The clockwise-rotating winds around the atmospheric “rotors” that are formed by the gigantic subtropical high pressures (see Circulation in the Tropics) drive similar clockwise rotors in the large oceans (anti-clockwise in the southern hemisphere).
What seems strange is that the atmospheric and oceanic rotors do not coincide. This can be seen in the Atlantic, where the atmospheric rotor created by the Azoric high pressure between the 20º and 40º latitudes is found between the Caribbean islands and North Africa.
The oceanic rotor is however found significantly to the west, with weaker currents on the eastern side and stronger on the western side. To understand this we have to look at the original description of the effects of the earth’s rotation, the Coriolis effect.
Variation of the Coriolis force with latitude
The Coriolis force deflects all movement into so called “inertial circles” with a radius proportional to the speed and inversely proportional to the Coriolis force. For 1 m/s the radius has a magnitude of 10 km.
If the Coriolis force did not vary with latitude then all motion would take the form of perfect circles. However the Coriolis force does vary with latitude, and is weaker in the south and more powerful in the north (in the Northern hemisphere).
This means that the inertial movement doesn’t follow a perfect circle but with a curve that opens up westwards. Persistent winds create currents in a spiral pattern, which slowly drift westwards.
There is a net drift of water from the oceans’ eastern parts towards the west. This redistribution of the water mass creates a water hill or “water high” centred over the western parts of the oceans.
This leads in turn to an increase in the speed of the water, giving a powerful flow towards the north east off the American East coast.
Seaweed and rubbish collect in water gyres
The Coriolis effect has further consequences. The winds over the seas set the sea surface in motion in the direction of the wind, and the water’s inner friction (viscosity) pulls the water from the layer below.
Because of the Coriolis effect this movement deviates to the right. The lower water layer in turn also deviates to the right and the movement continues downwards in a type of spiral until eventually the water moves in the opposite direction, although more slowly than at the surface.
The Coriolis effect moves the surface water inwards towards the centre of the rotor, so that floating seaweed and rubbish collects inside the “wheel”. This is what happens in the notorious Sargasso Sea south of Florida, where shipwrecked sailors have drowned, tangled in seaweed. One imaginative hypothesis states that ships disappear into the depths, where methane gas bubbles up from rotting silt on the sea floor.