The Oceans Role in Climate

·      Oceans influence atmospheric processes by albedo, moisture circulation and energy transfer.

• Atmospheric processes influence oceanic processes, for example waves receive their energy from the winds as they blow across the surface of water. The size of waves depends upon the velocity, fetch (how far it travels) and duration of wind.
• Energy is redistributed through oceans by thermohaline circulation. This helps to maintain the global climate.

Themohaline Circulation

Thermohaline Circulation

The thermohaline circulation, otherwise known as the ‘ocean gyre’ or ‘global ocean conveyor belt’, is large-scale oceanic circulation that is controlled by changes in water density (which in turn affects its buoyancy) caused by variations in temperature and salinity. Thermohaline circulation has a symbiotic relationship with global climate. We have already examined how heat is transferred around the Earth by the atmosphere now we must also consider the role of the oceans in the distribution of heat.

An easy way to remember what thermohaline means is by deconstructing the name:  thermo refers to temperature and haline refers to salt content.

Two factors determine water density:

Salinity: the saltier, the denser.

Temperature: the colder, the denser.

Ocean circulation is largely controlled by temperature gradients. Water cools and sinks at the poles and travels towards the equator along the seabed. At the equator these currents rise, warm and overturn (upwelling) to flow back towards the polar oceans.

 Seawater is 2-3 % denser than freshwater, therefore, pure freshwater floats.

How slanity and temperature affect the density of water.

Processes that increase salinity:
• Evaporation: removes pure fresh water.
• Freezing of sea ice: removes pure fresh water.
• Formation of glaciers: reduces runoff.
Processes that decrease salinity:
• Precipitation: adds pure fresh water.
• Runoff from land (including from melting glaciers): adds pure fresh water.
• Melting of sea ice: adds pure fresh water.

Why is the sea salty?

Salt in the sea or oceans originates from land. Chemical (solution) weathering of rocks and soils on land by aeolian, fluvial and anthropogenic frees up salt compounds that are then transported to the oceans via rivers.

Other important facts:

·        The ocean works for a state of equilibrium – any dissolved salt added to the ocean is removed into oceanic sediment.  

·        Temperature variations with depth: at the tropics and temperate regions a thermocline exists, where temperature reduces rapidly with depth. However, in the polar oceans the water is constantly cold from the surface to the seabed.

The Day After Tomorrow

The thriller ‘The Day After Tomorrow’ tells the sensationalised story of dramatically abrupt climate change brought about by the shutdown of the North Atlantic oceanic conveyor as a result of rapid melting of the ice caps disrupting the natural balance of fresh and saline water.

Although time scale is completely unrealistic, the process itself is not completely a fictional. There is strong scientific evidence indicating that rapid ice shelf collapse and increased precipitation in the northern hemisphere caused by the global warming could theoretically tip the balance and instigating the onset of an ice age. The influx of pure freshwater may lead to widespread reduction in North Atlantic surface salinity, thus a decline in thermohaline circulation. In the absence of the Gulf Stream (the surface current within the North Atlantic segment of the oceanic conveyor), Western Europe will loose a significant source of heat – this is believed to be the trigger an ice age or a temporary cold period such as the Younger Dryas. 

For further investigation into the theory of abrupt climate change I recommend the article 'Abrupt climate change and the thermohaline circulation: Mechanisms and predictability' by Jochem Marotzke. It can be accessed from the Proceedings of the National Academy of Sciences of the USA website:

http://www.pnas.org/content/97/4/1347.full