Introduction to Ocean Sciences

Chapter 10: Ocean Circulation

Guide to Reading and Learning

The water in the oceans is in constant motion. These motions can be separated into three categories: currents, waves, and tides. Chapter 10 reviews the different types of water movement that occur as ocean currents and the fundamental processes that control these motions. Waves and tides are described separately in their own chapters.

Before you begin the material in this chapter, you must understand, or at least know, the essential characteristics of the Coriolis effect and geostrophic flow. If you have just completed Chapter 9, no doubt you will already have read about these two critical concepts. However, even if you have done so, the beginning of this chapter is a good place to review these concepts.

Ocean currents can be separated into two types: those that are created by wind energy and those that are created by density differences. Chapter 10 starts by looking at how wind-driven currents are generated. This examination leads us to two rather strange facts that are all-important in understanding how the major ocean currents are created by the major climatic wind bands. The first concerns the concept of Ekman motion, which requires some understanding of the Coriolis effect but leads us to the most important piece of information in the chapter: when wind blows over water, the average movement of water in the upper water layers flows 90o to the right of the wind direction in the Northern Hemisphere and 90o to the left in the Southern Hemisphere. Be sure to understand and learn this concept. The second rather strange fact is that Ekman transport can move surface water across the ocean surface to create a sloping sea surface, but this causes the water column below the sea surface to be started in motion in a direction directly opposite to the direction of the Ekman transport in the surface layer. This is the basis for geostrophic currents.

Chapter 10 takes this information and describes the persistent geostrophic gyres that flow in all oceans except around Antarctica. These are the major currents that transport heat in the oceans and they dramatically affect climate. For example, did you realize that if the Gulf Stream (a geostrophic current) stopped flowing, the climate of most of northern Europe would be dramatically colder, becoming much more like Alaska? Did you also know that Alaska’s climate would be much warmer and more like Europe’s if the Aleutian Island chain and the submerged ridge that separates the North Pacific Ocean from the Bering Sea did not block the Kuroshio current, the Pacific equivalent of the Gulf Stream?

Although the subtropical geostrophic gyres and their currents are globally the most important wind-driven ocean currents, the chapter also reviews equatorial currents, high-latitude gyres, coastal currents, and other types of wind-driven current motion that are important in their own neighborhoods. Also, the chapter has a very important section on the creation of upwelling and downwelling by wind-driven water motions. Upwelling and downwelling are important in determining where and when abundant photosynthetic plankton populations are able to develop together with the fishes and other consumers that they support. Equatorial and coastal upwelling processes will become fundamentally important when we look at life in the oceans in many of the later chapters of this text.

Thermohaline circulation—currents created by high-density water sinking from the ocean surface into the depths, where it spreads out at its level of density equilibrium—is much simpler to understand than geostrophic currents, but these currents are not as well studied as surface currents although no less important. Did you know that cold water sinking at just a few locations near the poles flows through all the ocean basins and eventually returns in a conveyor belt–like circulation that includes the Gulf Stream? Did you know that this conveyor circulation has been shut down in the past with devastating and very rapid effects on climate, especially in Europe? Did you know that the rate of water sinking to drive this conveyor has slowed considerably in the past two decades and that ocean scientists believe it may slow even further as a result of global climate change?

Chapter 10 Essential to Know 

Critical Concepts used in this chapter

CC.1, CC.3, CC.6, CC.7, CC.9, CC.10, CC.11, CC.12, CC.13

10.1 Energy Sources

  • Water movements are caused by winds or by changes in salinity or water temperature that alter water density, but the primary energy source that drives the winds and causes temperature and salinity changes that alter density is the sun.
  • Winds are the primary source of energy for currents that flow horizontally in the ocean surface layers (less than 100–200 m deep).  These surface leyr currents are often called wind-driven currents.
  • Vertical movement of water and the currents that flow in the deep oceans below the surface layer are primarily driven by sinking of cold surface waters at some locations. The cold surface waters displace (push aside) existing deep waters.  Curents created by this mechanism are called thermohaline circulation.
  • Currents move large amounts of heat energy and dissolved substances within the oceans and are important influences on climate and ocean productivity.

10.2 Wind-Driven Currents

  • Winds create currents that can transport large volumes of water across the oceans.

