Chapter 8: Ocean Circulation

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 (remember momentum from physics)

geostrophic currents on a water-covered Earth

on a completely water-coveed 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 win 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

conservative properties of seawater

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)

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. Currents created by this mechanism are called thermohaline circulation

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.

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

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

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.

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 waves on density interfaces enhance mixing, especially as the currents flow over rough seafloor topography

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.

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.

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.

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 this energy is 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.

how does a TS diagram change when two water masses mix?

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

pycnocline and seafloor interruption of the Ekman spiral

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 45o 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. In the open ocean, where 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

high-latitude surface current

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.

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

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

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

Ekman transport in the equatorial region

The Doldrums migrate north and south seasonally but this region between the Northern and Southern Hemisphere trade wind bands is, on average displaced north of the equator. This produces a complex system of east west currents flowing east to west or west to east in the equatorial region between the two trade wind bands. Equatorial currents have the unique characteristic that they 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)

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

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

the Meridional Overturning Circulation (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 further slowing of the MOC and major climate changes, especially in Europe.

depth distribution of temperature and salinity

The layers of water masses separated by differences in density are extremely thin, sometimes 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.

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.

geostrophic currents

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

how does seasonal ice affect depth distribution?

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.

how do TS diagrams change when three water masses mix?

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

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

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.

generation of wind-driven 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

downwelling

Winds can create convergences at which the surface layer is thickened and downwelling may occur. Downwelling areas are depleted in nutrients and are areas of very low productivity

upwelling

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

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

except at high latitudes, the oceans have three primary layers, what are they?

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

TS diagrams

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.

vorticity

a pseudovector field that describes the local spinning motion of a continuum near some point (the tendency of something to rotate), as would be seen by an observer located at that point and traveling along with the flow

Where does downwelling occur?

at convergences. The principal open-ocean convergences are located along the equator and in a band around Antarctica. Convergences include the centers of the subtropical gyres. Convergences can be created when Ekman transport moves surface water toward the coast. This creates downwelling where productivity is low and fisheries are poor.

Where does upwelling occur?

at divergences. 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

Why are there no secondary gyres in the Southern Hemisphere?

because no continents block the geostrophic currents and they are free to flow around Antarctica

Why is there no high-latitude gyre in the Indian Ocean?

because the ocean does not extend far enough north

why can water masses be traced by their temperature and salinity?

because these are conservative properties

why are conservative properties of seawater particularly useful?

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

why do currents 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

nonconservative properties of seawater

can be altered in the body of the oceans. For example, oxygen is consumed in respiration and released in photosynthesis

mesoscale eddies

found throughout the oceans; weaker than the western boundary current eddies, but little else is known about them. Ocean circulation resembles atmospheric weather patterns. However, because 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 about 20 times the current speeds in ocean eddies.

pcynoclines

locations of strong vertical density gradient; they can be either thermoclines or haloclines

Langmuir circulation

strong winds create 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. It is important in wind mixing of the upper few meters of the water column.

wind-driven currents

surface layer currents; winds are the primary source of energy for currents that flow horizontally in the ocean surface layers (less than 100-200 m deep)

true or false: within the upper mixed layer of the oceans, shallow seasonal thermoclines may form in summer

true

open-ocean surface currents

wind-driven, initiated by Ekman transport, and maintained as geostrophic 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

what causes water movements?

winds changes in salinity or water temperature that alter water density the primary source that dries the winds and causes temperature and salinity changes that alter density: the sun