WHAT ARE SOME OF EARTH’S MAJOR OCEAN SYSTEMS?
T
he relatively uniform appearance of the world’s oceans belie the complex interactions that produce ocean circulation, eddies, and tides. Some of these systems have
been stable for millions of years while others change over months or hours. The
interplay of each of these systems is fundamentally important for ocean science and have
interesting implications for life on our planet.
This theme (Oceans - Systems and Interactions) addresses general ocean circulation processes,
how ocean eddies are formed, and ocean tides.
Related Themes:
• The role of winds and Earth’s rotation on ocean systems is discussed in Oceans Energy.
• The affect of salinity, temperature, and density on the oceans’ vertical structure is
featured in Oceans - Scale and Structure.
• Using satellites to study ocean systems is discussed in Oceans - Measurements.
• How ocean eddies and tides affect people’s lives is described Ocean - Human Interactions.
• How ocean circulation, upwelling, and downwelling affect climate is addressed in
Climate - Systems and Interactions.
• The influence of ocean upwelling, downwelling, and eddies on biological productivity is featured in Life - Systems and Interactions.
• How shifts in coastal upwelling off of Peru are tied to the climate phenomenon El
Niño can be found in Climate - Process of Change.
Related Activities:
• Wind-Driven Ocean Currents
• Ocean Eddies
• Global Winds and Ocean Currents
• Sea Level Slope and Surface Currents
• Timing the Tides
INTRODUCTION
There are many ocean systems on Earth and they are often global and dynamic. Forces are
needed to start up and maintain these systems. Atmospheric winds drive ocean circulation.
Moreover, the oceans and atmosphere forces are interdependent because wind patterns themselves are influenced by ocean circulation. In the case of ocean tides, astronomical bodies (i.e.,
the Sun and the Moon) exert gravitational forces on Earth’s oceans in a complex yet predictable
manner.
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a) Global wind patterns.
b) Global ocean wind-driven surface current patterns.
Figure 1. Maps of global wind and surface current patterns for January. Note the similarities and differences between large-scale wind and current patterns.
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HORIZONTAL OCEAN CIRCULATION
Wind-driven
Surface ocean currents, in the
upper 1000 meters of the ocean,
are driven by winds and are
strongly influenced by change in
density between layers of water.
These ocean currents tend to follow the winds in the lower atmosphere, except where they are diverted by continents or deflected
because of Earth’s rotation (see
Coriolis effect below). It is interesting to compare and contrast
global wind and ocean current
circulation patterns [Fig. 1].
Movie 1. The Coriolis effect on a merry-go-round. This demonAs winds blow across the sur- stration shows how, from above a spinning merry-go-round the path
face of the ocean, they drag along the ball travels appears to be straight. However, from the perspecthe water, setting it into motion. tive on the merry-go-round the ball appears to curve to the left as it
But because ocean currents are moves from person-to-person. Likewise, to a person sitting on the
moving on a rotating Earth, they rotating Earth the path of moving objects appears to be deflected.
will tend to be deflected from
their initial path, as will a marble
rolled on a merry-go-round
[Movie 1]. This tendency of moving objects to deflect in a rotating
reference frame is known as the
Coriolis effect. Earth’s rotation
causes ocean currents to deflect
to the right in the northern hemisphere and to the left in the southern hemisphere [Fig. 2].
The overall result of the interaction between winds and the
Coriolis effect is that surface currents tend to travel at significant
angles to the prevailing winds
(except along the equator where
east-west or west-east running Figure 2. The Coriolis effect. As Earth turns toward the east, parcurrents are not deflected by the ticles in motion (such as ocean or air currents) in the northern hemisphere are deflected to the right from their initial path. Currents in
Coriolis effect).
the southern hemisphere are deflected to the left.
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Picture the ocean as a series of
layers of water. Wind blowing
across the ocean’s surface drags the
uppermost layer with it. Just below, friction and the Coriolis effect
cause the next layer down to deflect
its direction relative to the layer
above it. With increasing depth,
each layer of ocean water is deflected a little more than the layer
above it. Also, because of friction,
each layer moves slower than the
layer above it. At the beginning of
the twentieth century, a physicist
named Vagn Walfrid Ekman realized that this balance between friction and the Coriolis effect forms a
spiral of successive layers of moving water. The net result of the Ekman Spiral is an overall deflection
of ocean currents at 90º angles to
prevailing winds in the top 100
meters or so of the ocean [Fig. 3].
The term Ekman transport is used to
describe the horizontal motion of
water at right angles to the wind
in the upper ocean where friction
is important.
Figure 3. Ekman spiral. This diagram shows how the Coriolis
effect and friction cause wind-driven currents to deflect to the
right in successively deeper layers (northern hemisphere). Current speeds also decrease with depth. Note that the resultant surface flow is at 45° to the right of the wind. Also, the net water
transport is 90° to the right of the wind.
Geostrophic (Earth turning)
Below the Ekman layer, friction is not important, and water
flows around highs and lows of
pressure [Movie 2]. The ocean
pressure differences are set up by
the winds.
