Ekman spiral

The Ekman spiral is a structure of currents or winds near a horizontal boundary in which the flow direction rotates as one moves away from the boundary. It derives its name from the Swedish oceanographer Vagn Walfrid Ekman. The deflection of surface currents was first noticed by the Norwegian oceanographer Fridtjof Nansen during the Fram expedition (1893–1896) and the effect was first physically explained by Vagn Walfrid Ekman.[1]

Ekman spirale
Ekman spiral effect.
1. Wind
2. force from above
3. Effective direction of the current
4. Coriolis effect

Theory

The effect is a consequence of the Coriolis effect which subjects moving objects to an apparent force to the right of their direction of motion in the northern hemisphere (and to the left in the Southern Hemisphere). Thus, when a persistent wind blows over an extended area of the ocean surface in the northern hemisphere, it causes a surface current which accelerates in that direction, which then experiences a Coriolis force and acceleration to the right of the wind: the current will turn gradually to the right as it gains speed. As the flow is now somewhat right of the wind, the Coriolis force perpendicular to the flow's motion is now partly directed against the wind. Eventually, the current will reach a top speed when the force of the wind, of the Coriolis effect, and the resistant drag of the subsurface water balance, and the current will flow at a constant speed and direction as long as the wind persists. This surface current drags on the water layer below it, applying a force in its own direction of motion to that layer, repeating the process whereby that layer eventually becomes a steady current even further to the right of the wind, and so on for deeper layers of water, resulting in a continuous rotation (or spiraling) of current direction with changing depth. As depth increases, the force transmitted from the driving wind declines and thus the speed of the resultant steady current decreases, hence the tapered spiral representation in the accompanying diagram. The depth to which the Ekman spiral penetrates is determined by how far turbulent mixing can penetrate over the course of a pendulum day.[2]

The diagram above attempts to show the forces associated with the Ekman spiral as applied to the Northern hemisphere. The force from above is in red (beginning with the wind blowing over the water surface), the Coriolis force (which is shown at right angles to the force from above when it should in fact be at right angles to the actual water flow) is in dark yellow, and the net resultant water movement is in pink, which then becomes the force from above for the layer below it, accounting for the gradual clockwise spiral motion as you move down.

Observation

The first documented observations of an oceanic Ekman spiral were made in the Arctic Ocean from a drifting ice floe in 1958.[3] More recent observations include:

  • SCUBA diving observations during a study of upwelling water transport through a kelp forest on the west coast of South Africa in 1978 [4]
  • The 1980 Mixed Layer Experiment[5]
  • Within the Sargasso Sea during the 1982 Long-Term Upper Ocean Study [6]
  • Within the California Current during the 1993 Eastern Boundary Current experiment [7]
  • Within the Drake Passage region of the Southern Ocean [8][9]
  • North of the Kerguelan Plateau during the 2008 SOFINE experiment [10]

Common to several of these observations spirals were found to be 'compressed', displaying larger estimates of eddy viscosity when considering the rate of rotation with depth than the eddy viscosity derived from considering the rate of decay of speed.[6][7][8] Though in the Southern Ocean the 'compression', or spiral flattening effect disappeared when new data permitted a more careful treatment of the effect of geostrophic shear.[9][10]

The classic Ekman spiral has been observed under sea ice,[3] but observations remain rare in open-ocean conditions. This is due both to the fact that the turbulent mixing in the surface layer of the ocean has a strong diurnal cycle and to the fact that surface waves can destabilize the Ekman spiral. Ekman spirals are also found in the atmosphere. Surface winds in the Northern Hemisphere tend to blow to the left of winds aloft.

