Differential Heating

Ever wonder why it is warmer at the equator and colder at high latitudes? 

It is related to how the sun's rays strike the surface of the Earth.

At the equator, the rays strike the Earth almost perpendicular to its surface, warming up a small area. At high latitudes, the curve of the Earth causes the rays to strike a larger surface area, so the same amount of heat is distributed over a larger area. That means it does not warm up as much as at the equator.

This difference in temperature between the equator and the poles causes our atmosphere and ocean to circulate.

But in the ocean there is another effect. The sun at the equator also warms the surface ocean and causes water to evaporate. This makes the ocean more salty. In other areas, where there is a lot of rain, the ocean is less salty.

It is the combination of differences in temperature and salinity that causes the deep ocean beneath the reach of the wind to circulate.