The subject of Ocean Currents is a large and complex one. The study of the many forces that work on our planet's seas causing the local and the global current systems makes up the larger part of the science of oceanography. In simple terms, there are two types of marine currents each caused by a range of distinct drivers.
Non tidal ocean currents, such as the Gulf Stream differ from tidal streams in many ways. Firstly, they are huge in terms of water flow and truly global in terms of their dynamic impact. The complex system of currents is powered by many forces, the crux though is the playoff between the joint forces of solar heating of tropical surface waters and the polar contributions of cold fresh water ice-melt flooding into the sea and the general cooling of the salty sea water.
Map of the main Ocean Currents
These are currents that operate at an ocean scale, compared to the localised nature of tidally driven currents. These are driven by salinity and temperature gradients and the ocean rotation impact of the earth's spinning motion known as the "Coriolis Force".
Warm surface waters are driven from the tropics towards their respective pole. These currents are matched by the cold deep polar waters moving along the ocean floor towards the equator. Prevailing on shore or offshore wind can act to either amplify or dull the effects of these currents.
This complex interaction of water movements is one of the most powerful drivers of the world's climate. What has become known as the "El Nino" effect, is linked to perturbations of the Pacific Ocean currents, failing to deliver the rich cool waters which form the basic prerequisite for the entire marine food chain.
These deep waters are the stores of the oceans' nutrients and minerals and where they are caused to upwell and rise to surface, they cause the largest explosions of primary plant growth anywhere. The cool nutrient rich water forced by a combination of ocean currents and offshore winds to upwell along the eastern pacific shore, in particular the hot desert coast of Peru, create a rich planktonic soup which, until over fished supported the largest fishery in all the world’s oceans.
These currents have evolved along with the geological development of the oceans themselves, the continental marching resulting from the growth and movement of tectonic plates over the billions of years since the first oceans condensed and started to swill around the planet's first seas. These currents are now embedded in the genetic code of most marine organisms, from turtles to eels or herring. Ancient reptiles swim back to remote islands, which have proven successful launching grounds for their offspring. The European Eel migrates back to the Sargasso Sea to spawn, endorsing their race’s faith in the ocean currents eastward motion which conveyed them and hopefully their future generations, to the rich adult feeding ground of the river systems of Europe.