Physical oceanography is the study of
physical conditions and physical processes within
the
ocean, especially the motions and physical
properties of ocean waters.
Physical oceanography is one of several sub-domains into which
oceanography is divided; others include
biological,
chemical and
geological oceanographies.
The physical setting
The pioneering oceanographer
Matthew Maury said in 1855
"Our
planet is invested with two great oceans; one visible, the other
invisible; one underfoot, the other overhead; one entirely
envelopes it, the other covers about two thirds of its
surface." The fundamental role of the oceans in shaping Earth
is acknowledged by
ecologists,
geologists,
meteorologists,
climatologists,
geographers and others interested in the physical
world. An Earth without oceans would truly be unrecognizable.
Roughly 97% of the planet's water is in its oceans, and the oceans
are the source of the vast majority of
water
vapor that condenses in the atmosphere and falls as
rain or
snow on the continents. The
tremendous
heat capacity of the oceans
moderates the planet's
climate, and its
absorption of various gases affects the composition of the
atmosphere. The ocean's influence extends even to
the composition of
volcanic rocks through
seafloor
metamorphism, as well as
to that of volcanic gases and
magmas created
at
subduction zones.
Vertical and horizontal dimensions
The oceans are far deeper than the
continents are tall; examination of the Earth's
hypsographic curve shows that the
average elevation of Earth's landmasses is only , while the ocean's
average depth is . Though this apparent discrepancy is great, for
both land and sea, the respective extremes such as
mountains and
trenches are
rare.
Temperature, salinity and density
Because the vast majority of the world ocean's volume is deep
water, the mean temperature of seawater is low; roughly 75% of the
ocean's volume has a temperature from 0° – 5°C (Pinet 1996). The
same percentage falls in a salinity range between 34–35 ppt
(3.4–3.5%) (Pinet 1996). There is still quite a bit of variation,
however. Surface temperatures can range from below freezing near
the poles to 35°C in restricted tropical seas, while salinity can
vary from 10 to 41 ppt (1.0–4.1%).
The vertical structure of the temperature can be divided into three
basic layers, a surface
mixed layer,
where gradients are low, a
thermocline
where gradients are high, and a poorly stratified abyss.
In terms of temperature, the ocean's layers are highly
latitude-dependent; the
thermocline is pronounced in the tropics, but
nonexistent in polar waters (Marshak 2001). The
halocline usually lies near the surface, where
evaporation raises salinity in the tropics, or meltwater dilutes it
in polar regions. These variations of salinity and temperature with
depth change the density of the seawater, creating the
pycnocline.
The general circulation of the ocean
Density-driven thermohaline
circulation
The ultimate energy source for the ocean circulation (and for the
atmospheric circulation) is the sun. The amount of sunlight
absorbed at the surface varies strongly with latitude, being
greater at the equator than at the poles, and this engenders fluid
motion in both the atmosphere and ocean that acts to redistribute
heat from the equator towards the poles, thereby reducing the
temperature gradients that would exist in the absence of fluid
motion. Perhaps three quarters of this heat is carried in the
atmosphere; the rest is carried in the ocean.
The atmosphere is heated from below, which leads to convection, the
largest expression of which is the
Hadley circulation. By contrast the ocean
is heated from above, which tends to suppress convection. Instead
ocean deep water is formed in polar regions where cold salty waters
sink in fairly restricted areas. This is the beginning of the
thermohaline
circulation.
Oceanic currents are largely driven by the surface wind stress;
hence the large-scale
atmospheric circulation is important
to understanding the ocean circulation. The Hadley circulation
leads to Easterly winds in the tropics and Westerlies in
mid-latitudes, which creates an anticyclonic wind stress curl over
the subtropical ocean. This leads to slow equatorward flow
throughout most of a subtropical ocean basin (the
Sverdrup balance). The return flow occurs
in an intense, narrow, poleward
western boundary current. Like the
atmosphere, the ocean is far wider than it is deep, and hence
horizontal motion is in general much faster than vertical motion.
In the southern hemisphere there is a continuous belt of ocean, and
hence the mid-latitude westerlies force the strong
Antarctic Circumpolar Current.
In the northern hemisphere the land masses prevent this and the
ocean circulation is broken into smaller
gyres
in the Atlantic and Pacific basins.
