8: Ocean Circulation
- Page ID
- 45478
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CC1 Density and Layering in Fluids: Water in the oceans is arranged in layers according to the water density. Many movements of water masses in the oceans, especially the movements of deep water masses, are driven by differences in water density.
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CC3 Convection and Convection Cells: Fluids, including ocean water, that are cooled from above, sink when their density is increased. This establishes convection processes that are a primary cause of vertical movements and the mixing of ocean waters. These processes are also important in transporting and distributing heat and carbon dioxide between the atmosphere and oceans and between regions of the globe.
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CC6 Salinity, Temperature, Pressure, and Water Density: Sea water density is controlled by temperature, salinity, and, to a lesser, extent pressure. Density is higher at lower temperatures, higher salinities, and higher pressures. Movements of water below the ocean surface layer are driven primarily by density differences.
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CC7 Radioactivity and Age Dating: Some elements have naturally occurring radioactive (parent) isotopes that decay at precisely known rates to become a different (daughter) isotope. Radioactive isotopes, especially those that were released during the period of atmospheric testing of nuclear weapons, are useful as tracers that can reveal the movements of water masses in the ocean, especially the rates at which deep water masses are formed.
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CC9 The Global Greenhouse Effect: The oceans and atmosphere are both important in studies of the greenhouse effect, as heat and carbon dioxide and other greenhouse gases are exchanged between the atmosphere and oceans at the sea surface. The oceans store large amounts of heat and carbon dioxide both in solution and in carbonates.
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CC10 Modeling: The complex interactions between the oceans and atmosphere can best be studied by using mathematical models. The motions of water masses within the body of the oceans, especially motions below the surface layer, are also studied extensively using mathematical models because they are extremely difficult to observe directly.
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CC11 Chaos: The nonlinear nature of ocean-atmosphere interactions makes at least parts of this system behave in sometimes unpredictable ways. It also makes it possible for changes in ocean circulation to occur in rapid, unpredictable jumps between one set of conditions and a different set of conditions, and these changes can affect climate.
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CC12 The Coriolis Effect: Water and air masses move freely over the Earth and ocean surface while objects on the Earth’s surface, including the solid Earth itself, are constrained to move with the Earth in its rotation. This causes moving water or air masses to appear to follow curving paths across the Earth’s surface. The apparent deflection, called the Coriolis effect, is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The deflection is at a maximum at the poles, is reduced at lower latitudes, and becomes zero at the equator.
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CC13 Geostrophic Flow: Air and water masses flowing on horizontal pressure gradients are deflected by the Coriolis effect until they flow across the gradient such that the pressure gradient force and Coriolis effect are balanced, a condition called geostrophic flow. This causes ocean currents and winds to flow around high and low pressure regions (regions of elevated or depressed sea surface height) in near circular paths. The circular gyres that dominate the global circulation of ocean waters are the result of water masses flowing geostrophically.
Throughout the oceans, from surface to seafloor, water is in continuous motion, ranging in scale from movements of individual molecules to oceanwide movements of water by tidal forces and winds. Oceanographers generally group ocean-water movements into three categories: waves, tides, and ocean circulation. Waves (Chap. 9) and tides (Chap. 10) are primarily short-term (lasting from seconds to hours) oscillating motions that move water in orbital paths around a single location.
Seawater properties such as salinity, temperature, and chemical concentrations vary with location and depth. However, large volumes of water have nearly uniform properties in many areas and these are called water masses. Water masses are transported across the oceans by currents, and vertically up or down by convection (CC3) or by interaction with seafloor topography. As this circulation occurs, water masses mix with other water masses at their interface (usually a horizontal interface between layers; CC1). Mixing is the exchange of water between adjacent water masses, which results in a new water mass with a proportional combination of the properties of the individual water masses before mixing. Currents and mixing are the major processes that transport and distribute heat, dissolved chemicals, suspended sediments, and planktonic bacteria, algae, and animals in the oceans.
Chapter 7 describes how ocean surface currents can affect local climates. This chapter examines the physical processes that control present-day ocean circulation, reviews the major features of circulation, and describes some aspects of the relationship between ocean circulation and global climate, both past and present. Subsequent chapters describe how ocean circulation controls and/or affects the distribution and abundance of life in the oceans. Although waves and tides have relatively little influence on large-scale ocean circulation, they contribute substantially to local currents, particularly in coastal waters and estuaries, and to vertical mixing, particularly in the upper several hundred meters of the water column (Chaps. 9, 10).