See also

Notes

  1. ^ Ekman, V. W. 1905. On the influence of the Earth's rotation on ocean currents. Arch. Math. Astron. Phys., 2, 1-52. [1]
  2. ^ "AMS Glossary". Archived from the original on 2007-08-17. Retrieved 2007-06-28.
  3. ^ a b Hunkins, K. (1966). "Ekman drift currents in the Arctic Ocean". Deep-Sea Research. 13 (4): 607–620. Bibcode:1966DSROA..13..607H. doi:10.1016/0011-7471(66)90592-4.
  4. ^ Field, J. G., C. L. Griffiths, E. A. S. Linley, P. Zoutendyk and R. Carter (1981). Wind-induced water movements in a Benguela kelp bed. Coastal Upwelling. F. A. Richards (Ed.), Washington D.C., American Geophysical Union: 507-513. ISBN 0-87590-250-2
  5. ^ Davis, R.E.; de Szoeke, R.; Niiler., P. (1981). "Part II: Modelling the mixed layer response". Deep-Sea Research. 28 (12): 1453–1475. Bibcode:1981DSRI...28.1453D. doi:10.1016/0198-0149(81)90092-3.
  6. ^ a b Price, J.F.; Weller, R.A.; Schudlich, R.R. (1987). "Wind-Driven Ocean Currents and Ekman Transport". Science. 238 (4833): 1534–1538. Bibcode:1987Sci...238.1534P. doi:10.1126/science.238.4833.1534. PMID 17784291.
  7. ^ a b Chereskin, T.K. (1995). "Direct evidence for an Ekman balance in the California Current". Journal of Geophysical Research. 100 (C9): 18261–18269. Bibcode:1995JGR...10018261C. doi:10.1029/95JC02182.
  8. ^ a b Lenn, Y.-D.; Chereskin, T.K. (2009). "Observation of Ekman Currents in the Southern Ocean". Journal of Physical Oceanography. 39 (3): 768–779. Bibcode:2009JPO....39..768L. doi:10.1175/2008jpo3943.1.
  9. ^ a b Polton, J.A.; Lenn, Y.-D.; Elipot, S.; Chereskin, T.K.; Sprintall, J. (2013). "Can Drake Passage Observations Match Ekman's Classic Theory?". Journal of Physical Oceanography. 43 (8): 1733–1740. Bibcode:2013JPO....43.1733P. doi:10.1175/JPO-D-13-034.1.
  10. ^ a b Roach, C.J.; Phillips, H.E.; Bindoff, N.L.; Rintoul, S.R. (2015). "Detecting and Characterizing Ekman Currents in the Southern Ocean". Journal of Physical Oceanography. 45 (5): 1205–1223. Bibcode:2015JPO....45.1205R. doi:10.1175/JPO-D-14-0115.1.

References

  • AMS Glossary, mathematical description
  • A. Gnanadesikan and R.A. Weller, 1995 · "Structure and instability of the Ekman spiral in the presence of surface gravity waves" · Journal of Physical Oceanography  25(12), pp. 3148–3171.
  • J.F. Price, R.A. Weller and R. Pinkel, 1986 · "Diurnal cycling: Observations and models of the upper ocean response to diurnal heating, cooling and wind mixing" · Journal of Geophysical Research  91, pp. 8411–8427.
  • J.G. Richman, R. deSzoeke and R.E. Davis, 1987 · "Measurements of near-surface shear in the ocean" · Journal of Geophysical Research  92, pp. 2851–2858.
  • Field, J. G., C. L. Griffiths, E. A. S. Linley, P. Zoutendyk and R. Carter, 1981 Wind-induced water movements in a Benguela kelp bed. Coastal Upwelling. F. A. Richards (Ed.), Washington D.C., American Geophysical Union: 507-513. ISBN 0-87590-250-2
Bahama Banks

The Bahama Banks are the submerged carbonate platforms that make up much of the Bahama Archipelago. The term is usually applied in referring to either the Great Bahama Bank around Andros Island, or the Little Bahama Bank of Grand Bahama Island and Great Abaco, which are the largest of the platforms, and the Cay Sal Bank north of Cuba. The islands of these banks are politically part of the Bahamas. Other banks are the three banks of the Turks and Caicos Islands, namely the Caicos Bank of the Caicos Islands, the bank of the Turks Islands, and wholly submerged Mouchoir Bank. Further southeast are the equally wholly submerged Silver Bank and Navidad Bank north of the Dominican Republic.

Dead water

Dead water is the nautical term for a phenomenon which can occur when a layer of fresh or brackish water rest on top of denser salt water, without the two layers mixing. A ship powered by direct thrust under the waterline (such as a propeller), traveling in such conditions may be hard to maneuver or can even slow down almost to a standstill. Much of the energy from the ship's propeller only results in waves and turbulence between the two layers of water, leaving a ship capable of traveling at perhaps as little as 20% of its normal speed.The phenomenon was first described by Fridtjof Nansen, the Norwegian Arctic explorer.

Nansen wrote the following from his ship Fram in August 1893 in the Nordenskiöld Archipelago near the Taymyr Peninsula:

"When caught in dead water Fram appeared to be held back, as if by some mysterious force, and she did not always answer the helm. In calm weather, with a light cargo, Fram was capable of 6 to 7 knots. When in dead water she was unable to make 1.5 knots. We made loops in our course, turned sometimes right around, tried all sorts of antics to get clear of it, but to very little purpose."This phenomenon is observable where glacier runoff flows into salt water without much mixing, such as in fjords.

Ekman

Ekman is a surname of Swedish origin which may refer to

Ekman layer

The Ekman layer is the layer in a fluid where there is a force balance between pressure gradient force, Coriolis force and turbulent drag. It was first described by Vagn Walfrid Ekman. Ekman layers occur both in the atmosphere and in the ocean.

There are two types of Ekman layers. The first type occurs at the surface of the ocean and is forced by surface winds, which act as a drag on the surface of the ocean. The second type occurs at the bottom of the atmosphere and ocean, where frictional forces are associated with flow over rough surfaces.

