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| [[File:Ekman layer.jpg|thumb|350px|The Ekman layer is the layer in a fluid where the flow is the result of a balance between pressure gradient, Coriolis and turbulent drag forces. In the picture above, the wind blowing North creates a surface stress and a resulting [[Ekman spiral]] is found below it in the column of water.]]
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| The '''Ekman layer''' is the layer in a [[fluid]] where there is a [[force]] balance between [[pressure gradient force]], [[Coriolis force]] and [[Drag (physics)|turbulent drag]]. It was first described by [[Vagn Walfrid Ekman]].
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| ==History==
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| Ekman developed the theory of the Ekman layer after [[Fridtjof Nansen]] observed that [[iceberg|ice]] drifts at an angle of 20°-40° to the right of the [[wind|prevailing wind]] direction while on an [[Arctic]] expedition aboard the [[Fram]]. Nansen asked his colleague, [[Vilhelm Bjerknes]] to set one of his students upon study of the problem. Bjerknes tapped Ekman, who presented his results in 1902 as his [[doctoral thesis]].<ref name="Cushman">{{cite book |last=Cushman-Roisin |first=Benoit |authorlink=http://engineering.dartmouth.edu/faculty/regular/benoitroisin.html |title=Introduction to Geophysical Fluid Dynamics |edition=1st |year=1994 |publisher=Prentice Hall |pages=76–77 |chapter=Chapter 5 - The Ekman Layer |isbn=0-13-353301-8 }}</ref>
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| ==Mathematical formulation==
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| The mathematical formulation of the Ekman layer can be found by assuming a neutrally stratified fluid, with horizontal momentum in balance between the forces of pressure gradient, Coriolis and turbulent drag.
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| :<math>
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| \begin{align}
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| -fv &= -\frac{1}{\rho_o} \frac{\part p}{\part x}+K_m \frac{\part^2 u}{\part z^2}, \\
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| fu &= -\frac{1}{\rho_o} \frac{\part p}{\part y}+K_m \frac{\part^2 v}{\part z^2}, \\
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| 0 &= -\frac{1}{\rho_o} \frac{\part p}{\part z},
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| \end{align}
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| </math>
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| where <math>\ u </math> and <math>\ v </math> are the velocities in the <math>\ x </math> and <math>\ y </math> directions, respectively, <math>\ f </math> is the local [[Coriolis parameter]], and <math>\ K_m </math> is the diffusive eddy viscosity, which can be derived using [[mixing length theory]]. | |
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| ===Boundary conditions===
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| There are many regions where an Ekman layer is theoretically plausible; they include the bottom of the atmosphere, near the surface of the earth and ocean, the bottom of the ocean, near the [[sea floor]] and at the top of the ocean, near the air-water interface.
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| Each of the different regions will have different [[boundary conditions]]. We will consider boundary conditions of the Ekman layer in the upper ocean:<ref name="Vallis">{{cite book |last=Vallis |first=Geoffrey K. |authorlink=http://www.princeton.edu/~gkv/ |title=Atmospheric and Oceanic Fluid Dynamics |edition=1st |year=2006 |publisher=Cambridge University Press |location=Cambridge, UK |pages=112–113 |chapter=Chapter 2 – Effects of Rotation and Stratification |isbn=0-521-84969-1 }}</ref>
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| :<math>
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| \text{at } z = 0 :
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| \quad A \frac{\part u}{\part z} = \tau^x \quad \text{and}
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| \quad A \frac{\part v}{\part z} = \tau^y,
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| </math>
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| where <math>\ \tau^x</math> and <math>\ \tau^y</math> are the components of the surface stress, <math>\ \tau </math>, of the wind field or ice layer at the top of the ocean and <math>\ u_g </math> and <math>\ v_g </math> are the [[geostrophic]] flows in the <math>\ x</math> and <math>\ y</math> directions – as <math>\ z \to \infty : u \to u_g, v \to v_g.</math>
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| ===Solution===
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| [[Image:EkmanSpiral.svg|thumb|right|Three views of the wind-driven Ekman layer at the surface of the ocean in the Northern Hemisphere. The geostrophic velocity is zero in this example.]]
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| These differential equations can be solved to find:
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| :<math>
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| \begin{align}
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| u &= u_g + \frac{\sqrt{2}}{fd}e^{z/d}\left [\tau^x \cos(z/d - \pi/4) - \tau^y \sin(z/d - \pi/4)\right ],
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| \\
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| v &= v_g + \frac{\sqrt{2}}{fd}e^{z/d}\left [\tau^x \sin(z/d - \pi/4) + \tau^y \cos(z/d - \pi/4)\right ],
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| \\
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| d &= \sqrt{2 K_m/f}.
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| \end{align}
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| </math>
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| This variation of horizontal velocity with depth (<math>-z</math>) is referred to as the [[Ekman spiral]], diagrammed above and at right.
