nUnderstanding what causes currents and where they flow is fundamental to all marine sciences. It helps explain how heat, sediments, nutrients, and organisms move within the seas.
Causes of Currents
nThree major factors drive ocean currents.
Ø1. Wind.
nIf the wind blows long enough
in one direction,
it will cause a water current to develop.
nThe current continues to flow
until internal friction,
or friction with the sea floor, dissipates its energy.
Ø2. Changes in sea level.
nSea level is the
average level of the sea’s surface at its mean
height between high and low tide.
nThe ocean’s surface is never flat, ocean circulation cause slopes to develop. The steeper the “mound” of water, the larger and faster the current. The force that drives this current is the pressure gradient force.
Ø3. Variations in water density.
nDifferences in water density also cause horizontal differences in water pressure. When the density of seawater in one area is greater than another, the horizontal pressure gradient between the two areas initiates a current that flows below the surface.
Gyres
nThe combination of westerlies, trade winds, and the Coriolis effect results in a circular flow in each ocean basin. This flow is called a gyre.
nThere are five major gyres – one in each major ocean basin:
Ø1. North Atlantic Gyre
Ø2. South Atlantic Gyre
Ø3. North Pacific Gyre
Ø4. South Pacific Gyre
Ø5.
nThe flow of currents in all
parts of the
ocean is a balance of various factors,
including the pressure gradient force,
friction, and the Coriolis effect.
Ekman Transport
nThe Ekman
transport is an interesting phenomenon
discovered in the 1890s by Fridtjof Nansen.
nThe wind and the Coriolis effect influences water
well below the surface because water tends to flow
in what can be imagined as layers.
ØDue to friction, the upper water currents push the deep water below it. This deep layer pushes the next layer below it. The process continues in layers downward. Each water layer flows to the right of the layer above causing a spiral motion.
ØThis spiraling effect of water
layers pushing slightly to the right from the one above (to
the left in the Southern Hemisphere) is called the Ekman
spiral.
nThere is a net motion imparted to the water column down to friction depth. This motion is called the Ekman transport.
ØThe net effect, averaging of all the speeds and directions of the Ekman spiral, is to move water 90° to the right of the wind in the Northern Hemisphere, or to the left in the Southern Hemisphere.
Western and Eastern Boundary Currents
nSatellite images show that the oceans are really “hilly,” not calm or flat.
ØThese images show that water piles up where currents meet. Where currents diverge, “valleys” form.
nThere is a dynamic balance between the clockwise deflection of the Coriolis effect (attempting to move water to the right) and the pressure gradient created by gravity (attempting to move the water to the left).
ØThe balance keeps the gyre
flowing around the outside of
the ocean basin.
nGeostrophic
currents are
created by the Earth’s rotation.
ØThis current results from the
balance between the pressure
gradient force and the
Coriolis effect.
Western and Eastern Boundary Currents (continued)
nWestern boundary currents are
found on
the east coasts of the continents and are
stronger and faster than eastern boundary
currents due to western intensification.
Western boundary currents flow through
smaller areas than eastern boundary currents.
ØTrade winds blow along the
equator pushing
water westward, causing it to “pile up” on the western edge of ocean basins
before it turns to the poles. The Earth’s rotation tends to shift the higher
surface level in the center of the gyre westward. The higher surface level is
now west of center and forces the current to “squeeze” through a
narrower area.
ØTotal water volume balances
out.
Western boundary currents handle the
same volume, but through smaller areas,
so water must move more rapidly.
Countercurrents
nCountercurrents and undercurrents are water flows that differ from the major ocean currents.
nCountercurrents are associated with equatorial currents – it runs opposite of its adjacent current.
ØIt is hypothesized they
develop in equatorial regions because of the doldrums. Without wind
pushing water westward, water driven in from the east enters the basin more
quickly than it exits. This causes a
countercurrent to develop.
nUndercurrents flow
beneath the
adjacent current and are found
beneath most major currents.
ØThey can significantly
affect land masses
and land temperatures.
Upwelling and Downwelling
nUpwelling is an upward vertical current that brings deep water to the surface. Downwelling is a downward vertical current that pushes surface water to the bottom.
nCoastal upwellings occur when the wind blows offshore or parallel to shore. In the Northern Hemisphere this wind blowing southward will cause an upwelling only on a west coast.
