Vertical & Horizontal Temperature Distribution of Oceans

Vertical & Horizontal Temperature Distribution of Oceans

The temperature of the oceanic water is impor­tant for marine organisms including plants (phytoplanktons) and animals (zooplanktons). The tem­perature of sea water also affects the climate of coastal lands and plants and animals therein.

The study of both, surface and subsurface temperature of sea water is significant. Standard type of thermometer is used to measure the surface temperature while reversing thermometers and thermographs are used to measure the subsurface temperature.

• Oceans absorb more than 80 percent of the solar radiation that reaches the earth. Moreover, water has a remarkable capacity for absorbing heat.
• The uppermost 10 percent of the oceans contain more heat than the entire atmosphere.
• The temperature of the oceans is not uniform. It differs from latitude to latitude and from the surface to the bottom.

Sources of Ocean Heat

Radiation from the sun and sky:

• The short-wave radiation that reaches the sea surface comes partly directly from the sun and partly from the sky as reflected or scattered radiation.
• The amount of radiation energy which is absorbed per unit volume in the sea depends upon the amount of energy that reaches the sea surface, the reflection from the sea surface, and the absorption coefficients for total energy.
• The incoming radiation depends mainly upon the altitude of the sun, the absorption in the atmosphere, and the cloudiness.
• With a clear sky and a high sun, about 85 per cent of the radiation comes directly from the sun and about 15 per cent from the sky, but with a low sun the proportion from the sky is greater, reaching about 40 per cent of the total with the sun 10 degrees above the horizon.

Processes that Heat Ocean Water

• Absorption of radiation from the sun and the sky
• Convection of heat through the ocean bottom , from the interior of the earth.
• Transformation of kinetic energy to heat.
• Heating due to chemical processes.
• Convection of sensible heat from the atmosphere.
• Condensation of water vapor.

Processes That Cool Ocean Water

• Back radiation from the sea surface
• Convection of sensible heat to the atmosphere
• Evaporation

The ocean water is heated by three processes.

1. Absorption of sun’s radiation.
2. The conventional currents: Since the temperature of the earth increases with increasing depth, the ocean water at great depths is heated faster than the upper water layers. So, convectional oceanic circulations develop causing circulation of heat in water.
3. Heat is produced due to friction caused by the surface wind and the tidal currents which increase stress on the water body.

The ocean water is cooled by

1. Back radiation (heat budget) from the sea surface takes place as the solar energy once received is reradiated as long wave radiation (terrestrial radiation or infrared radiation) from the seawater.
2. Exchange of heat between the sea and the atmosphere if there is temperature difference.
3. Evaporation: Heat is lost in the form of latent heat of evaporation (atmosphere gains this heat in the form of latent heat of condensation).

Vertical Temperature Distribution of Oceans

• Photic or euphotic zone extends from the upper surface to ~200 m. The photic zone receives adequate solar insolation.
• Aphotic zone extends from 200 m to the ocean bottom; this zone does not receive adequate sunrays.

Thermocline

• The profile shows a boundary region between the surface waters of the ocean and the deeper layers.
• The boundary usually begins around 100 – 400 m below the sea surface and extends several hundred of meters downward.
• This boundary region, from where there is a rapid decrease of temperature, is called the thermocline. About 90 per cent of the total volume of water is found below the thermocline in the deep ocean. In this zone, temperatures approach 0° C.

Three-Layer System

• The temperature structure of oceans over middle and low latitudes can be described as a three-layer system from surface to the bottom.
• The first layer represents the top layer of warm oceanic water and it is about 500m thick with temperatures ranging between 20° and 25° C. This layer, within the tropical region, is present throughout the year but in mid-latitudes it develops only during summer.
• The second layer called the thermocline layer lies below the first layer and is characterized by rapid decrease in temperature with increasing depth. The thermocline is 500 -1,000 m thick.
• The third layer is very cold and extends up to the deep ocean floor. Here the temperature becomes almost stagnant.

Both energy and sunlight decrease with depth in the oceans.