Generation of Currents

  • Winds blowing over the ocean surface transfer kinetic energy to the surface layer of the water column, and this energy is transferred downward through the water column by internal friction.
  • Winds that blow steadily for a period of time can generate surface currents of about 1–3% of the wind speed.
  • As the wind blows across the surface, the surface layer is started in motion and  kinetic energy associated with this motion is transferred downward so that water below the surface is also started in motion.. However, a small amount of frictional energy is lost with increasing depth, causing the current speed to become slower with depth below the surface.  As a result, wind-driven currents are restricted to no more than the upper 100–200 m of the oceans.

Restoring Forces and Steering Forces

  • Once current motion is started it will continue for some time because the water has momentum—just as a bicycle does not stop immediately when the rider stops pedaling.
  • Currents also continue to flow after winds abate because winds move the surface layers of water horizontally to create sloping sea surfaces although the slopes are very small. This causes a horizontal pressure gradient to form at all depths under the sloped surface, with higher pressure under the high point of the sloping sea surface. Even after the winds stop, water flows horizontally from the high-pressure region toward the low-pressure region until the sea surface is eventually restored to a level configuration.
  • Currents are subject to the Coriolis deflection and can be blocked or deflected by coasts.

10.3 Ekman Motion

  • Once set in motion, the speed and direction of flowing water is affected by internal friction, the Coriolis effect, the presence of land masses, and horizontal pressure gradients.
  • However, the direction of motion of the surface water is not the same as the wind direction because the current is deflected by the Coriolis effect.

The Ekman Spiral

  • When winds blow over the ocean surface, each progressively deeper layer of water that is set in motion is deflected by the Coriolis effect and does not flow in the direction of the layer above.
  • The current flows in directions that progressively deflected cum sole (to the right in the Northern Hemisphere and to the left in the Southern Hemisphere) and become progressively slower to form a spiral that extends down into the water column. This is called the Ekman spiral.
  • Winds create Ekman spiral motion if they blow for long enough and if there is no shallow sea floor or shallow pycnocline.
  • Ideally the surface current flows at 45o cum sole to the wind and the average transport direction within the Ekman spiral depth (called Ekman transport) is at 90o cum sole to the wind, but in most instances the deflections are somewhat smaller.

Pycnocline and Seafloor Interruption of the Ekman Spiral

  • Pycnoclines are locations of strong vertical density gradient and they can be either thermoclines or haloclines.
  • In most parts of the open oceans there is a permanent pycnocline with an upper boundary at a depth of about 10–500m.
  • In areas where a pycnocline or the seafloor is shallower than the depth of the full Ekman spiral motion, the spiral is modified and both the surface current and average transport take place at angles shallower than 45 o and 90o cum sole to the wind respectively.
  • In shallow water less than a few meters deep, the Ekman spiral does not develop and wind-driven currents generally flow along the contours of the bottom topography.
  • Ekman transport can either elevate or lower the sea surface on a coastline.
  • When Ekman transport from separate wind bands brings surface water together from two directions this creates a convergence, whereas if the transport moves water away in different directions a divergence is formed.

10.4 Geostrophic Currents

  • When Ekman transport creates sloping sea surfaces, geostrophic currents develop that flow on the horizontal pressure gradient beneath the sloping surface.

Ekman Transport, Sea Surface Slope, and Pressure Gradient

  • Below a sloping sea surface there is a horizontal pressure gradient with the area of higher pressure being located under the elevated area of sea surface.
  • Water is set in motion and accelerated on this gradient.
  • The horizontal pressure gradients that are established below sloping sea surface created by Ekman transport generally set water in motion in the opposite direction from the Ekman transport.
  • Ekman transport occurs only in the wind-driven upper layer (a few tens of meters deep), but the horizontal pressure gradient beneath the sloping sea surfaces and the currents that flow on these gradients extend to depths beyond the wind-driven layer and are not restricted by shallow pycnoclines.

Balance between Pressure Gradient and Coriolis Effect

  • Currents flow on a horizontal pressure gradient, initially from the high-pressure zone toward the low-pressure zone, but they are deflected by the Coriolis effect.
  • The currents accelerate and are deflected until they flow along the contours of equal pressure across the gradient, with the pressure gradient force approximately balanced by the Coriolis deflection.
  • Geostrophic currents continue to flow for as long as the horizontal pressure gradient persists even if there is no wind.