Ekman transport

Ekman transport, part of Ekman motion theory first investigated in 1902 by Vagn Walfrid Ekman. Winds are the main source of energy for ocean circulation, and Ekman Transport is a component of wind-driven ocean current.. Ekman transport occurs when ocean surface waters are influenced by the friction force acting on them via the wind. As the wind blows it casts a friction force on the ocean surface that drags the upper 10-100m of the water column with it.. However, due to the influence of the Coriolis effect, the ocean water moves at a 90° angle from the direction of the surface wind.. The direction of transport is dependent on the hemisphere: in the northern hemisphere, transport occurs at 90° clockwise from wind direction, while in the southern hemisphere it occurs at a 90° counterclockwise.. This phenomenon was first noted by Fridtjof Nansen, who recorded that ice transport appeared to occur at an angle to the wind direction during his Arctic expedition during the 1890s.. Ekman transport has significant impacts on the biogeochemical properties of the world’s oceans. This is because they lead to upwelling (Ekman suction), downwelling (Ekman pumping) in order to obey mass conservation laws. Mass conservation, in reference to Ekman transfer, requires that any water displaced within an area must be replenished, this can be done by either Ekman suction or Ekman pumping depending on wind patterns..

Ekman velocity

In oceanography, Ekman velocity – also referred as a kind of the residual ageostropic velocity as it derivates from geostrophy – is part of the total horizontal velocity (u) in the upper layer of water of the open ocean. This velocity, caused by winds blowing over the surface of the ocean, is such that the Coriolis force on this layer is balanced by the force of the wind.

Typically, it takes about two days for the Ekman velocity to develop before it is directed at right angles to the wind. The Ekman velocity is named after the Swedish oceanographer Vagn Walfrid Ekman (1874–1954).

Ekman water bottle

The Ekman water bottle is a sea water temperature sample device. The cylinder is dropped at the desired depth, the trap door below is opened to let the water enter and then closed tightly. This can be repeated at different depths as each sample goes to a different chamber of the insulated bottle.

It was used for greater depths during the research cruises of 1903 and 1904. The first instruments made, however, were too delicate; after being used for some time, the brass rods which press the lids towards both ends of the cylinder and close the water-bottle, became bent and therefore did not work sufficiently well. For this reason the instruments had to be frequently tested and repaired. As they are now made, they work very well and are very easily handled.

Geophysical fluid dynamics

Geophysical fluid dynamics, in its broadest meaning, refers to the fluid dynamics of naturally occurring flows, such as lava flows, oceans, and planetary atmospheres, on Earth and other planets.Two physical features that are common to many of the phenomena studied in geophysical fluid dynamics are rotation of the fluid due to the planetary rotation and stratification (layering). The applications of geophysical fluid dynamics do not generally include the circulation of the mantle, which is the subject of geodynamics, or fluid phenomena in the magnetosphere.

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This is a list of Wave topics.

List of Lund University people

This is a list of notable people affiliated with Lund University, either as students or as researchers and academic teachers (or both). Lund University, located in the town of Lund in Skåne, Sweden, was founded in 1666.

Ocean current

An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, shoreline configurations, and interactions with other currents influence a current's direction and strength. Ocean currents are primarily horizontal water movements.

An ocean current flows for great distances and together they create the global conveyor belt, which plays a dominant role in determining the climate of many of Earth’s regions. More specifically, ocean currents influence the temperature of the regions through which they travel. For example, warm currents traveling along more temperate coasts increase the temperature of the area by warming the sea breezes that blow over them. Perhaps the most striking example is the Gulf Stream, which makes northwest Europe much more temperate than any other region at the same latitude. Other example is Lima, Peru, where the climate is cooler, being sub-tropical, than the tropical latitudes in which the area is located, due to the effect of the Humboldt Current.

Oceanic plateau

An oceanic or submarine plateau is a large, relatively flat elevation that is higher than the surrounding relief with one or more relatively steep sides.There are 184 oceanic plateaus covering an area of 18,486,600 km2 (7,137,700 sq mi), or about 5.11% of the oceans. The South Pacific region around Australia and New Zealand contains the greatest number of oceanic plateaus (see map).

Oceanic plateaus produced by large igneous provinces are often associated with hotspots, mantle plumes, and volcanic islands — such as Iceland, Hawaii, Cape Verde, and Kerguelen. The three largest plateaus, the Caribbean, Ontong Java, and Mid-Pacific Mountains, are located on thermal swells. Other oceanic plateaus, however, are made of rifted continental crust, for example Falkland Plateau, Lord Howe Rise, and parts of Kerguelen, Seychelles, and Arctic ridges.

