The atmosphere and ocean are intimately intertwined. They freely exchange water, gases, heat and momentum. While the transfer of momentum from the atmosphere to the ocean by wind stress is the most important forcing of the ocean circulation, the heat and water exchanges affect the horizontal and vertical temperature gradients of the lower atmosphere and the upper ocean, which, in turn, modify wind and ocean currents and maintain equilibrium of the climate system. The sea surface exchanges are the fundamental processes of the coupled atmosphere–ocean system. An accurate knowledge of the flux variability is critical to our understanding and prediction of the changes of global weather and climate.
Maintenance of the earth’s climate depends on a balance between the absorption of heat from the sun and the loss of heat through radiative cooling. 40 % of the solar energy entering the atmosphere is absorbed by the ocean, compared to only 20 % that is absorbed by the atmosphere. Much of this oceanic heat is transferred back to the atmosphere by the ocean-to-air heat flux. The geographical variation of this atmospheric heating drives the winds and associated weather systems. The wind, in turn transfers its momentum to the sea causing waves and the wind-driven currents. Major ocean currents transport heat polewards and at higher latitudes the sea-to-air heat flux significantly ameliorates the climate. Thus the heat and momentum fluxes through the ocean surface form a crucial component of the earth’s climate system. Some of the highlights of the ocean-atmosphere coupling are:
1. Global winds in each hemisphere are arranged into large circulation cells that are caused by wind responding to pressure gradients and Coriolis deflection. These cells result in zonal wind flow, including the westerlies of the midlatitudes and the trade winds of the subtropics.
2. Ocean circulation can be divided into wind induced surface currents, which influence about 10 percent of the total ocean’s volume of water, and density-driven subsurface flow, which affects the remaining 90 percent.
3. Under the influence of a steadily blowing wind, an Ekman spiral can develop, whereby the speed of the current decreases with depth and the direction spirals around to the right in the Northern Hemisphere and to the left in the Southern Hemisphere because of Coriolis deflection. The overall result of the Ekman spiral is net transport at 90 degrees to the right of the wind in the Northern Hemisphere and to the left of the wind in the Southern Hemisphere.
4. The large wind-powered circulation gyres in each ocean consist of a system of geostrophic currents that rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. These currents, rather than flowing downslope, actually flow around a central mound of water parallel to the water slope. Geostrophic currents represent a dynamic balance between Coriolis deflection and pressure gradient.
5. In both hemispheres, the pattern of current flow around the gyres is asymmetrical, with narrow, deep, swift transport to the west (western-boundary intensification) and broad, shallow, slow drift to the east. Because Coriolis deflection increases with latitude, more water is driven into westward equatorial flow, which piles up along the western edge of ocean basins. Therefore, geostrophic flow accelerates along the western side of all circulation gyres.
6. The flow axis of western-boundary currents, such as the Gulf Stream, typically meanders like a river. At times, closed meander loops become separated from the main current and create rings that have either a warm-water core or a cold-water core. These rings persist for several months to several years before they are absorbed back into the main current flow.
7. Subsurface flow, known as thermohaline circulation, results from density variations produced by a difference in the temperature and salinity of water masses. When water masses converge, the more dense water sinks below and buoys up the less dense water. Most water that fills the depths of the oceans is near freezing and originated near the surface of the polar seas. Residence time for water in the deep sea is about 500 to 1,000 years.
8. Ocean basins exchange water on a regular basis. A preliminary “conveyer belt” model of this exchange suggests that warm water from the Pacific Ocean and Indian Ocean is exported to the Atlantic Ocean along the surface, where it cools and sinks, returning at depth back to the Pacific and Indian Oceans.
9. Climate exerts strong control on the water circulation in semi-enclosed seas. As a consequence of intense evaporation, which creates dense, salty water, the Mediterranean Sea has surface inflow and subsurface outflow over its still surface. By contrast, the Black Sea, with its wet climate, has surface outflow and subsurface inflow over its sill. The water chemistry and the marine biology in each of these basins is influenced greatly by their respective circulation patterns.
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Department of Geology
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