Dynamic Topography

  • Geostrophic current speeds and directions can be estimated by dynamic height calculations using measured salinity-temperature-depth distributions, and details of current distribution can be estimated from sea surface height variations measured by satellites.

Energy Storage

  • Geostrophic currents are generated initially by winds that create a sloping sea surface. However, they continue to flow after the winds abate until the sea surface slope is gone. Thus, energy is stored when winds blow and Ekman transport occurs and then released over time as the geostrophic currents flow.
  • The length of time during which geostrophic currents flow after the winds stop varies, but it is longer for higher-surface slopes or for slopes that extend over greater distances. Geostrophic currents continue to flow for only a few hours if they are created by individual storms, but where they are created by climatic winds that are reasonably consistent and that blow over large areas, they can flow continuously provided that any gaps during which there is no wind or much reduced wind strength are not prolonged.

10.5 Open-Ocean Surface Currents

  • Open-ocean surface currents are wind-driven, initiated by Ekman transport, and maintained as geostrophic currents.
  • Surface currents generally extend to depths of several hundred meters and can extend to depths of as much as 2000 m.
  • The interaction of climatic winds, Ekman transport, and the blocking effect of continents create subtropical geostrophic gyres in each hemisphere of each ocean, less well-defined gyres at higher latitudes in the Northern Hemisphere, and circumpolar currents around Antarctica.

Geostrophic Currents on a Water-Covered Earth

  • On a completely water-covered Earth, there would be an ocean surface water divergence at the equator created by the Ekman transport due to the trade winds in each hemisphere.  Ekman transport would also create convergences between the trade wind and westerly wind bands.
  • Geostrophic currents would flow east to west on both sides of the equator in the trade wind band.
  • Higher-latitude geostrophic currents would flow west to east in the westerly wind band.
  • These currents exist on the real Earth but, except for the high-latitude current in the Southern Hemisphere, they are blocked by the continents and the currents are deflected north or south along the coastlines to form the closed current loops called geostrophic gyres.

Geostrophic Gyres

  • The continents provide a western boundary that deflects currents that flow east to west and an eastern boundary that deflect currents that flow west to east.
  • In subtropical regions, the westward-flowing geostrophic currents north and south of the equator are deflected away from the equator when they meet the western boundary of the continents. They then flow toward the pole until they join the eastward-flowing geostrophic currents in the westerly wind zone. The current then flows eastward until it meets the eastern boundary of the continent, where it is deflected back toward the equator to complete the loop. These are the subtropical gyres
  • Similar but less-well-defined gyres are located at higher latitudes in the Northern Hemisphere but not in the Southern Hemisphere, where geostrophic currents are free to flow around Antarctica unimpeded by continents.

Westward Intensification of Boundary Currents

  • Within the subtropical gyres western boundary currents are narrower, faster, and deeper than eastern boundary currents, so coastal upwelling is more likely in eastern boundary current areas.
  • The reasons for the westward intensification are somewhat complex but the major factor involved is the variation of planetary vorticity with latitude.

10.6 Equatorial Surface Currents<

  • The Northern and Southern Hemisphere trade wind zones are separated by the Doldrums, where winds are very light and variable.
  • Equatorial currents are complex because the intertropical convergence zone does not lie exactly on the equator.

Ekman Transport in the Equatorial Region

  • On either side of the equator the main geostrophic currents of the subtropical gyres, the North and South Equatorial Currents, flow east to west, driven by the trade winds
  •  However, the Southern Hemisphere trade wind band extends across the equator into the Northern Hemisphere. This causes a complex sea surface slope. South of the equator the surface slopes down toward the equator but north of the equator the slope first slopes upward to a maximum at about 3o north of the equator, then downward until about 7o north of the equator before again sloping upward toward the pole.

Geostrophic Currents in the Equatorial Region      

  • This sea surface height configuration results in west to east Ekman transport in a narrow band between the equator and about 3–5o north. This is opposite in direction to the North Equatorial Current of the Northern Hemisphere subtropical gyres.  The sea surface height configuration also casues a west to east equatorial countercurrent to flow in the region just north of the equator.
  • Another west to east geostrophic current, the Equatorial Undercurrent, also flows on the pressure gradient created by Ekman transport of surface water to the west by the trade winds.
  • Equatorial currents can flow directly across the entire ocean without deflection by the Coriolis effect.
  • Equatorial currents are complex and variable and are important in the El Niño Southern Oscillation (ENSO).

