ABSTRACT
Ocean water, like air in the atmosphere, circulates. The dominant paths of flow are ocean currents. Prevailing surface winds drive nearly all large currents at the top of the ocean. The main feature of the surface-ocean circulation is a set of gyres (Figure 17a). These are slowmoving vortices that occur in the oceans of the Northern and Southern Hemispheres. They rotate clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere, tracking the passage of air around the subtropical high-pressure cells. A large gyre, or circumpolar current, also moves around Antarctica. Near the equator, ocean currents travel westwards. On approaching land, they are diverted to the north and south to form warm currents that run parallel to the coastline. In the Pacific Ocean, some of the water returns eastwards, approximately along the equator, as an equatorial counter-current. Once in middle latitudes, the warm currents are forced eastwards by the westerly winds. They travel across the oceans until they reach the western edges of continents, which they keep relatively warm in winter. They then join either the Arctic or the Antarctic circulation, or else they return towards the equator as cool currents. They are often associated with cold-water upwellings along continental margins, as with the Humboldt Current that runs down the coastline of Peru and Chile. The presence of cold currents in tropical waters and warm currents in temperate and polar waters can affect regional and even global climates. The warm North Atlantic Current (North Atlantic Drift) moderates winter temperatures in the British Isles and northwest Europe. The cold Humboldt Current off western Ecuador, Peru, and Chile reduces temperatures and increases aridity in those countries, and especially in Chile. Periodically, the water in the current becomes warmer as part of the El Niño-Southern Oscillation, which can affect climates around the world (see teleconnections).