In Earth’s oceans, wind systems from both the high- and
low-latitude regions generally
move surface water toward the
subtropical regions, around 30º
latitude. These wind patterns
thus form subtropical gyres where
surface waters converge to form
subtle hills. At these zones of con-
Movie 2. How the Coriolis effect deflects wind and ocean currents. When the Coriolis effect deflects winds and currents until
they are parallel to isobars (lines of equal pressure), then the pressure gradient balances Coriolis effect.
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vergence or hills, the balance between gravity and the Coriolis effect causes geostrophic currents to
flow in a predictable direction. Subtropical gyres located north of the equator rotate clockwise
and those found in the southern hemisphere move counterclockwise.
Sub-polar gyres are found above 40º latitude in the Atlantic and Pacific Oceans. They are
caused by large-scale divergence where regional wind patterns blow surface waters away from
the area. In these zones of divergence or valleys, deeper layers move upward to replace water
that has moved away from the gyre’s center. Geostrophic currents associated with these valleys
rotate in the opposite direction of ocean topographic hills. The Antarctic Circumpolar Current is
a sub-polar gyre whose entire path is unblocked by landmasses. This fast-moving clockwisecirculating current corresponds to a distinct band of very low sea level [Fig. 4].
Sea-surface hills and valleys - also known as ocean topography - can be measured to calculate
the speed and direction of geostrophic currents. Geostrophic ocean currents flow around these
high and low centers of water pressure and their speed depends on the slope of the ocean surface.
Figure 4. Global ocean topography. This three-dimensional view shows the hills and valleys of
Earth’s oceans caused by geostrophic currents. The total relief is about 2 meters.
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Figure 5. Causes of ocean topography. Top: In the northern hemisphere, clockwise winds cause surface
ocean waters to converge, creating a sea-surface “hill.” Counterclockwise winds create “valleys” of water, with cold water--marked by the thermocline--moving upward. Bottom: Side view of two “valleys” of
diverging water between a “hill” of converging water.
The thermocline is found at the depth at which an abrupt change in water temperature is
found. The thermocline separates the wind-stirred upper layer from the colder layers that are
moved by deep ocean currents. The convergence of surface ocean water creates a sea-surface hill
that pushes denser, colder water downward. Therefore, beneath such hills, the thermocline is
relatively deep. On the other hand, the center of ocean topographic valleys are associated with
cold water moving upward from depth [Fig. 5].
VERTICAL OCEAN CIRCULATION
Global Thermohaline (Temperature and Salt driven) Circulation
Large-scale vertical circulation in the oceans is primarily caused by differences in the density
of surface waters. The density of water is a function of its temperature and salinity, i.e., salt con6
tent. Cooler water is more dense than warmer water, saline water is more dense than pure water.
Although the highest salinity water is found in the subtropics (around 30º latitude), the water
temperatures there are high enough to keep the surface waters afloat. In a few important locations in the Atlantic (e.g., Weddell Sea, Greenland Sea, Labrador Sea), unusually dense water is
produced by intense cooling and evaporation at the ocean’s surface. In these regions, water tends
to sink until it reaches the depth where its density is the same as the surrounding water. Sometimes it is heavy enough to sink all the way to the ocean floor. This sinking motion is called
thermohaline circulation because it is a result of water’s temperature and salinity.
Local Coastal Upwelling and Downwelling
On some coasts, steady winds and Ekman transport (i.e., surface currents that flow 90º from
the wind direction) combine to produce vertical circulation of seawater. Upwelling occurs where
surface waters are carried out to sea by winds blowing parallel to the coastline. These surface
waters are replaced by cooler, often more nutrient-rich, water from below. Downwelling, on the
other hand, is caused by winds blowing along the coast that push surface water toward the
shore.
Coastal upwelling and
downwelling depend on
the direction of the alongshore winds and whether
the coast is located in
Earth’s northern or southern hemisphere. For example, because Peru is located in the southern
hemisphere, the Coriolis
effect drives Ekman transport to the left of the wind
direction. Thus, coastal
upwelling occurs along
Peru’s west coast when
steady southerly (i.e.,
from south-to-north)
winds blow [Fig. 6]. Conversely, northerly (i.e.,
north-to-south blowing)
winds induce local
downwelling
along
Peru’s west coast.
Figure 6. Upwelling and downwelling along the western coast of Peru.
Driven by southerly winds, surface ocean currents move offshore - to the
left of wind direction - and are replaced by upwelled water from below.
Northerly alongshore winds drive surface currents toward the shore and
thereby induce local downwelling.
Coastal upwelling and downwelling strongly influence local climates and biological productivity. Interestingly, the weather-disrupting El Niño phenomenon derives its name from historical shifts in upwelling and downwelling along the west coast of Peru.
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OCEAN “WEATHER”: EDDIES
Ocean flow is turbulent and can thus spin off large eddies. Eddies can range in size from tens
to hundreds of kilometers, and sometimes extend to depths of 1,000 meters. Like larger scale
ocean currents, eddies transport heat, salt, and nutrients to different parts of the ocean. Eddies
are highly changeable structures, most form and dissociate in just a few months, but some persist
for even a year or two.