10.7 High-Latitude Surface Currents

  • In the North Atlantic and North Pacific Oceans there are secondary gyres at high latitudes above the subtropical gyres.
  • The high-latitude gyres rotate in the opposite direction to the subtropical gyres and are more variable and less well-defined, primarily due the complex configurations of the coasts.
  • There is no high-latitude gyre in the Indian Ocean because the ocean does not extend far enough north.
  • There are no secondary gyres in the Southern Hemisphere because no continents block the geostrophic currents and they are free to flow around Antarctica.

10.8 Upwelling and Downwelling

  • Winds can create divergences at which upwelling of cold, nutrient-rich water from below the pycnocline may occur, and convergences at which the surface layer is thickened and downwelling may occur.
  • Upwelling is important because it brings the cold deep water that is rich in nutrients such as nitrogen and phosphorus into the surface layer of the oceans, where it is used by photosynthetic organisms to support their growth. Upwelling area are areas of high primary productivity and, therefore, abundant fisheries.
  • In contrast, downwelling areas are depleted in nutrients and are areas of very low productivity.

Locations of Upwelling and Downwelling

  • Upwelling occurs at divergences. Downwelling at convergences.
  • The principlal open-ocean convergences are located along the equator and in a band around Antarctica.
  • Convergences include the centers of the subtropical gyres.
  • Divergences can be created when Ekman transport by winds moves surface water away from a coastline. This creates coastal upwelling where productivity is high and fisheries are abundant.
  • Convergences can be created when Ekman transport moves surface water toward the coast. This creates downwelling where productivity is low and fisheries are poor.

Boundary Currents and Upwelling or Downwelling

  • Western boundary currents are fast, narrow, and deep and flow along the edge of the continental shelf. As a result, if Ekman transport causes offshore transport on the western boundary (east coasts) of the continents, the upwelled water comes from nutrient-poor shallow continental shelf water or warm water from the western boundary current that also has low nutrient concentrations.
  • Eastern boundary currents are broad and shallow and extend onto the continental shelf. In these regions, offshore Ekman transport causes upwelling of cold nutrient-rich subpycnocline water.
  • Consequently, coastal upwelling of cold nutrient-rich water is more frequent, widespread, and persistent on the west coasts of the continents.

10.9 Coastal Currents

  • Coastal currents generally flow parallel to the coast in a direction determined by coastal winds, which are usually variable in both speed and direction, and Ekman transport.
  • Coastal currents are temporally and spatially highly variable, may flow in the opposite direction to the boundary currents, and may reverse seasonally.

10.10 Eddies

  • In the same way that the atmosphere has climatic winds that are relatively invariable and the much-more-variable small variations (in time and space) called weather, the oceans too have a long-term mean circulation and much more variability on smaller time and space scales.

Satellite Observations of Eddies

  • Little was known about short-term and limited-spatial-scale variations in ocean currents until satellite observations began.
  • Satellites can provide much more detailed views of ocean surface properties than can be gathered from surface vessels, and can repeat those observations often enough to reveal short-term temporal changes.
  • Satellites are limited in the parameters they measure in the oceans and the depth to which they can “see.” However, they are able to observe the color of the ocean surface by measuring backscattered light and to measure ocean surface temperatures by tracking infrared (heat) radiation.
  • Variations in these parameters across the oceans are very small but their differences are magnified by computers in color-enhanced satellite images.

Gulf Stream Rings

  • Complex eddies form in western boundary currents including the Gulf Stream.
  • These eddies begin as meanders that flow with the current and are then pinched off and develop into cold-core and warm-core rings.
  • Cold-core rings are counterclockwise-spinning rings of cold water from the continental shelf that are spun off into the warm surface waters of the subtropical gyre.
  • Warm-core rings are clockwise-spinning rings of warm water from the Gulf Stream (or other boundary current) that are spun off into the cold surface waters of the continental shelf. They move progressively southward in the coastal currents and are mixed with shelf water, but some reattach to the Gulf Stream before they are destroyed by mixing.
  • The rings are 100–300 km across and are encircled by swift-flowing currents.
  • The position of the rings is important to fishers, as fishes tend to be concentrated in the ring—warmer-water species in the warm-core rings and colder-water species in the cold-core rings.

