World ocean bathymetry.
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 oceanography.
Physical oceanography may be subdivided into descriptive and dynamical physical oceanography.
Descriptive physical oceanography seeks to research the ocean
through observations and complex numerical models, which describe the
fluid motions as precisely as possible.
Dynamical physical oceanography focuses primarily upon the
processes that govern the motion of fluids with emphasis upon
theoretical research and numerical models. These are part of the large
field of Geophysical Fluid Dynamics (GFD) that is shared together with meteorology. GFD is a sub field of Fluid dynamics describing flows occurring on spatial and temporal scales that are greatly influenced by the Coriolis force.
Physical setting
Perspective
view of the sea floor of the Atlantic Ocean and the Caribbean Sea. The
purple sea floor at the center of the view is the Puerto Rico Trench.
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.
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 840
metres (2,760 ft), while the ocean's average depth is 3,800 metres
(12,500 ft). Though this apparent discrepancy is great, for both land
and sea, the respective extremes such as mountains and trenches are rare.
| Body | Area (106km²) | Volume (106km³) | Mean depth (m) | Maximum (m) |
| Pacific Ocean | 165.2 | 707.6 | 4282 | -11033 |
| Atlantic Ocean | 82.4 | 323.6 | 3926 | -8605 |
| Indian Ocean | 73.4 | 291.0 | 3963 | -8047 |
| Southern Ocean | 20.3 |
|
|
-7235 |
| Arctic Ocean | 14.1 |
|
1038 |
|
| Caribbean Sea | 2.8 |
|
|
-7686 |
Temperature, salinity and density
WOA surface 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.
Circulation
Density-driven thermohaline circulation
Energy for the ocean circulation (and for the atmospheric
circulation) comes from solar radiation and gravitational energy from
the sun and moon.
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. 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.
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). 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 10 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.
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 6 m (20 ft) deep) alternating rotating
clockwise and counterclockwise. In the convergence zones debris, foam and seaweed accumulates, while at the divergence
zones plankton are caught and carried to the surface. If there are many
plankton in the divergence zone fish are often attracted to feed on
them.
Ocean–atmosphere interface
Hurricane Isabel east of the Bahamas on 15 September 2003
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
- 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.
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 and Southern hemispheres, 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 that year's 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.
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
- Deep ocean (density-driven)
- Western boundary currents
- Eastern Boundary currents
Antarctic circumpolar
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
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
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 and Labrador 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.
Heat flux
Heat storage
Ocean heat flux is a turbulent and complex system which utilizes atmospheric measurement techniques such as eddy covariance to measure the rate of heat transfer expressed in the unit of joules or watts per second. Heat flux
is the difference in temperature between two points through which the
heat passes. Most of the Earth's heat storage is within its seas with
smaller fractions of the heat transfer in processes such as evaporation,
radiation, diffusion, or absorption into the sea floor. The majority of
the ocean heat flux is through advection
or the movement of the ocean's currents. For example, the majority of
the warm water movement in the south Atlantic is thought to have
originated in the Indian Ocean.
Another example of advection is the nonequatorial Pacific heating which
results from subsurface processes related to atmospheric anticlines. Recent warming observations of Antarctic Bottom Water in the Southern Ocean is of concern to ocean scientists because bottom water changes will effect currents, nutrients, and biota elsewhere. The international awareness of global warming has focused scientific research on this topic since the 1988 creation of the Intergovernmental Panel on Climate Change. Improved ocean observation, instrumentation, theory, and funding has increased scientific reporting on regional and global issues related to heat.
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 2081-2100, global warming will lead to a sea level rise of 260 to 820 mm.
Rapid variations
Tides
The Bay of Fundy is a bay located on the Atlantic coast of North America, on the northeast end of the Gulf of Maine between the provinces of New Brunswick and Nova Scotia.
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 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 520 m (1,710 ft) high and is the biggest
tsunami ever measured, almost 90 m (300 ft) taller than the Sears Tower in Chicago and about 110 m (360 ft) taller than the former World Trade Center in New York.
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.
