Major currents and their effects

Major currents follow the stresses of the prevailing winds and the Coriolis force, so ocean circulation roughly matches atmospheric circulation:

• In middle latitudes, air circulation over the oceans is mainly anticyclonic (more so in the southern hemisphere), and currents follow the same pattern.
• In higher latitudes, where winds are mostly cyclonic, currents follow that.
• Where the monsoon is strong (as in the north Indian Ocean), monsoon winds steer the currents, even reversing them with the seasons.
• Warm currents from low latitudes turn right in the northern hemisphere and left in the southern.
• Like the atmosphere, ocean circulation carries heat between latitudes: cold Arctic and Antarctic water moves towards the tropics, and warm low-latitude water moves poleward.

Effects of ocean currents:

• West coasts in tropical and subtropical latitudes (except near the equator) are bordered by cool water: temperatures are fairly low, with narrow daily and yearly ranges; fog is common, but the land is generally arid.
• West coasts in middle and high latitudes are bordered by warm water, giving a marine climate: cool summers, mild winters, small annual range (as in north-west Europe).
• Warm currents along east coasts in tropical and subtropical latitudes give warm, rainy climates; these areas lie on the western edges of the subtropical anticyclones.
• Where warm and cold currents mix, oxygen is replenished and plankton (the primary food of fish) thrives. The world's best fishing grounds are mainly in these mixing zones.

Example (new): Norway's west coast ports, though near the Arctic Circle, stay largely free of ice in winter because warm Atlantic water flows along that coast, while places at the same latitude in north-east Canada freeze over.