The Coriolis Effect
The
Coriolis effect results in a
deflection of fluid flows (to the right in the Northern Hemisphere
and left in the Southern Hemisphere). Because the distance around
the Earth decreases as one moves away from the equator, and because
the Earth rotates in a counter clockwise direction as seen from the
north pole, air and water masses are deflected to the east as they
move from the equator to the poles, and to the west as they move
from the poles to the equator. This has profound effects on the
flow of the oceans. In particular it means the flow goes
around high and low pressure systems, permitting them to
persist for long periods of time. As a result, tiny variations in
pressure can produce measurable currents. A slope of one part in
one million in sea surface height, for example, will result in a
current of 1 cm/s at mid-latitudes. The fact that the Coriolis
effect is largest at the poles and weak at the equator results in
sharp, relatively steady western boundary currents which are absent
on eastern boundaries. Also see
secondary circulation effects.
The Coriolis effect is also responsible for coastal
upwelling as
wind-driven
currents tend to forced to the right of the winds in the Northern
Hemisphere and to the left of the winds in the Southern Hemisphere.
When winds blow either equatorward along an eastern ocean boundary
or poleward along a western ocean boundary, water is driven away
from the coasts (the so called
Ekman
transport), and denser water rises from below to replace
it.
Ekman Transport
Ekman Transport results in the net transport of surface water 90
degrees to the right of the wind in the Northern Hemisphere, and 90
degrees to the left of the wind in the Southern Hemisphere. As the
wind blows across the surface of the ocean, it "grabs" onto a thin
layer of the surface water. In turn, that thin sheet of water
transfers motion energy to the thin layer of water under it, and so
on. However, because of the Coriolis Effect, the direction of
travel of the layers of water slowly move farther and farther to
the right as they get deeper in the Northern Hemisphere, and to the
left in the Southern Hemisphere. In most cases, the very bottom
layer of water affected by the wind is at a depth of 100 m – 150 m
and is traveling about 180 degrees, completely opposite of the
direction that the wind is blowing. Overall, the net transport of
water would be 90 degrees from the original direction of the
wind.
Langmuir Circulation
Langmuir circulation results in
the occurrence of thin, visible stripes, called
windrows on the surface of the ocean parallel to
the direction that the wind is blowing. If the wind is blowing with
more than 3 m s
-1, it can create parallel
windrows alternating upwelling and downwelling about 5–300 m
apart. These windrows are created by adjacent ovular water cells
(extending to about deep) alternating rotating clockwise and
counterclockwise. In the
convergence zones debris, foam and seaweed
accumulates, while at the
divergencezones
plankton are caught and carried to the surface. If there are many
plankton in the divergencezone fish are often attracted to feed on
them.
Ocean - Atmosphere Interface
At the ocean-atmosphere interface, the ocean and atmosphere
exchange fluxes of heat, moisture and momentum.
- Heat
The important
heat terms at the surface are the
sensible heat
flux, the latent heat flux, the
incoming
solar radiation and the
balance of long-wave (
infrared)
radiation. In general, the tropical oceans will
tend to show a net gain of heat, and the polar oceans a net loss,
the result of a net transfer of energy polewards in the
oceans.
The oceans' large heat capacity moderates the climate of areas
adjacent to the oceans, leading to a
maritime climate at such locations. This
can be a result of heat storage in summer and release in winter; or
of transport of heat from warmer locations: a particularly notable
example of this is
Western Europe,
which is heated at least in part by the
north atlantic drift.
- Momentum
Surface winds tend to be of order meters per second; ocean currents
of order centimeters per second. Hence from the point of view of
the atmosphere, the ocean can be considered effectively stationary;
from the point of view of the ocean, the atmosphere imposes a
significant wind
stress on its
surface, and this forces large-scale currents in the ocean.
Through the wind stress, the wind generates
ocean surface waves; the longer waves
have a
phase velocity tending towards
the
wind speed.
Momentum of the surface winds is transferred into
the energy
flux by the ocean surface waves. The
increased
roughness of the ocean surface,
by the presence of the waves, changes the wind near the
surface.
- Moisture
The ocean can gain
moisture from
rainfall, or lose it through
evaporation.
Evaporative loss leaves the ocean saltier;
the Mediterranean
and Persian
Gulf
for example have strong evaporative loss; the
resulting plume of dense salty water may be traced through the
Straits of
Gibraltar
into the Atlantic Ocean
. At one time, it was believed that
evaporation/
precipitation was a major driver
of ocean currents; it is now known to be only a very minor
factor.