Mesoscale Eddies

  • There are numerous eddies called mesoscale eddies throughout the oceans. They are weaker than the western boundary current eddies but little else is known about them.
  • Ocean circulation resembles atmospheric weather patterns, but ocean eddy currents flow more slowly than winds, eddies in ocean circulation are smaller and more numerous than those in atmospheric circulation, making observation much more difficult.
  • Mesocale eddies range in diameter from about 25–200 km and drift a few kilometers a day compared to atmospheric eddies, which are about 1000 km across and drift about 1000 km per day. Wind speeds in atmospheric eddies are also about 20 times the current speeds in ocean eddies.

10.11 Inertial Currents

  • Once set in motion, water masses may flow in circular patterns called inertial currents even if the wind that created them no longer blows.

10.12 Langmuir Circulation

  • Strong winds can create Langmuir circulation, which is a series of laterally extended side-by-side cells within which water moves in a corkscrew-like motion.
  • Langmuir circulation can be observed visually because foam and floating materials are concentrated in the long linear downwelling zones between adjacent cells.
  • Langmuir circulation is important in wind mixing of the upper few meters of the water column.

10.13 Thermohaline Circulation

  • Wind-driven currents dominate water motions in the upper layer above the pycnocline. Below the pycnocline currents are predominantly driven by differences in density. This is called thermohaline circulation because the principal determinants of density are temperature and salinity.
  • Thermohaline circulation begins when a surface water mass becomes dense enough to sink. It sinks until it finds its equilibrium density level and then spreads out horizontally. These horizontal motions are deep ocean currents.

Depth Distribution of Temperature and Salinity

  • Except at high latitudes, the oceans have three primary layers: a mixed layer in which density is almost uniform, a pycnocline layer in which density increases rapidly with depth primarily due to the rapid decrease of temperature with depth in the pycnocline, and a deep layer in which density increases slowly with depth.
  • The layers of water masses separated by differences in density are extremely thin, sometimes just a less than 10 m thick, but they stretch across thousands of kilometers of ocean.
  • At high latitudes the entire water column is cold and there is no thermocline. However, a halocline forms in some areas where river runoff and ice melt lower the salinity of the surface layer.
  • When seasonal ice forms in winter, dissolved salts are left behind in a process called ice exclusion and seawater salinity is increased so that it sinks. When the ice melts it releases fresh water and lowers the salinity of the surface layer forming a halocline.
  • Within the upper mixed layer of the oceans shallow seasonal thermoclines may form in summer.

Formation of Deep Water Masses

  • Deep water masses are formed in only a few high-latitude regions where there is no halocline and where intense cooling raises the water density and causes it to sink.
  • The water sinks to its density equilibrium level and spreads out from its area of formation.

Locations of Deep Water-Mass Formation

  • Deep water mass formation occurs primarily in a limited area near Greenland in the North Atlantic Ocean and in several areas surrounding the Antarctic continent.
  • The deep water formed around Greenland—the North Atlantic Deep Water—is less dense than the Antarctic Bottom Water formed around Antarctica.

Water Masses at Intermediate Depths

  • Currents and water masses at intermediate depths are caused by sinking of cool, high-density water at the Antarctic convergence and by sinking of warm, but high-salinity water at the subtropical convergences.
  • In the Atlantic Ocean, the warm, high-salinity water flowing out from the Mediterranean Sea also forms an intermediate-depth water layer.
  • Cold deep water is mixed slowly but progressively upward as it travels through the oceans.
  • The primary mixing process is molecular diffusion, which is very slow, but turbulent diffusion and internal wave on density interfaces enhance mixing, especially as the currents flow over rough seafloor topography.

10.14 Ocean Circulation and Climate

  • Ocean circulation carries massive quantities of heat from one area of the planet to another and, therefore, has a major influence on regional climates.

