Geography Reference
In-Depth Information
most dominant surface features of the world's oceans with their
centres located at 30° north or south. The centres of the gyres were
often avoided by sailors of the past due to their calm wind and
calm ocean current conditions. Interestingly, the centres of these
gyres have become collecting areas for the floating rubbish we have
discarded into the world's oceans (Box 1.1). In each ocean there is
a westerly current in the temperate mid-latitudes with a cold
current flowing back towards the equator on the western edge of
the continents. There is also a circumpolar current around Antarc-
tica around 60° south. This is not mirrored around the Arctic as
there are too many land masses in the region acting as a barrier to
water flow.
Box 1.1 The oceanic floating plastic rubbish dump
The United Nations estimates that 10 per cent of the plastic we produce
ends up being dumped in the oceans with around 18,000 pieces per
square kilometre. The ocean currents move the floating debris of plastic
bags, bottles, cartons, caps and so on which can later collect in the calm
centres of the gyres. It is thought that in the centre of the North Pacific
gyre the plastic is concentrated to around 334,000 pieces per square
kilometre which may be equivalent to around 100,000 tonnes of plastic
waste. Sea creatures can become entangled in the plastic. Plastic slowly
breaks down into smaller pieces under sunlight, wave action and
rubbing but does not fully degrade. This smaller plastic gets eaten by
sea creatures and small fish may even mistake the particles for the tiny
plants known as plankton that normally forms their diet. In some parts
of the centre of the gyre there may be six times as many tonnes of
plastic than plankton. The plastic accumulates harmful toxic chemicals
which poisons sea creatures.
While surface currents are largely driven by wind, deep ocean cur-
rents are driven by differences in water density. This deep ocean
circulation system is called the
thermohaline circulation
system
(Figure 1.4). There are two important areas where deep water cur-
rents form. The first is in the North Atlantic/Arctic ocean and the
second is in the Antarctic ocean. In the far North Atlantic, salty
water from the Gulf Stream moves north into the Arctic and cools.
Being saline and cool it is denser than surrounding waters and so it