Planetary Waves in the Ocean
- Kelvin Waves
A
Kelvin wave is any
progressive wave that is channeled between two
boundaries or opposing forces (usually between the
Coriolis force and a
coastline or the
equator).
There are two types, coastal and equatorial. Kelvin waves are
gravity driven and non-dispersive,
meaning that the
phase speed of the wave
at any one
frequency will equal the
group speed of the wave energy for all
frequencies. This means that Kelvin waves can retain their shape
and direction over long periods of time. They are usually created
by a sudden shift in the wind, such as the change of the
trade winds at the beginning of the
El Niño-Southern
Oscillation.
Coastal
Kelvin waves follow shorelines and will
always propagate in a counterclockwise direction in the Northern
hemisphere
(with the shoreline to the
right of the direction of travel) and clockwise in the Southern hemisphere
.
Equatorial
Kelvin waves propagate to the east in the Northern
hemisphere and to the west in the Southern
hemisphere, using the equator as a
guide.
Kelvin waves are known to have very high speeds, typically around
2–3 meters per second. They have
wavelengths of thousands of kilometers and
amplitudes in the tens of meters.
- Rossby Waves
Rossby waves, or
planetary waves are huge, slow waves
generated in the
troposphere by
temperature differences between the
ocean and the
continents.
Their major
restoring force is the
change in
Coriolis force with
latitude. Their wave
amplitudes are usually in the tens of meters and
very large
wavelengths. They are usually
found at low or mid latitudes
There are two types of Rossby waves,
barotropic and
baroclinic. Barotropic Rossby waves have the
highest speeds and do not vary vertically. Baroclinic Rossby waves
are much slower.
The special identifying feature of Rossby waves is that the
phase velocity of each individual
wave always has a westward component, but the
group velocity can be in any direction.
Usually the shorter Rossby waves have an eastward group velocity
and the longer ones have a westward group velocity.
Climate variability
December 1997 chart of ocean surface temperature anomaly [°C]
during the last strong El Niño
The interaction of ocean circulation, which serves as a type of
heat pump, and biological effects such as the concentration of
carbon dioxide can result in global
climate changes on a time scale of
decades. Known climate oscillations resulting from these
interactions, include the
Pacific decadal oscillation,
North Atlantic
oscillation, and
Arctic
oscillation. The oceanic process of
thermohaline circulation is a
significant component of heat redistribution across the globe, and
changes in this circulation can have major impacts upon the
climate.
La Niña – El Niño
and
Antarctic Circumpolar Wave
This is a
coupled ocean/atmosphere wave that
circles the Southern
Ocean
about every eight years. Since it is a
wave-2 phenomenon (there are two peaks and two troughs in a
latitude circle) at each fixed
point in space a signal with a
period of four years is seen. The
wave moves eastward in the direction of the
Antarctic Circumpolar
Current.
Ocean currents
Among the most important
ocean
currents are the:
Antarctic Circumpolar Current
The ocean body surrounding the
Antarctic
is currently the only continuous body of water where there is a
wide latitude band of open water.
It interconnects the Atlantic, Pacific
and Indian
oceans, and provide an uninterrupted stretch for
the prevailing westerly winds to significantly increase wave
amplitudes. It is generally accepted that these prevailing
winds are primarily responsible for the circumpolar current
transport. This current is now thought to vary with time, possibly
in an oscillatory manner.
Deep ocean currents (abyssal circulation)
In the
Norwegian
Sea
evaporative cooling is predominant, and the sinking
water mass, the North Atlantic
Deep Water (NADW), fills the basin and spills southwards
through crevasses in the submarine
sills that connect Greenland
, Iceland
and Britain
. It then flows along the western boundary of
the Atlantic with some part of the flow moving eastward along the
equator and then poleward into the ocean basins. The NADW is
entrained into the Circumpolar Current, and can be traced into the
Indian and Pacific basins.
Flow from the Arctic Ocean
Basin into the Pacific, however, is blocked by the
narrow shallows of the Bering Strait
.
Also see
marine
geology about that explores the
geology of the ocean floor including
plate tectonics that create deep ocean
trenches.
