Physical oceanography is the study ofphysical conditions and physical processes within theocean, especially the motions and physical properties of ocean waters.
Physical oceanography is one of several sub-domains into whichoceanography is divided. Others includebiological,chemical andgeological oceanography. Like the study ofatmospheric physics, physical oceanography is founded upon principles ofthermodynamics andfluid mechanics.
Physical oceanography may be subdivided intodescriptive anddynamical physical oceanography.[1]
Descriptive physical oceanography seeks to research the ocean through observations and complex numerical models that describe temperature, salinity, density, currents and other ocean features as accurately and completely 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 ofGeophysical Fluid Dynamics (GFD) that is shared together withmeteorology. GFD is a sub field offluid dynamics describing flows occurring on spatial and temporal scales that are greatly influenced by theCoriolis force.
| External image | |
|---|---|
 Space and time scales of physical oceanographic processes.[2] |
Roughly 97% of the planet's water is in its oceans, and the oceans are the source of the vast majority ofwater vapor that condenses in the atmosphere and falls asrain orsnow on the continents.[3][4] The tremendousheat capacity of the oceans moderates the planet'sclimate, and its absorption of various gases affects the composition of theatmosphere.[4] The ocean's influence extends even to the composition ofvolcanic rocks through seafloormetamorphism, as well as to that of volcanic gases andmagmas created atsubduction zones.[4]
From sea level, the oceans are far deeper than thecontinents are tall; examination of the Earth'shypsographic 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 asmountains andtrenches are rare.[3]
| 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 |
.png)
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 and 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%).[5]
The vertical structure of the temperature can be divided into three basic layers, a surfacemixed layer, where gradients are low, athermocline where gradients are high, and a poorly stratified abyss.
In terms of temperature, the ocean's layers are highlylatitude-dependent; thethermocline is pronounced in the tropics, but nonexistent in polar waters (Marshak 2001). Thehalocline usually lies near the surface, where evaporation raises salinity in the tropics, or meltwater dilutes it in polar regions.[5] These variations of salinity and temperature with depth change the density of the seawater, creating thepycnocline.[3]
The temperature of ocean water varies significantly across different regions and depths. As mentioned, the vast majority of ocean water (around 75%) lies between 0° and 5°C, mostly in the deep ocean, where sunlight does not penetrate. The surface layers, however, experience far greater variability. In polar regions, surface temperatures can drop below freezing, while in tropical and subtropical regions, they may reach up to 35°C. This thermal stratification results in a vertical temperature gradient that divides the ocean into distinct layers.
Salinity, a measure of the concentration of dissolved salts in seawater, typically ranges between 34 and 35 parts per thousand (ppt) in most of the world’s oceans. However, localized factors such as evaporation, precipitation, river runoff, and ice formation or melting cause significant variations in salinity. These variations are often most evident in coastal areas and marginal seas.
The salt in the oceans originates from runoff from terrestrial sources as well as hydrothermal vents.[6] It has been estimated that the salinity of oceans was greater in the distant past than it is today.[7]
The combination of temperature and salinity variations leads to changes in seawater density. Seawater density is primarily influenced by both these factors—colder, saltier water is denser than warmer, fresher water. This variation in density creates stratification in the ocean and is key to understanding ocean circulation patterns.
Understanding the complex interactions between temperature, salinity, and density is essential for predicting ocean circulation patterns, climate change effects, and the health of marine ecosystems. These factors also influence marine life, as many species are sensitive to the specific temperature and salinity ranges of their habitats.
Energy for the ocean circulation (and for the atmospheric circulation) comes from solar radiation and gravitational energy from the Sun and Moon.[8] 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 theHadley 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 thethermohaline circulation.
Oceanic currents are largely driven by the surface wind stress; hence the large-scaleatmospheric 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 (theSverdrup balance). The return flow occurs in an intense, narrow, polewardwestern 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 strongAntarctic Circumpolar Current. In the northern hemisphere the land masses prevent this and the ocean circulation is broken into smallergyres in the Atlantic and Pacific basins.
TheCoriolis 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 goesaround 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 seesecondary circulation effects.
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 results in the occurrence of thin, visible stripes, calledwindrows 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 theconvergence zones debris, foam and seaweed accumulates, while at thedivergence 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.
At the ocean-atmosphere interface, the ocean and atmosphere exchange fluxes of heat, moisture and momentum.
The importantheat terms at the surface are the sensible heatflux, the latent heat flux, the incomingsolar 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 amaritime 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 isWestern Europe, which is heated at least in part by thenorth Atlantic drift.
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 windstress on its surface, and this forces large-scale currents in the ocean.
Through the wind stress, the wind generatesocean surface waves; the longer waves have aphase velocity tending towards thewind speed.Momentum of the surface winds is transferred into the energyflux by the ocean surface waves. The increasedroughness of the ocean surface, by the presence of the waves, changes the wind near the surface.
The ocean can gainmoisture fromrainfall, or lose it throughevaporation. Evaporative loss leaves the ocean saltier; theMediterranean andPersian Gulf for example have strong evaporative loss; the resulting plume of dense salty water may be traced through theStraits of Gibraltar into theAtlantic Ocean. At one time, it was believed thatevaporation/precipitation was a major driver of ocean currents; it is now known to be only a very minor factor.
AKelvin wave is anyprogressive wave that is channeled between two boundaries or opposing forces (usually between theCoriolis force and acoastline or theequator). There are two types, coastal and equatorial. Kelvin waves aregravity driven andnon-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 thetrade winds at the beginning of theEl Niño-Southern Oscillation.
