Ocean Currents and salinity

 The sea is salty. If all the oceans evaporated and the salt spread out evenly, it would form a solid layer of75 meter thick. It is likely that that the first oceans were of almost fresh water. Four billion years of rain, rivers and erosion have progressively washed more and more soluble material out of continental rocks and down to the sea.

The salinity varies widely from place to place. In the Baltic there is low evaporation and a regular input of fresh water from melting snow and its salinity is about 5000 parts per million. In the Red Sea and Persian Gulf, however, it can exceed 40,000 ppm. Salt plays an important role in the global transport of ocean currents.As water evaporates, the sea becomes saltier and as a result denser. Eventually, particularly if it cools, the salt water sinks downwards. In this way, salt and sunshine drive great conveyor belts that carry heat from equatorial waters in surface currents and return salt in deep currents.

The best example of this is in the North Atlantic, where the Gulf Stream brings rain and warm weather to western Europe and the salt returns south at depth. If this conveyor belt is disrupted, it can trigger an ice age.Ocean currents depend on the positions of the continents, and continental drift has caused major climatic change in the past.

About 30 million years ago an eastward circumpolar current established itself around Antarctica, isolating the continent from other weather systems and leading to the development of the ice cap.  A modern example of the effects of changing currents is El Nino (means The Child, so called because it occurs at Christmas) a warm current that can develop in the pacific and move towards the coast of Peru. It causes disruption to fisheries and triggers equatorial drought and tropical storms.