Western boundary currents
An idealised subtropical ocean basin forced by winds circling
around a high pressure (anticyclonic) systems such as the
Azores-Bermuda high develops a
gyre circulation
with slow steady flows towards the equator in the interior. As
discussed by
Henry Stommel, these
flows are balanced in the region of the western boundary, where a
thin fast polewards flow called a
western boundary current develops.
Flow in the real ocean is more complex, but the
Gulf stream, Agulhas and
Kuroshio are examples of such currents. They are
narrow (approximately 100 km across) and fast (approximately
1.5 m/s).
Equatorwards western boundary currents occur in tropical and polar
locations, e.g. the East Greenland andLabrador currents, in the
Atlantic and the
Oyashio. They are forced by
winds circulation around low pressure (cyclonic)
- Gulf stream
The Gulf
Stream, together with its northern extension, North Atlantic Current, is a
powerful, warm, and swift Atlantic ocean current that originates in
the Gulf of
Mexico
, exits through the Strait of Florida, and follows
the eastern coastlines of the United States and Newfoundland to the
northeast before crossing the Atlantic Ocean.
- Kuroshio
The
Kuroshio Current is an ocean current found
in the western Pacific Ocean off the east coast of Taiwan
and flowing
northeastward past Japan
, where it
merges with the easterly drift of the North Pacific Current. It is
analogous to the Gulf Stream in the Atlantic Ocean, transporting
warm, tropical water northward towards the polar region.
Oceanic heat flux and the climate connection
Heat storage
Heat storage and transfer in the ocean is very uneven.
Sea level change
Tide gauges and satellite altimetry suggest an increase in sea
level of 1.5–3 mm/yr over the past 100 years.
The
IPCC
predicts that by 2100,
global warming
will lead to a sea level rise of 110 to 880 mm.
Rapid variations in the ocean
Ocean tides
The rise and fall of the oceans due to tidal effects is a key
influence upon the coastal areas. Ocean tides on the planet Earth
are created by the gravitational effects of the
Sun and
Moon. The tides produced by
these two bodies are roughly comparable in magnitude, but the
orbital motion of the Moon results in tidal patterns that vary over
the course of a month.
The ebb and flow of the tides produce a cyclical current along the
coast, and the strength of this current can be quite dramatic along
narrow estuaries. Incoming tides can also produce a
tidal bore along a river or narrow bay as the
water flow against the current results in a wave on the
surface.
Tide and Current (Wyban 1992) clearly illustrates the
impact of these natural cycles on the lifestyle and livelihood of
Native Hawaiians tending coastal
fishponds.
Aia ke ola ka hana meaning . . .
Life is in
labor.
Tidal
resonance occurs in the Bay of Fundy
since the time it takes for a large wave to travel from the mouth of the bay to the opposite end, then reflect and travel back to
the mouth of the bay coincides with the timing between this
repeating wave that is also reinforced by the tidal rhythm
producing the world's highest tides.
As the surface tide oscillates over topography, such as submerged
seamounts or ridges, it generates
internal waves at the tidal frequency, which
are known as
internal tides.
Tsunamis
A series of surface waves can be generated due to large-scale
displacement of the ocean water. These can be caused by sub-marine
landslides, seafloor deformations due to
earthquakes, or the impact of a large
meteorite.
The waves can travel with a velocity of up to several hundred
km/hour across the ocean surface, but in mid-ocean they are barely
detectable with
wavelengths spanning
hundreds of kilometers.
Tsunamis, originally called tidal waves, were renamed because they
are not related to the tides. They are regarded as
shallow-water waves, or waves in
water with a depth less than 1/20 their wavelength. Tsunamis have
very large periods, high speeds, and great wave heights.
The primary impact of these waves is along the coastal shoreline,
as large amounts of ocean water are cyclically propelled inland and
then drawn out to sea. This can result in significant modifications
to the coastline regions where the waves strike with sufficient
energy.
The
tsunami that occurred in Lituya Bay
, Alaska on July 9, 1958 was high and is the biggest
tsunami ever measured, almost taller than the Sears Tower
in Chicago and about taller than the World Trade
Center
in New York.
Ocean surface waves
The wind generates ocean surface waves, which have a large impact
on
offshore structures,
ships,
coastal erosion and
sedimentation, as well as
harbours. After their generation by the wind, ocean
surface waves can travel (as
swell)
over long distances.
See also
References
Further reading
External links
Top topics
Wikis
Quiz
Map
Search