The pattern of air movement over the oceans results from solar heating of the atmosphere and Earth’s rotation. This pattern of winds is modified by linked areas of low and high pressure (cyclones and anticyclones), which continually move over the oceans’ surface. Near coasts, additional onshore and offshore breezes are common. These are caused by differences in the capacity of sea and land to absorb heat.
Solar heating causes the air in Earth’s atmosphere to cycle around the globe in three sets of giant loops, called atmospheric cells. Hadley cells are produced by warm air rising near the equator, cooling in the upper atmosphere, and descending to the surface around subtropical latitudes (30°N and S). Then the air moves back toward the equator. Ferrel cells are produced by air rising around subpolar latitudes (60°N and S), cooling and falling in the subtropics, and then moving toward the poles. Polar cells are caused by air descending at the poles and moving toward the equator.
The coriolis effect
The atmospheric cells cause air to move in a north–south direction. This is altered by the coriolis effect, which is a consequence of Earth’s spin. Because Earth turns continuously underneath the airflow as it travels, the air appears to be deflected from its straight north–south course. It veers to the east when moving away from the equator, and to the west when moving toward it. The coriolis effect is an apparent, not a true, force. No actual force is exerted on the wind.
The winds produced by pressure differences and modified by the coriolis effect are called the prevailing winds. In the tropics and subtropics, the air movements toward the equator in Hadley cells are deflected to the west. These are known as trade winds. They comprise the northeasterly trades in the Northern Hemisphere, and southeasterly trades in the south. At higher latitudes, the surface winds in Ferrel cells deflect to the east, producing the westerlies. In the Southern Hemisphere, these winds blow from west to east without meeting land. Those around latitudes of 40°S are known as the Roaring Forties. In polar regions, winds deflect to the west as they move away from the poles. These are known as polar northeasterlies and southeasterlies.
In any area of ocean where air sinks—often at subtropical latitudes—a zone of high atmospheric pressure, or anticyclone, develops. Where warm air rises, areas of low pressure, called cyclones or depressions, occur. These often develop near the equator and subpolar latitudes. Cyclones and anticyclones create linked, circulating wind patterns, which continually move and change. In the Northern Hemisphere, there is a clockwise movement of air around an anticyclone, and a counterclockwise motion around a cyclone. This pattern is reversed in the Southern Hemisphere. Local pressure systems can affect the general pattern of prevailing winds. In particular, cyclones move swiftly over the ocean and can produce rapid changes in wind strength and direction.
Local winds, called onshore and offshore breezes, are generated near coasts, especially in sunny climes. Onshore breezes—sometimes called sea breezes—develop during the day. These are caused by the land heating up more quickly than the sea, as both absorb solar radiation. This occurs because the sea absorbs large quantities of heat energy with only a small rise in temperature, whereas the same amount of heat energy is likely to cause the land temperature to rise sharply.