Coastal Kelvin waves followshorelines and will always propagate in acounterclockwise direction in theNorthern Hemisphere (with theshoreline to the right of the direction of travel) andclockwise in theSouthern Hemisphere.
Equatorial Kelvin waves propagate to the east in both theNorthern Hemisphere andSouthern Hemisphere, using theequator as aguide.
Kelvin waves are known to have very high speeds, typically around 2–3 meters per second. They havewavelengths of thousands of kilometers andamplitudes in the tens of meters.
Rossby waves, orplanetary waves are huge, slow waves generated in thetroposphere bytemperature differences between theocean and thecontinents. Their majorrestoring force is the change inCoriolis force withlatitude. Their waveamplitudes are usually in the tens of meters and very largewavelengths. They are usually found at low or mid latitudes.
There are two types of Rossby waves,barotropic andbaroclinic. 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 thephase velocity of each individual wave always has a westward component, but thegroup 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.
The interaction of ocean circulation, which serves as a type ofheat pump, and biological effects such as the concentration ofcarbon dioxide can result in globalclimate changes on a time scale of decades. Knownclimate oscillations resulting from these interactions, include thePacific decadal oscillation,North Atlantic oscillation, andArctic oscillation. The oceanic process ofthermohaline circulation is a significant component of heat redistribution across the globe, and changes in this circulation can have major impacts upon the climate.
This is a coupledocean/atmospherewave that circles theSouthern Ocean about every eight years. Since it is a wave-2 phenomenon (there are two peaks and two troughs in alatitude circle) at each fixed point in space a signal with aperiod of four years is seen. The wave moves eastward in the direction of theAntarctic Circumpolar Current.
Among the most importantocean currents are the:
The ocean body surrounding theAntarctic is currently the only continuous body of water where there is a wide latitude band of open water. It interconnects theAtlantic,Pacific andIndian 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.
In theNorwegian Sea evaporative cooling is predominant, and the sinking water mass, theNorth Atlantic Deep Water (NADW), fills the basin and spills southwards through crevasses in thesubmarine sills that connectGreenland,Iceland andBritain. 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 theArctic Ocean Basin into the Pacific, however, is blocked by the narrow shallows of theBering Strait.
Also seemarine geology about that explores thegeology of the ocean floor includingplate tectonics that create deep ocean trenches.
An idealised subtropical ocean basin forced by winds circling around a high pressure (anticyclonic) systems such as the Azores-Bermuda high develops agyre circulation with slow steady flows towards the equator in the interior. As discussed byHenry Stommel, these flows are balanced in the region of the western boundary, where a thin fast polewards flow called awestern boundary current develops. Flow in the real ocean is more complex, but theGulf Stream, Agulhas andKuroshio 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 theOyashio. They are forced by winds circulation around low pressure (cyclonic).
The Gulf Stream, together with its northern extension,North Atlantic Current, is a powerful, warm, and swift Atlantic Ocean current that originates in theGulf 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.
TheKuroshio Current is an ocean current found in the western Pacific Ocean off the east coast ofTaiwan and flowing northeastward pastJapan, where it merges with the easterly drift of theNorth Pacific Current. It is analogous to the Gulf Stream in the Atlantic Ocean, transporting warm, tropical water northward towards the polar region.
Ocean heat flux is a turbulent and complex system which utilizes atmospheric measurement techniques such aseddy covariance to measure the rate of heat transfer expressed in the unit of orpetawatts.[9]Heat flux is the flow of energy per unit of area per unit of time. 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 throughadvection 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.[10] Another example of advection is the nonequatorial Pacific heating which results from subsurface processes related to atmospheric anticlines.[11] Recent warming observations ofAntarctic bottom water in theSouthern Ocean is of concern to ocean scientists because bottom water changes will effect currents, nutrients, and biota elsewhere.[12] The international awareness of global warming has focused scientific research on this topic since the 1988 creation of theIntergovernmental Panel on Climate Change. Improved ocean observation, instrumentation, theory, and funding has increased scientific reporting on regional andglobal issues related to heat.[13]
Tide gauges and satellite altimetry suggest an increase in sea level of 1.5–3 mm/yr over the past 100 years.
TheIPCC predicts that by 2081–2100,global warming will lead to a sea level rise of 260 to 820 mm.[14]
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 theSun andMoon. 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 atidal 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 ofNative Hawaiians tending coastal fishponds.Aia ke ola ka hana meaning . . .Life is in labor.
Tidal resonance occurs in theBay of Fundy since the time it takes for a largewave to travel from the mouth of thebay 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 generatesinternal waves at the tidal frequency, which are known asinternal tides.
A series of surface waves can be generated due to large-scale displacement of the ocean water. These can be caused by sub-marinelandslides, seafloor deformations due toearthquakes, or the impact of a largemeteorite.
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 withwavelengths spanning hundreds of kilometers.
Tsunamis, originally called tidal waves, were renamed because they are not related to the tides. They are regarded asshallow-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 inLituya 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 theSears Tower in Chicago and about 110 m (360 ft) taller than the formerWorld Trade Center in New York.[15]
The wind generates ocean surface waves, which have a large impact onoffshore structures,ships,Coastal erosion andsedimentation, as well asharbours. After their generation by the wind, ocean surface waves can travel (asswell) over long distances.
{{cite book}}: CS1 maint: multiple names: authors list (link)| International | |
|---|---|
| National | |
| Other | |