Meridional Overturning Circulation: The Conveyor Belt

  • A conveyor belt–like circulation exists in which a deep water mass is formed in the North Atlantic, moves south along the Atlantic Ocean floor, and spreads around Antarctica, where it mixes with deep water masses formed in this region. It then spreads north into the Indian and Pacific Oceans, is slowly mixed upward back to the surface layer, and returns to the North Atlantic through a complex pattern of surface currents culminating in the Gulf Stream. This is the Meridional Overturning Circulation (MOC).
  • The conveyor belt circulation transports heat from low to high latitudes in the Atlantic Ocean.

The MOC Climate Switch

  • The conveyor belt has been “switched” on and off at various times in the past, resulting in major climate changes in Europe and probably elsewhere.
  • The annual rate of formation of North Atlantic Deep Water, the start of the conveyor belt, has decreased substantially in the past two decades compared to the decade before. There is concern that this slowdown may be related to the enhanced greenhouse effect and, if continued or accelerated, may lead to substantial slowing of the MOC and major climate changes, especially in Europe.

10.15 Tracing Water Masses

  • It is very difficult to measure currents directly in the deep layers of the oceans so ocean scientists study these movements by following the distributions of properties of the water masses called tracers that can be used to track their movements.

Conservative and Nonconservative Properties

  • Conservative properties of seawater are properties that are not changed by biological, chemical, or physical processes within the body of the oceans. Conservative properties include heat content (temperature) and sodium and chloride concentrations (and salinity).
  • Nonconservative properties are altered in the body of the oceans. For example, oxygen is consumed in respiration and released in photosynthesis.
  • Conservative properties are particularly useful as tracers because when two water masses mix the value of the conservative property in the mixed water mass is determined by the relative proportions of the two water masses in the mixture.

TS Diagrams

  • Water masses can be traced by their temperature and salinity because these are conservative properties.
  • TS diagrams are a plot of the salinity versus temperature for a series of water samples, usually from different depths at the same location.
  • Within a given water mass all points will plot in the same location on the TS diagram.
  • When two water masses mix, the mixed water mass will plot on a straight line between the points at which the two individual water masses lie and the proportions of each water mass in the mixture can be calculated by where they fall on that line.
  • When three water masses mix so none of the original middle layer water still exists other than in mixtures with one of the other two, the characteristics of the fully mixed water mass can be deduced and the proportions of the mixture calculated by extending the portions of the straight line mixing curve that still exist.

Tracers

  • Other characteristics including oxygen and radionuclide concentrations and the concentrations of other constituents can be used to trace water masses.
  • Two of the more useful tracers, especially for studying deep water mass formation, are tritium and chlorofluorocarbons. Tritium occurs naturally at very low concentrations but very large amounts were released primarily in the Arctic Ocean during the nuclear bomb–testing era. Chlorofluorocarbons are non-naturally occurring organic compounds synthesized only in recent decades. Because the time of origin of tritium and chlorofluorocarbons is known and because they enter the deep oceans only through transport dissolved in the ocean water, these compounds are good tracers of the rate of formation of deep water masses and their rate of movement through the oceans.

 

Critical Concept Reminders:

CC.1 Density and Layering in Fluids (pp. 240, 243, 262, 263, 265)

  • 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. To read CC1 go to page 2CC.

CC.3 Convection and Convection Cells (pp. 240, 262)

  • Fluids, including ocean water, that are cooled from above, sink because 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. To read CC3 go to page 10CC.

CC.6 Salinity, Temperature, Pressure, and Water Density (pp. 240, 263, 265, 270)

  • 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. To read CC6 go to page 16CC.

CC.7 Radioactivity and Age Dating(p. 272)

  • 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. To read CC7 go to page 18CC.

CC.9 The Global Greenhouse Effect (p. 268)

  • 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. To read CC9 go to page 22CC.

CC.10 Modeling (pp. 263, 270, 273)

  • 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. To read CC10 go to page 26CC.

CC.11 Chaos (p. 268)

  • 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. To read CC11 go to page 28CC.

CC.12 The Coriolis Effect (pp. 242, 243, 245, 250, 255, 259, 261)

  • 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. To read CC12 go to page 32CC.

CC.13 Geostrophic Flow (pp. 242, 244, 245, 246, 255)

  • 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 seasurface height) in near circular paths. The circular gyres that dominate the global circulation of ocean waters are the result of water masses flowing geostrophically. To read CC13 go to page 43CC.

 

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