• Only about 45 percent of light energy striking the ocean surface reaches a depth of about one meter, and only 16 percent reaches a depth of 10 meters.
• On the basis of the temperature, the ocean depths may be divided into the following three zones:
  • Surface Zone or Mixed Zone: This is also known as the Photic zone or Euphoric zone. It is the upper layer of the ocean. In this layer, the temperature and salinity are relatively constant. It contains about 2 percent of the total volume of water in the ocean. It is limited to a depth of about 100 meters.
  • Thermocline: It lies between 100 metres and 1000 metres. It contains about 18 percent of the total volume of water in the ocean. There is a steep fall in temperature in this zone. The density of water increases with increasing depth.
  • Deep Zone: This zone lies below 1000 metres in the mid-latitudes. This zone contains about 80 percent of the total volume of water in the ocean. The temperature in this zone remains constant. The ocean bottom always has a temperature which is one or two degrees Celsius above the freezing point.

General behavior

• In the Arctic and Antarctic circles, the surface water temperatures are close to 0° C and so the temperature change with the depth is very slight (ice is a very bad conductor of heat). Here, only one layer of cold water exists, which extends from surface to deep ocean floor.

The rate of decrease of temperature with depths is greater at the equator than at the poles.

• The surface temperature and its downward decrease is influenced by the upwelling of bottom water (Near Peruvian coast during normal years).
• In cold Arctic and Antarctic regions, sinking of cold water and its movement towards lower latitudes is observed.
• In equatorial regions the surface, water sometimes exhibits lower temperature and salinity due to high rainfall, whereas the layers below it have higher temperatures.
• The enclosed seas in both the lower and higher latitudes record higher temperatures at the bottom.
• The enclosed seas of low latitudes like the Sargasso Sea, the Red Sea and the Mediterranean Sea have high bottom temperatures due to high insolation throughout the year and lesser mixing of the warm and cold’ waters.
• In the case of the high latitude enclosed seas, the bottom layers of water are warmer as water of slightly higher salinity and temperature moves from outer ocean as a sub-surface current.
• The presence of submarine barriers may lead to different temperature conditions on the two sides of the barrier. For example, at the Strait of Bab-el-Mandeb, the submarine barrier (sill) has a height of about 366 m. The subsurface water in the strait is at high temperature compared to water at same level in Indian ocean. The temperature difference is greater than nearly 20° C.

Horizontal Temperature Distribution of Oceans

• In general, the temperature of the surface water in the lower latitudes is about 26 degrees Celsius which decreases towards poles.
• The oceans of the Northern Hemisphere record an average temperature of 19.4 degrees Celsius. However, the average temperature recorded at various latitudes also varies with 22 degrees Celsius recorded at 20 degrees latitude, and 14 degrees Celsius recorded at 40 degrees latitude in the Northern Hemisphere.
• At the poles, the temperature drops to 0 degrees Celsius.
• The maximum and minimum annual temperatures of ocean water in the Northern Hemisphere are in the months of August and February respectively.
• The average annual range of temperature is about 12 degrees Celsius.
• The highest annual range of temperature is recorded in the North Atlantic Ocean. Moreover, the annual range of temperature is higher for the inland seas as compared to the open oceans.

Range of Ocean Temperature

• The oceans and seas get heated and cooled slower than the land surfaces. Therefore, even if the solar insolation is maximum at noon, the ocean surface temperature is highest at 2 p.m.
• The average diurnal or daily range of temperature is barely 1 degree in oceans and seas.
• The highest temperature in surface water is attained at 2 p.m. and the lowest, at 5 a.m.
• The diurnal range of temperature is highest in oceans if the sky is free of clouds and the atmosphere is calm.
• The annual range of temperature is influenced by the annual variation of insolation, the nature of ocean currents and the prevailing winds.
• The maximum and the minimum temperatures in oceans are slightly delayed than those of land areas (the maximum being in August and the minimum in February [Think why intense tropical cyclones occur mostly between August and October – case is slightly different in Indian Ocean due to its shape]).
• The northern Pacific and northern Atlantic oceans have a greater range of temperature than their southern parts due to a difference in the force of prevailing winds from the land and more extensive ocean currents in the southern parts of oceans.
• Besides annual and diurnal ranges of temperature, there are periodic fluctuations of sea temperature also. For example, the 11-year sunspot cycle causes sea temperatures to rise after a 11- year gap.