Ocean currents and Upwelling
Ocean currents-Upwelling, worldwide there are ocean currents that follow along predictable paths because of the constraints of the immovable continents and rotation of the earth. The position of the fixed land masses or continents and the precise rotation of the earth in space give rise to the worlds ocean currents-upwelling, they are the constants. The prevailing winds and temperature variations drive ocean surface currents, but these have little effect on the large-scale movements and the mixing of seawater. The major currents in the sea are caused by the differences in density owing to variation in temperature and/or salinity – cold seawater is denser than warm seawater, while full-strength seawater is denser than that diluted by freshwater. The same as with air molecules, water molecules behave in the same manner. In a given sample of air at sea level like a balloon that contains a fixed number of air molecules, the balloon will occupy a certain volume, if the balloon is taken to a higher elevation where the atmospheric pressure is reduced; Low-pressure areas have less atmospheric mass above their location, the balloon will expand and occupy a larger volume with a reduction in temperature, conversely – high pressure areas have more atmospheric mass above their location.
Air molecules that are heated occupy a larger volume because the molecules move about and the distance between them increases, like in a hot air balloon, and again water molecules do the same as air molecules. But remember that air at sea level, is 784 times less dense than water. Expressed in another way, a volume of air at sea level has 0.1275% of the density of the same volume of water.
The point I am making is that the density of seawater is greatly affected by temperature and pressure. Warm equatorial seawater is less dense than cold polar seawater, just as surface temperature of ocean seawater is greater than the seawater at great depth.
The Southern African coast line is 5500 km long and extends from Angola (Cunene River mouth)in the west to the Zambezi delta in the east, the border of Tanzania. The South African coastline is 3200 km from Oranjemund on the west coast to the Mozambique border at Ponta do Oura on the east coast. The greatest majority of the 343 rivers occur on the east where they are fast flowing and deposit 400 tons of sediment per square kilometer per year. The large influx of fresh water dilutes the density of the seawater as compared to full-strength polar seawater. Thus the temperature changes between the equator and the poles, and the salinity changes because of rain, evaporation, freezing of seawater and the melting of ice, all play a role in ocean circulation and ocean currents-upwelling.
Global surface currents
On a global scale, the earth’s rotation and the position of the continents constrain large ocean currents to follow an almost circular pattern, called a gyre. Each of the 3 southern hemisphere gyre’s is made up of a westward-flowing equatorial current, an intense poleward-flowing western boundary current, an eastward-flowing sub polar current, and a diffuse equator ward-flowing eastern boundary current. In other words, gyres flow clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere. This can be attributed to the Coriolis Effect, caused by the earth’s rotation to the east. The precise position of the polar current is determined by the direction of the macro-scale wind direction. The equatorial currents are warm while polar currents are cold.
Equatorial currents flow at 3-6 km per day and usually extend 100-200 m below the surface. The equatorial counter current, which flows towards the east, is a partial return of water carried westward by the north and south equatorial currents. In El Nino years, this current intensifies in the Pacific Ocean.
Western boundary currents
Western boundary currents flowing along the east coast of continents, such as the Gulf Stream, Kuro Shio, Brazil, East Australia and the Agulhas currents, are all warm water currents with usually narrow, jet-like flows travelling at speeds of 40-120 km per day, and extending down to depths of 1000 m.
The Agulhas current follows the edge of the continental shelf as it flows down the east coast of South Africa, transporting about 75 million cubic meters per second. Between Port Shepstone and East London, where the shelf is narrow, the current reaches its maximum velocity of 2.5 meters per second. The Agulhas current flows between 2.2 up to 9.3 Km/H depending on the continental shelf.
Eastern boundary currents
Eastern boundary currents, including the Canary, California, Peru, West Australia and Benguela currents, are generally broad, shallow-moving flows that travel at speeds of 3-7 km per day.
The sub polar currents
The sub polar currents in the northern hemisphere – the east flowing North Pacific Current and North Atlantic Drift move water back across the starting point of the gyres. In the southern hemisphere this function is performed by the South Pacific, South Indian and South Atlantic Currents – all associated with the Antarctic Circumpolar (West Wind Drift) that flows in a continuous motion around Antarctica.
For example, in the South Atlantic Gyre the warm-water Brazil Current flows south along the east coast of South America before meeting up with the north-flowing Falklands Current, where both currents turn to the east and together become the South Atlantic Current. Near the southern tip of Africa this current splits, some of the flow going northward and the rest continues into the Indian Ocean. The northward flow forms the Benguela Current, up the west coast of South Africa, which joins the South Equatorial Current System to reconnect to the Brazil Current, thus closing the gyre.
Ocean currents-Upwelling. Upwelling as he name suggests, is the movement of water from relatively deep in the ocean into surface layers. Three processes may induce upwelling.
- Deep currents meeting an obstacle such as a mid-ocean ridge will deflect water upwards.
- In areas of divergence, for example immediately north and south of the equator where surface water is moved apart because of the Coriolis force, water upwells to fill the resulting hole.
- Most importantly, when surface water is driven away from a coastline by wind action, water upwells to replace it.
Both the north-east trade winds in the northern hemisphere and the south east trade winds in the southern hemisphere have the effect of driving water away from the western coasts of continents. Wind-induced upwelling therefore occurs along the western coasts of the USA, central South America, Australia, and north-west and South-West Africa. The west coast of South Africa experiences the strongest upwelling in the world, ten to fifteen times that found off California and Chile.
Upwelling on the west coast of Southern Africa
Along the west coast of southern Africa, conditions favourable for upwelling occur as far north as southern Angola. But the south-easter the prevailing wind from spring to autumn – is stronger and blows from a slightly different direction in some areas because of topographic variation. Maximum upwelling therefore occurs where the winds are the strongest and the continental shelf is the narrowest and deepest. These so-called “centres of upwelling” are found at Cape Frio, Luderitz, Hondeklip Bay, Cape Columbine and the western seaboard of the Cape Peninsular.
The south-easter blows parallel to the west coast, pushing the surface layer of seawater before it in a northerly direction. However, because of the Coriolis force (generated by the earth’s rotation to the east) this surface water is deflected offshore, and cold, deep water upwells to replace it (Ocean currents-Upwelling). Freshly upwelled water is clear, turquoise in colour because it contains no phytoplankton, the microscopic plant life of the sea. Phytoplankton cannot survive in the dark depths as it requires light for photosynthesis. However, the upwelled water is rich in nutrients, especially nitrogen in the form of nitrates. This is a by-product of the decomposition of organic material that has rained down on the seafloor.
Branch, G. & Branch, M 1981
Living Shores of Southern Africa
Sue Matthews September 2000