. However, on the coast of North America, isotherms bend slightly northward under the influence of the warm Kuroshio current, and along the coast of Japan, isotherms are closely spaced due to the cold Oyashio current.
• In the equatorial region of the western part of the Pacific, high temperatures are recorded as the warm equatorial current flows southward. In the eastern part of the Pacific, low temperatures prevail due to the influence of the cold Peru Current. In the South Pacific, isotherms make minor loops due to the warm Peru or Humboldt Current.
• In the Indian Ocean, the isotherms of 25°C, 27°C, and 28°C occupy the central location of the ocean. There is no significant difference between the southern Indian Ocean and the southern Pacific Ocean, as the isotherms roughly follow the parallels, except for a minor loop near the Cape of Good Hope due to the cold Agulhas current. The isotherms bend southward near the coast of North Africa due to a cold current flowing southwestward from Cape Guardafui.
• The same isotherm bends northward in the Arabian Sea when it enters the Indian Peninsula, but in the Bay of Bengal, it bends southward due to the effect of monsoon drift. Enclosed water bodies like the Red Sea have higher temperatures toward the south due to the mixture of open ocean water. The Persian Gulf records lower temperatures than the Indian Ocean under the influence of cold land area.
• In August, the isothermal conditions are markedly different from those in February. In the Atlantic, the Arctic ice melts away, resulting in a northward shift of all isotherms in the Davis Strait. The sharp northward bends of isotherms on the Norwegian coast are absent in August. On average, the isotherms in the North Atlantic shift northward in August. The southern Pacific Ocean shows isothermal lines and latitudes placed parallel to each other. Towards the west, the adjacent ocean of the Australia-Asia region witnesses temperatures as high as 28°C as the westerly flowing equatorial current draws warm water toward the western Pacific.
• In the Indian Ocean, the highest surface temperature of 28°C is recorded over the Arabian Sea and the Bay of Bengal. In August, enclosed seas like the Red Sea and the Persian Gulf show higher temperatures (30° to 33°C) than the open ocean due to their contact with warm land areas.

Vertical Distribution of Temperature

• The temperature of the ocean water changes as you go deeper, typically decreasing as you descend. About 90% of the sun's heat is absorbed within the top 60 feet (15.6 meters) of water. The water's temperature generally matches the surface temperature up to a depth of around 100 meters, but it tends to decrease with further depth.
• In tropical oceans and seas, there are three distinct layers of water temperature from the surface to the bottom. The first layer is approximately 500 meters thick, with temperatures ranging from 20°C to 25°C. In mid-latitude regions, this top layer is only present during the summer months. The second layer, known as the thermodine layer, is located just below the first one. 
• This layer is characterized by a rapid decrease in temperature as depth increases. The third and final layer is very cold and extends all the way down to the ocean floor.

Salt Budget

The salt budget, also known as the salt cycle, encompasses the various processes by which salt travels between the ocean, the lithosphere (Earth's solid outer layer), and to a lesser extent, the atmosphere.

• The movement of water, including groundwater, dissolves minerals from rocks through surface erosion. This mineral-rich water then flows into rivers and streams, eventually making its way to the ocean and contributing to its salinity. Over time, some of the salts in the ocean water settle to the bottom through sedimentation, forming mineralized rocks. These rocks can be raised above the ocean surface over millions of years due to plate tectonics or volcanic activity, reintroducing the salt to the lithosphere in the form of minerals (rocks).
• Additionally, ocean salt can be transferred to the atmosphere through the action of wind, creating salt spray. This salt then falls back to the lithosphere, mixed with precipitation. However, this process accounts for only a small portion of the salt exchange between land and sea.
• Overall, the salt cycle operates over an extensive period of time, involving the continuous transfer of salt between the ocean, lithosphere, and atmosphere.

Conclusion

The sun is the primary source of heat for the world's oceans, with additional contributions from the Earth's interior. Heat is transferred through various mechanisms such as absorption of solar radiation, convection currents, and friction from winds and currents. The oceans lose heat through back radiation, heat exchange with the atmosphere, and evaporation. The distribution of ocean temperatures varies depending on factors such as latitude, ocean currents, and prevailing winds. The salt budget, or salt cycle, involves the continuous transfer of salt between the ocean, lithosphere, and atmosphere, affecting the ocean's salinity and overall chemical composition.

Frequently Asked Questions (FAQs) of Heat & Salt Budgets

What is the primary source of heat for the world's oceans?

The primary source of heat for the world's oceans is the sun, which provides energy not only for the oceans but for all life on Earth. Additionally, heat from the Earth's interior also contributes to warming the ocean water.

How is heat transferred within the ocean?

Heat is transferred within the ocean through absorption of solar radiation, convection currents, and friction from winds and currents.

How do the oceans lose heat?

The ocean loses heat through back radiation, heat exchange with the atmosphere, and evaporation.

What factors influence the temperature range and distribution in the oceans?

The temperature range and distribution in the oceans are influenced by the annual variation in solar radiation, the nature of ocean currents, the prevailing winds, and the location of landmasses.

What is the salt budget, and how does it operate?

The salt budget, also known as the salt cycle, encompasses the various processes by which salt travels between the ocean, the lithosphere (Earth's solid outer layer), and to a lesser extent, the atmosphere. The salt cycle operates over an extensive period of time, involving the continuous transfer of salt between the ocean, lithosphere, and atmosphere through processes such as erosion, sedimentation, and plate tectonics.