The Role of Oceans

"Ocean water and currents affect climate. Because it takes far more energy to change the temperature of water than land or air, water warms up and cools off much more slowly than either. As a result, inland climates are subject to more extreme temperature ranges than coastal climates, which are insulated by nearby water.

Over half the heat that reaches the earth from the sun is absorbed by the ocean's surface layer, so surface currents move lots of heat. Currents that originate near the equator are warm; currents that flow from the poles are cold." (Smithsonian Ocean Planet)

"The planetary water supply is dominated by the oceans (see Table 8b-1). Approximately 97 % of all the water on the Earth is in the oceans. The other 3 % is held as freshwater in glaciers and icecaps, groundwater, lakes, soil, the atmosphere, and within life." (Michael J. Pidwirny)

Table 8b-1: Inventory of water at the Earth's surface.

Volume (cubic km x 10,000,000)
Percent of Total
Ice Caps and Glaciers
Soil Moisture
Streams and Rivers




 "Water is continually cycled between its various reservoirs. This cycling occurs through the processes of evaporation, condensation, precipitation, deposition, runoff, infiltration, sublimation, transpiration, melting, and groundwater flow. Table 8b-2 describes the approximate residence times of water in the major reservoirs. On average water is renewed in rivers once every 16 days. Water in the atmosphere is completely replaced once every 8 days. Slower rates of replacement occur in large lakes, glaciers, ocean bodies and groundwater. Replacement in these reservoirs can take from hundreds to thousands of years. Some of these resources (especially groundwater) are being used by humans at rates that far exceed their renewal times. This type of resource use is making this type of water effectively nonrenewable." (Michael J. Pidwirny)

Table 8b-2: Approximate residence time of water found in various reserviors.


 Approximate Residence Time


 40 years

 Seasonal Snow Cover

 0.4 years

 Soil Moisture

 0.2 years

 Groundwater: Shallow

 200 years

 Groundwater: Deep

 10,000 years


 100 years


 0.04 years


1.Area: covers 70% of the Earth’s surface
Volume: 97% of all the water on the Earth
Depth: 4 kilometers
3.Density: 1034-1035 kg/m3 (Pure water: 1000 kg/m3) over 90% of the ocean. Depends on temperature and salinity.
cold water ā high density
loss of water by evaporation ā increase salinity ā high density
precipitation and river discharge ā decrease salinity ā low density
4.Heat capacity: high compared to land
5.Temperature: less variable than in the atmosphere
6.Freezing point:  – 1.9°C, not at 0°C because of salinity
7.Surface is not level due to currents, waves, atmospheric pressure differences, and variations in gravity.
8.Two main forms of circulation:
wind-driven circulation (horizontal, surface waters, fast)
thermohaline circulation (vertical, deep waters, slow)
  • Well over half of the solar radiation reaching the Earth’s surface is absorbed by the oceans, so that it figures prominently in the regulation of the global climate.
  • Furthermore, the mean temperature of the global ocean is only ~3.6oC thus it represents a huge heat sink in the global climate system.
  • The primary heat source for the oceans is solar radiation entering through the ocean surface. Almost all this insolation (incoming solar radiation) is absorbed in the top 100 metres. Turbulent mixing in the surface layer is promoted by the interaction of wind and waves.
  • In the mixed layer of the ocean, convection and turbulence are so effective that the temperature and salinity are almost uniform (constant) with depth.
  • The global average thickness of the mixed layer is 70-100 metres.
  • The base of the mixed layer is marked by a horizon where the water properties change rapidly with depth – this is called the thermocline layer.

    This is a mechanically stable region, within which vertical motions are strongly resisted (since water density increases with declining temperature).

    Accordingly, the waters of the deep ocean are thermally "isolated" from the atmosphere, and the exchange of heat is very slow.

  • Below the thermocline layer, the ocean is thermally stratified and tends to a uniform temperature (since the density of sea water is greatest at around 4oC.

    The deep ocean comprises about 80% of the total volume of the oceans.

    Below the thermocline layer, in the deep ocean, there exist slow circulations driven primarily by density gradients. These currents are quite weak, and it is estimated that the time required for complete