ØThe same wind on the east
coast in
the Northern Hemisphere sends
surface water toward shore causing
a downwelling.
nThese currents have strong
biological effects:
ØUpwelling tends to bring
deepwater
nutrients up into shallow water.
ØUpwellings also relate to significant weather patterns.
ØDownwellings are important in carrying and cycling nutrients to the deep ocean ecosystems and sediments.
Heat Transport and Climate
nCurrents play a critical role by transporting heat from warm areas to cool areas and affects climate by moderating temperatures. Without currents moving heat, the world’s climates would be more extreme.
El Niño
Southern Oscillation (ENSO)
nEl Niño tremendously
affects world
weather patterns.
ØThis brings low pressure and
high
rainfall in the Western Pacific.
ØThe opposite happens in the
Eastern
Pacific with high pressure and
less rainfall.
El Niño (continued)
nFor reasons still not clear, every 3 to 8 years a rearrangement of the high- and low-pressure systems occur.
nHigh pressure builds in the Western Pacific and low pressure in the Eastern Pacific. Trade winds weaken or reverse and blow eastward – the southern oscillation.
ØThis causes warm water of the
west to migrate east to the coast of
ØOver the eastern Pacific, humid air rises causing precipitation in normally arid regions. Flooding, tornados, drought and other weather events can lead to loss of life and property damage.
Thermohaline Circulation
and Water Masses
nThermohaline circulation is water motion caused by differing water densities.
ØIn the deep-ocean layers, water
density variation, not wind, is the primary cause
of current.
ØCirculation drives most of the
vertical motion of seawater and
the ocean’s overall circulation.
nThermohaline
circulation works because water
density increases due to cooling, increased
salinity or both.
ØWhen water becomes dense,
it sinks, causing a downward flow.
ØThis means water in some
other place must rise to replace
it, causing an upward flow.
ØDensity differences drive the
slow
circulation of deep water.
How Deep Water Forms
nThe intermediate, deep, and
bottom water
masses form primarily, but not entirely, at
high latitudes (around 70° North and South).
ØThe densest ocean waters, Antarctic
Bottom Waters form in the
Antarctic in winter, sink to the bottom
and spread along the ocean floor to
about 40° north latitude.
ØIn the
Waters form, but often get trapped
there by the topography of the ocean basin.
ØIn the Northern Hemisphere
along the east coast of the
ØMediterranean Deep Waters
form due to evaporation rather than cooling, with a salinity of 38‰. Flowing
out of the
Deep-Water Flow Patterns
nThe enormous water quantities
sinking at the poles and in the
ØDense water descends into low areas and bottom water upwell to compensate.
ØThe rising warm water enters wind-driven currents and is carried to the poles. There it cools, becomes more dense, and sinks again, repeating the process.
The Ocean Conveyor Belt
nThe interconnected flow of currents that redistribute heat is called the ocean conveyor belt or the Earth’s “air conditioner.”
ØThe ocean conveyor belt is important because it moderates the world’s climate. This marriage of surface and deep water circulation carries heat away from the tropics and, in turn, keeps the tropics from getting too hot.
ØSome scientists hypothesize that some Ice Ages may have resulted from a disruption of the conveyor belt.
Two Distinct Approaches
nThere are two main approaches to study currents:
Ø1. Lagrangian method, also called the float method.
nStudying the current by tracking a drifting object. This involves floating something in the current that records the information as it drifts.
Ø2. Eulerian method, also called the flow method.
nStudying the current by staying in one place and measuring changes to the velocity of the water as it flows past. This method uses fixed instruments that meter/sample the current as it passes.
Instrumentation and Methods
nThere are five examples of
instruments or methods
that scientists apply for studying
currents.
nFor Lagrangian study methods researchers use:
Ø1. A drogue. The advantage over a simple
surface
float is that the “holey sock” ensures that the current and
not the wind determine where it drifts.
Instrumentation and Methods (continued)
Ø2. The Argo float drifts at depth before periodically rising to the surface to transmit to a satellite a temperature and salinity profile of the water it rose through.
nFor Eulerian study methods researchers use:
Ø3. Various types of flow
meters. These devices
use impellors and vanes to measure and record
current speed and direction. The information gathered
is either transmitted immediately or stored for
retrieval later.
Ø4. A more sophisticated device is the Doppler Acoustic
Current Meter. This instrument determines current
direction and speed.
Ø5. Oceanographers can now use satellites
to help them.
Although they are primarily used for studying the surface,
these instruments use laser and photography to study currents.