Plateaus formed by large igneous provinces were formed by the equivalent of continental flood basalts such as the Deccan Traps in India and the Snake River Plain in the United States.

In contrast to continental flood basalts, most igneous oceanic plateaus erupt through young and thin (6–7 km (3.7–4.3 mi)) mafic or ultra-mafic crust and are therefore uncontaminated by felsic crust and representative for their mantle sources.

These plateaus often rise 2–3 km (1.2–1.9 mi) above the surrounding ocean floor and are more buoyant than oceanic crust. They therefore tend to withstand subduction, more-so when thick and when reaching subduction zones shortly after their formations. As a consequence, they tend to "dock" to continental margins and be preserved as accreted terranes. Such terranes are often better preserved than the exposed parts of continental flood basalts and are therefore a better record of large-scale volcanic eruptions throughout Earth's history. This "docking" also means that oceanic plateaus are important contributors to the growth of continental crust. Their formations often had a dramatic impact on global climate, such as the most recent plateaus formed, the three, large, Cretaceous oceanic plateaus in the Pacific and Indian Ocean: Ontong Java, Kerguelen, and Caribbean.

Storm surge

A storm surge, storm flood, tidal surge or storm tide is a coastal flood or tsunami-like phenomenon of rising water commonly associated with low pressure weather systems (such as tropical cyclones and strong extratropical cyclones). Its severity is affected by the shallowness and orientation of the water body relative to storm path, as well as the timing of tides. Most casualties during tropical cyclones occur as the result of storm surges. It is a measure of the rise of water beyond what would be expected by the normal movement related to tides.

The two main meteorological factors contributing to a storm surge are a long fetch of winds spiraling inward toward the storm, and a low-pressure-induced dome of water drawn up under and trailing the storm's center.

Subsurface currents

A subsurface current is an oceanic current that runs beneath surface currents. Examples include the Equatorial Undercurrents of the Pacific, Atlantic, and Indian Oceans, the California Undercurrent, and the Agulhas Undercurrent, the deep thermohaline circulation in the Atlantic, and bottom gravity currents near Antarctica. The forcing mechanisms vary for these different types of subsurface currents.

Undersea mountain range

Undersea mountain ranges are mountain ranges that are mostly or entirely underwater, and specifically under the surface of an ocean. If originated from current tectonic forces, they are often referred to as a mid-ocean ridge. In contrast, if formed by past above-water volcanism, they are known as a seamount chain. The largest and best known undersea mountain range is a mid-ocean ridge, the Mid-Atlantic Ridge. It has been observed that, "similar to those on land, the undersea mountain ranges are the loci of frequent volcanic and earthquake activity".

Upwelling

Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The nutrient-rich upwelled water stimulates the growth and reproduction of primary producers such as phytoplankton. Due to the biomass of phytoplankton and presence of cool water in these regions, upwelling zones can be identified by cool sea surface temperatures (SST) and high concentrations of chlorophyll-a.The increased availability of nutrients in upwelling regions results in high levels of primary production and thus fishery production. Approximately 25% of the total global marine fish catches come from five upwellings that occupy only 5% of the total ocean area. Upwellings that are driven by coastal currents or diverging open ocean have the greatest impact on nutrient-enriched waters and global fishery yields.

Vagn Walfrid Ekman

Vagn Walfrid Ekman (3 May 1874 – 9 March 1954) was a Swedish oceanographer.Born in Stockholm to Fredrik Laurentz Ekman, himself an oceanographer, he became committed to oceanography while studying physics at the University of Uppsala and, in particular, on hearing Vilhelm Bjerknes lecture on fluid dynamics.

During the expedition of the Fram, Fridtjof Nansen had observed that icebergs tend to drift not in the direction of the prevailing wind but at an angle of 20°-40° to the right. Bjerknes invited Ekman, still a student, to investigate the problem. Later, in 1905, Ekman published his theory of the Ekman spiral which explains the phenomenon in terms of the balance between frictional effects in the ocean and the Coriolis force, which arises from moving objects in a rotating environment, like planetary rotation.

On completing his doctorate in Uppsala in 1902, Ekman joined the International Laboratory for Oceanographic Research, Oslo where he worked for seven years, not only extending his theoretical work but also developing experimental techniques and instruments such as the Ekman current meter and Ekman water bottle.

From 1910 to 1939 he continued his theoretical and experimental work at the University of Lund, where he was professor of mechanics and mathematical physics. He was elected a member of the Royal Swedish Academy of Sciences in 1935.

A gifted amateur bass singer, pianist, and composer, he continued working right up to his death in Gostad, near Stockaryd, Sweden.

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The wave base, in physical oceanography, is the maximum depth at which a water wave's passage causes significant water motion. For water depths deeper than the wave base, bottom sediments and the seafloor are no longer stirred by the wave motion above.

Weddell Polynya

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