Eddies cause topographic highs and lows in the ocean which are shown in TOPEX/Poseidon
satellite data [Fig. 7]. Measuring eddy activity from space benefits people who work at sea including marine biologists. Studies indicate that some marine mammals frequent the edges of
eddies where high concentrations of microscopic plant life are brought to the surface, creating a
rich habitat. Also, monitoring the swift currents associated with eddies is important for people
who work on oil-production platforms.
Figure 7. Ocean eddies as measured by TOPEX/Poseidon. The swirls of water spun off of a main
current or forced by the wind are like atmospheric storms. TOPEX/Poseidon tracks these vortices and
their time-varying sea-surface height. The most rapidly swirling eddies cause the greatest changes in sea
height - over 25 centimeters - indicated in this image by white.
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TIDES
Gravity is the force of attraction that keeps the planets moving in their orbits around the Sun.
It is also the force of attraction that pulls everything on Earth’s surface toward its center. Any
object with mass exerts a gravitational force proportional to its mass. The strength of the gravitational force between two bodies decreases with the square of the distance separating the bodies
(so twice as far away, the force is four times weaker).
The Sun and the Moon each exert gravitational pulls on Earth’s solid body and its oceans.
The term tides commonly refers to the periodic rise and fall of ocean water levels resulting from
the gravitational pull of the Moon and the Sun. The Moon is not nearly as massive as the Sun, but
because the Moon is so much closer to Earth than the Sun its influence on the tides is greater (in
fact, about twice greater).
Figure 8. Idealized view of tides. These four diagrams show what the effect of the Sun and the Moon
would be if Earth’s oceans were very deep and covered the entire surface. The gravitational pull of the
Moon and Earth’s rotation combine to produce two bulges, one on the side close to the Moon, the other on
the side 90º away. High tide is experienced at the bulges, low tide ninety degrees away from the bulges.
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The Earth-Moon system revolves around a common center of mass that is located inside
Earth. This turning motion, combined with the Moon’s gravitational attraction, causes egg-shaped
bulging of the ocean surface [Fig. 8]. Areas near the bulging portions of the oceans experience
high tide. Regions 90º away (that is, a quarter of the way around the globe) from the bulges
experience low tide. (Another way to think about it is that the solid Earth rotates beneath the
water bulges induced by the Moon.) The highest and lowest tides each month occur when the
Sun, Earth, and Moon are aligned.
Ideally, we expect coasts to have two high tides and two low tides during a day. However,
once created by astronomical factors, tides are modified by ocean bottom topography and the
orientation of coastlines. Also, because the Moon’s monthly revolution around Earth is in the
same direction as our planet’s rotation, a point on Earth takes about 24 hours and 50 minutes to
catch up with the advancing Moon. This period of time is called a lunar day.
The result of these complex interactions gives us the three common coastal
tidal patterns [Fig. 9]:
• diurnal tides: a single high and
low water each lunar day.
• semi-diurnal tides: two high
and two low waters each lunar
day with each successive high
and low water having about
the same height.
• mixed tides: combination of
both diurnal and semi-diurnal
tides with successive high
and/or low waters having
large differences in water
heights.
Figure 9. Types of tides. Types of tides observed along
Although the factors that influence lo- the coasts of North America and northern South America.
cal tides is complex, the effect of tides on
coastal water levels is well-known from tide station data. To predict tides accurately, tide stations must gather data for a minimum of 18.6 years to experience most major tide-generating
configurations of the Earth-Moon system. Coastal tides have been monitored for a long time
because of their impact on human activities such as navigation, fisheries, and recreation.
Until the development of Earth-orbiting satellites, studying tides in the open ocean (i.e., away
from coastlines) was difficult. Today, considerable research is focused on the global nature of
tides, their influence on ocean circulation and the generation of waves. Many researchers use
ocean height data from satellites such as TOPEX/Poseidon.
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CONCLUSION
The influence of the Sun and the Moon on Earth’s oceans is profound. Energy from the Sun
creates atmospheric winds that force ocean circulation and help to spin off ocean eddies. The
gravitational pull of the Sun and the Moon exert great influence on Earth’s ocean tides. Moreover, ocean circulation, eddies, and tides are complicated by the rotation of Earth itself, although
in a predictable manner. Scientific research of our ocean systems and interactions has been improved with global data sets collected by Earth-orbiting satellites. Better understanding of these
oceanic systems and interactions will ultimately benefit those people whose lives and livelihood
are tied to Earth’s oceans.
VOCABULARY
Coriolis effect
divergence
eddy
El Niño
gravitational force
mixed tides
semi-diurnal tides
tide
convergence
downwelling
Ekman spiral
geostrophic
gyre
reference frame
thermocline
topography
diurnal tides
dynamic
Ekman transport
gradient
lunar day
salinity
thermohaline circulation
upwelling
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