Ocean currents and their causes

Ocean currents are like rivers in the ocean: a regular volume of water moving along a definite path and direction.

Forces: primary forces start the movement; secondary forces influence how currents flow. The primary forces are:

• Solar heating: water expands when heated, so near the equator the sea is about 8 cm higher than in middle latitudes, giving a very slight slope down which water tends to flow.
• Wind: pushes surface water along by friction.
• Gravity: pulls water down the 'pile' and creates gradients.
• Coriolis force: turns moving water to the right in the northern hemisphere and to the left in the southern. The large accumulations of water and the flow around them are gyres: large circular currents in every ocean basin.

Density drives vertical movement: saltier water and colder water are denser and sink; lighter water rises. Cold polar water sinks and slowly moves towards the equator; warm surface water flows poleward to replace it.

Types by depth:

• Surface currents: the upper 400 m, about 10 per cent of ocean water.
• Deep water currents: the other 90 per cent, moved by density and gravity; deep water sinks in high latitudes, where it is cold enough to become dense.

Types by temperature:

• Cold currents bring cold water into warm areas. They are usually on west coasts in low and middle latitudes (both hemispheres), and on east coasts in higher northern latitudes.
• Warm currents bring warm water into cold areas. They are usually on east coasts in low and middle latitudes (both hemispheres), and on west coasts in high northern latitudes.

Measuring currents: a current's speed is its drift, in knots. Currents are usually strongest at the surface (sometimes over five knots) and slow with depth (often under 0.5 knots). Most currents run at 5 knots or less.

Example (new): a sealed bottle dropped into the sea off southern Japan may be carried north-east by the warm Kuroshio current and then across the Pacific, travelling thousands of kilometres around the North Pacific gyre.