Physical oceanography
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Image:World11.jpg 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 five sub-domains into which oceanography is divided; the other fields being biological, chemical, geological and meteorologic oceanography.
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, geographers and others interested in the physical world. The uniqueness of the world is largely derived from the presence of oceans.
Roughly 97% of the planet's water is in its oceans, and it is the oceans that are the source of the vast majority of water vapor that eventually falls as rain or snow on the continents (Pinet 1996),(Hamblin 1998). The tremendous heat capacity of the oceans moderates the planet's climate, and its absorption of various gases affects the composition of the atmosphere (Hamblin 1998). 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 (Hamblin 1998). An Earth without oceans would truly be unrecognizable.
Vertical and horizontal dimensions
Image:Atlantic-trench.JPG The oceans are far deeper than the continents are tall; the average elevation of Earth's landmasses is only 840 meters, while the ocean's average depth is 3800 meters (Way, Hypsographic curve). Though this apparent discrepency is great, for both land and sea, the respective extremes such as mountains and trenches are rare (Pinet 1996).
| Body | Area (106km2) | Volume (106km3) | Mean depth (m) | Maximum (m) |
| Pacific Ocean | 165.2 | 707.6 | 4282 | -10911 |
| 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
Because 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 is true for salinity, with the same percentage falling between 34-35 ppt (3.4-3.5%).(Pinet 1996) There is still quite a bit of variation, however. Surface temperatures can range from freezing at the poles to 35°C in restricted tropical seas, while salinity can vary from 10 to 41 ppt (1.0-4.1%).(Marshak 2001)
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) Similarly, the halocline is limited to shallow waters, where evaporation raises salinity in the tropics, or meltwater dilutes it in polar regions.(Marshak 2001)
These variations of salinity and temperature with depth change the density of the seawater, creating the pycnocline.(Pinet 1996)
Density
Image:Thermohaline circulation.png
- Thermohaline circulation (Density-driven)
Also see:
The general circulation of the ocean
The ultimate energy source for the ocean circulation, like 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 in the ocean.
The atmosphere is heated from below, which tends 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 waters where cold salty waters sink in fairly restricted areas: the beginnings 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 circulatiion leads to Easterly winds in the tropics and Westerlies in mid-latitudes, which tends to force a corresponding gyre in the major ocean basins. 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 force
Image:Isabel 091503bm.jpg The Coriolis effect results in a deflection of fluid flows (to the right in the northern hemisphere). This has profound effects on the flow of the oceans, for example the sharp western boundary currents which are absent on eastern boundaries. Also see secondary circulation effects.
This is the key piece of physics that gives rise to 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.
Pressure-driven flows
Angular momentum and the ocean circulation
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 (infra red) 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 released 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.
Moisture
The ocean can gain moisture from rainfall, or lose it through evaporation. Evaporative loss leaves the ocean saltier; the mediterranean for example has strong evaporative losses; the resulting plume of dense salty water may be traced through the Straits of Gibraltar into the Atlantic. 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.
Equatorial effects
Planetary waves in the ocean
Kelvin Waves
Rossby Waves
Climate variability
Image:El-nino.gif 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
Template:Main La Niña
The Walker circulation is seen at the surface as easterly trade winds which move water and air warmed by the sun towards the west. This also creates ocean upwelling off the coasts of Peru and Ecuador and brings nutrient-rich cold water to the surface, increasing fishing stocks.
The western side of the equatorial Pacific is characterized by warm, wet low pressure weather as the collected moisture is dumped in the form of typhoons and thunderstorms. The ocean is some 60 cm higher in the eastern Pacific as the result of this motion.
The water and air are returned to the east. Both are now much cooler, and the air is much drier. An El Niño episode is characterised by a breakdown of this water and air cycle, resulting in relatively warm water and moist air in the eastern Pacific.
In the Pacific, La Niña is characterized by unusually cold ocean temperatures in the eastern equatorial Pacific, compared to El Niño, which is characterized by unusually warm ocean temperatures in the same area.
Antarctic Circumpolar Wave
Template:Main 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
These global thermodynamic forces drive ocean currents:
- Antarctic Circumpolar Current
- Deep ocean (density-driven)
- Western boundary currents
Antarctic Circumpolar Current
The ocean body surrounding the Antarctic is currently the only continuous body of water to circumnavigate the globe about the polar axis. 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 very slowly into the deep abyssal plains of the Atlantic, always in a southerly direction. 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 ocean basin, spun up from rest by a cyclonic wind stress, acquires a gyre circulation with slow steady flows everywhere except 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 also exist, e.g. the East Greenland current.
Gulf stream
The Gulf Stream, together with its northern extension, North Atlantic Drift, 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.
Overflows
Ocean eddies
Coastal and nearshore processes
Modeling the ocean general circulation
Oceanic heat flux and the climate connection
Heat storage
Sea level change
Template:Main 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
Template:Main 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 livlihood of Native Hawaiians tending coastal fishponds. Aia ke ola ka hana meaning . . . Life is in labor.
Image:Wpdms nasa topo bay of fundy.jpg
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.
Tsunamis
Template:Main A series of surface waves can be generated due to large-scale displacement of the ocean water. These can be caused sub-marine land slips, 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.
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.
References
- Gill, Adrian E. (1982) Atmosphere-Ocean Dynamics, Academic Press, ISBN 0-12-283520-0.
- Hamblin, W. Kenneth and Eric H. Christiansen (1998)
Earth's Dynamic Systems, 8th ed., Upper Saddle River: Prentice-Hall ISBN 0130183717 (8th ed.)
- Marshak, Stephen. (2001) Earth: Portrait of a Planet, New York: W.W. Norton & Company, ISBN 0393974235
- Maury, Matthew F. (1855) The Physical Geography of the Seas and Its Meteorology.
- Pinet, Paul R. (1996) Invitation to Oceanography, St. Paul, MN: West Publishing Co., ISBN 0763721360 (3rd ed.)
- Way, John H., Hypsographic curve, http://www.lhup.edu/jway/101/101.sg/hypsographic_curve.htm Accessed Jan. 10, 2006
- Wyban, Carol Araki (1992) Tide and Current: Fishponds of Hawai'I University of Hawaii Press :: ISBN 0-82481-396-0
See also
External links
- NASA Oceanography
- Ocean World (digital book)
- Pacific Disaster Center
- Pacific Tsunami Museum Hilo, Hawaii
- Science of Tsunami Hazards (journal)ko:물리해양학