  • The mixed layer, then, responds quickly to changes in the surface climate, whereas the deep ocean layer does not. The thermal capacity of the mixed layer implies response times to surface changes on the order of years.
  • The thermal properties of the deep ocean constitute a time lag in the climate system on the scale of 1000 years.
Additional Readings:
El Nino / Southern Oscillation
There has been a confusing range of uses for the terms El Niņo, La Niņa and ENSO by both the scientific community and the general public. I hope by taking this course students will have a better understanding of their meanings.
What is the Southern Oscillation? What is the Southern Oscillation Index (SOI)?
What is the Walker Circulation?
What is El Nino? How often do El Nino events occur?
What is La Nina?
What is ENSO? Oceanic characteristics of ENSO and atmospheric characteristics of ENSO. Links between atmosphere-ocean in ENSO.
How ENSO is developed? Nature and effects of oceanic Kelvin and Rossby waves in El Nino events.
How does ENSO influence the world climate?
Can ENSO be predicted?
How ENSO is monitored? Range and nature of indices used to indicate the state of ENSO (El Nino or La Nina).
Typical impacts of ENSO events (temperature and precipitation). ENSO and the climate in the USA in general, and in Texas in particular
El Niņo. "El Niņo translates from Spanish as 'the boy-child'. Peruvian anchovy fishermen traditionally used the term - a reference to the Christ child - to describe the appearance, around Christmas, of a warm ocean current off the South American coast, adjacent to Ecuador and extending into Peruvian waters. El Niņo affects traditional fisheries in Peru and Ecuador, In most years, colder nutrient-rich water from the deeper ocean is drawn to the surface near the coast (upwelling), producing abundant plankton, food source of the anchovy. However, when upwelling weakens in El Niņo years, and warmer low-nutrient water spreads along the coast, the anchovy harvest plummets. It was ruined in the four or five most severe El Niņo events this century." (Bureau of Meteorology, Australia)

"The South American El Niņo current is caused by large-scale interactions between the ocean and atmosphere. Nowadays, the term El Niņo refers to a sequence of changes in circulations across the Pacific Ocean and Indonesian archipelago when warming is particularly strong (on average every three to eight years). Characteristic changes in the atmosphere accompany those in the ocean, resulting in altered weather patterns across the globe." (Bureau of Meteorology, Australia)

The Walker Circulation. The Walker circulation is named after Sir Gilbert Walker, a Director-General of British observatories in India who, early this century, identified a number of relationships between seasonal climate variations in Asia and the Pacific region.

 "I cannot help believing that we shall gradually find out the physical mechanism by which these (relationships) are maintained..."

- Sir Gilbert T. Walker, 1918

The easterly trade winds are part of the low-level component of the Walker circulation. Typically, the trades bring warm moist air towards the Indonesian region. Here, moving over normally very warm seas, moist air rises to high levels of the atmosphere. The air then travels eastward before sinking over the eastern Pacific Ocean. The rising air is associated with a region of low air pressure, towering cumulonimbus clouds and rain. High pressure and dry conditions accompany the sinking air.

The Southern Oscillation. "By the Southern Oscillation is implied the tendency of pressure at stations in the Pacific ... to increase, while pressure in the region of the Indian Ocean ... decreases."

- Sir Gilbert T. Walker, 1924

This definition remains valid, but it now refers to the seesaw pattern of reversing surface air pressure at opposite ends of the Pacific Ocean. We now say that the Southern Oscillation occurs because of the large changes in the Walker circulation closely linked to the pattern of tropical Pacific sea temperatures. Because the pressure reversals and ocean warming are more or less simultaneous, we call this phenomenon the El Nino/Southern Oscillation or ENSO for short.

The Southern Oscillation Index. "The Southern Oscillation Index (SOI) gives us a simple measure of the strength and phase of the Southern Oscillation, and indicates the status of the Walker circulation. The SOI is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin (Equation = Tahiti – Darwin). The 'typical' Walker circulation Pattern shown in the diagram has an SOI close to zero (Southern Oscillation close to the long-term average state). When this pattern is strong the SOI is strongly positive (Southern Oscillation at one extreme of its range). When the Walker circulation enters its El Niņo phase, the SOI is strongly negative (Southern Oscillation at the other extreme of its range). Positive values of the SOI are associated with stronger Pacific trade winds and warmer sea temperatures to the north of Australia. Together these give a high probability that eastern and northern Australia will be wetter than normal. During El Niņo episodes, the Walker circulation weakens, seas around Australia cool, and slackened trade winds feed less moisture into the Australian/Asian region. There is then a high probability that eastern and northern Australia will be drier than normal." (Bureau of Meteorology, Australia)

La Niņa. SO tendencies for unusually low pressures west of the date line and high pressures east of the date line have also been linked to periods of anomalously cold equatorial Pacific sea surface temperatures (SSTs) sometimes referred to as La Niņa.

Another way to say the above. High SOI (large pressure difference) is associated with stronger than normal trade winds and La Niņa conditions, and low SOI (smaller pressure difference) is associated with weaker than normal trade winds and El Niņo conditions.

Sea surface temperatures. During non-El Nino and non-La Nina conditions sea surface temperatures are approximately 6-8 degrees Celsius warmer in the western tropical Pacific than in the eastern tropical Pacific. These temperature disparities typically occur because the easterly trade winds that blow across the tropical Pacific move the warm surface water with them from east to west. Thus, you could look at SST data to determine whether an El Nino event is occurring at present, or not.


Additional Readings

El Nino and US Climate

News and General Information

Impact Sites

Policy Sites

Full-Text Resources


Last updated on 01/05/10 03:25 PM by Zong-Liang Yang