OCEAN CIRCULATION

The Oceanic Conveyor Belt
The constant flow of water around the world's oceans is controlled by many factors, including the shape of the seabed (bathymetry) and many physical properties of the water itself. The dominant properties affecting where a water mass lies in the sea are temperature and salinity which regulate its density.
On the map below, the arrows show a theoretical path for ocean circulation, known as the conveyor belt of the oceans. The red arrows show surface waters (top 1000m) and the blue arrows show the movement of the water deep within the sea. A parcel of water following these arrows may take thousands of years to complete the circuit.

Finding the Temperature of the Sea from the Sky
Oceanographers make use of the electromagnetic (EM) spectrum using satellites. Different EM wavelengths reflect differently from the ocean surface, showing up various ocean properties.
The visible part of the spectrum (light) may be used to look at the apparent colour of the water, i.e. an area of green is probably chlorophyll, indicating levels of plankton. The infrared part of the spectrum is used to observe the sea surface temperature (the map above is an example of this with red meaning warm water and blue meaning cold). Higher EM frequencies (microwaves) are used to look at sea surface roughness, e.g. waves.

Layers in the ocean
The density of sea water is determined predominantly by its temperature and salinity. Salty water is denser than fresh, and cold water is denser than warm. Thus if two parcels of water have different temperatures and/or salinities they will be of different densities. Just as when oil lies on top of water (oil being less dense) different water masses lie one on top of each other within the ocean basins.
When surface waters reach the north or south poles they become very cold and more saline, as some fresh water freezes out as ice. The water becomes very dense and sinks to the bottom. Water in tropical regions may be very warm, but evaporation means it is saltier. In temperate regions, rain and river input freshen the surface waters as they travel northwards. In transporting warm water from the tropics to the poles, the ocean plays its part in the Earth's heat budget.

El Niño
The interaction between the atmosphere and the ocean has featured widely in the media in recent years. El Niño conditions occur when warm surface waters from the west Pacific move across to the east and stop the normal upwelling (where cool nutrient rich waters are forced to rise up to the surface as coastal winds push water offshore).
Linked to this is a shift in the atmospheric circulation. El Niño causes droughts in Australia and floods and storms in the Americas. Fish stocks may become depleted along the Peruvian coast, as the nutrient filled waters no longer upwell, cutting off the fishes' source of food.

The Attraction of the Moon
Tides are the result of gravitational attraction between the oceans and the sun and moon. This causes the oceans to distort. The tide is actually one of the longest waves in the ocean, with a period (time between two high tides) of just greater than 12 hours. However, tides also vary over the lunar cycle, according to how the pull of the sun and moon interact. When the moon and sun act in the same direction, the tide experiences its highest highs and lowest lows, called a spring tide. If the sun and the moon are acting at right angles, the tidal range is less extreme and is called a neap tide. This is shown in the diagram below. Tides also contribute to oceanic currents.

Waves
Waves on the sea surface are transverse waves. The most common type of wave is generated as wind blows across the ocean. Waves do not actually move the water, it is the disturbance which travels over it without any net transport. The water particles themselves undergo circular movements as the wave passes them, returning to where they started within a wavelength, as shown in the diagram below. When a wave reaches a coastline, the water shallows. Friction with the seabed causes the wave to change shape and steepen. When they become too steep, the waves break, and at this point the water particles move too. Surfers harness the wave energy using their boards.

   
This webpage is based on a poster created by Ivo Grigorov, a postgraduate student at the Southampton Oceanography Centre, with the help of pupils of St Anne's School, Southampton. Visit our website at www.soc.soton.ac.uk
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