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| By applying the continuity equation we can have the vertical velocity as following
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| :<math>w = \frac{1}{f\rho_o}\left [-\left (\frac{\partial \tau^x}{\partial x} + \frac{\partial \tau^y}{\partial y} \right )e^{z/d}\sin(z/d) + \left (\frac{\partial \tau^y}{\partial x} - \frac{\partial \tau^x}{\partial y} \right )(1-e^{z/d}\cos(z/d))\right ].</math>
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| Note that when vertically integrated the volume transport associated with the Ekman spiral is to the right of the wind direction in the Northern Hemisphere.
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| ==Experimental observations of the Ekman layer==
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| There is much difficulty associated with observing the Ekman layer for two main reasons: the theory is too simplistic as it assumes a constant eddy viscosity, which Ekman himself anticipated,<ref name="Ekman">{{cite journal | last = Ekman | first = V.W. | title = On the influence of the earth's rotation on ocean currents | journal = Ark. Mat. Astron. Fys. | volume = 2 | issue = 11 | pages = 1–52 | year = 1905}}</ref> saying
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| {{cquote|It is obvious that <math>\ \left[\nu \right]</math> cannot generally be regarded as a constant when the density of water is not uniform within the region considered}}
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| and because it is difficult to design instruments with great enough sensitivity to observe the velocity profile in the ocean.
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| ===Laboratory demonstrations===
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| The bottom Ekman layer can readily be observed in a rotating cylindrical tank of water by dropping in dye and changing the rotation rate slightly.[http://paoc.mit.edu/labguide/ekman.html] Surface Ekman layers can also be observed in rotating tanks.[http://dennou-k.gaia.h.kyoto-u.ac.jp/library/gfd_exp/exp_e/exp/ek/1/app.htm]
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| ===In the atmosphere===
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| In the atmosphere, the Ekman solution generally overstates the magnitude of the horizontal wind field because it does not account for the velocity shear in the [[surface layer]]. Splitting the boundary layer into the surface layer and the Ekman layer generally yields more accurate results.<ref name="Holton">{{cite book |last=Holton |first=James R. |authorlink=http://books.google.com/books?hl=en&id=fhW5oDv3EPsC |title=Dynamic Meteorology |edition=4th |series=International Geophysics Series |volume=88 |year=2004 |publisher=Elsevier Academic Press |location=Burlington, MA |pages=129–130 |chapter=Chapter 5 – The Planetary Boundary Layer |isbn=0-12-354015-1 }}</ref>
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| ===In the ocean===
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| The Ekman layer, with its distinguishing feature the Ekman spiral, is rarely observed in the ocean. The Ekman layer near the surface of the ocean extends only about 10 – 20 meters deep,<ref name="Holton" /> and instrumentation sensitive enough to observe a velocity profile in such a shallow depth has only been available since around 1980.<ref name="Vallis" /> Also, [[wind waves]] modify the flow near the surface, and make observations close to the surface rather difficult.<ref name = "Santala">{{cite journal | last1 = Santala | first1 = M. J.| first2 = E. A. | last2 = Terray| title = A technique for making unbiased estimates of current shear from a wave-follower | journal = Deep-Sea Res. | volume = 39 | pages = 607–622 | year = 1992 | doi = 10.1016/0198-0149(92)90091-7 | issue = 3–4 |bibcode = 1992DSRI...39..607S }}</ref>
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| ====Instrumentation====
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| Observations of the Ekman layer have only been possible since the development of robust surface moorings and sensitive current meters. Ekman himself developed a current meter to observe the spiral that bears his name, but was not successful.<ref name = "Rudnick">{{cite journal | last = Rudnick | first = Daniel | authorlink = chowder.ucsd.edu/ | title = Observations of Momentum Transfer in the Upper Ocean: Did Ekman Get It Right? | journal = Near-Boundary Processes and their Parameterization | publisher = School of Ocean and Earth Science and Technology | location = Manoa, Hawaii | year = 2003}}</ref>
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| The Vector Measuring Current Meter <ref name = "Weller">{{cite journal | last = Weller | first = R.A. | coauthors = Davis, R.E. | title = A vector-measuring current meter | journal = Deep-Sea Res. | volume = 27 | pages = 565–582 | year = 1980 | doi = 10.1016/0198-0149(80)90041-2 | issue = 7|bibcode = 1980DSRI...27..565W }}</ref> and the [[Acoustic Doppler Current Profiler]] are both used to measure current.
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| ====Observations====
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| The first documented observations of an Ekman-like spiral were made in the Arctic Ocean from a drifting ice flow in 1958.<ref name = "Hunkins">{{cite journal | last1 = Hunkins | first1 = K. | title = Ekman drift currents in the Arctic Ocean | journal = Deep-Sea Res. | volume = 13 | pages = 607–620 | year = 1966 }}</ref> More recent observations include:
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| * The 1980 [[Mixed Layer Experiment]]<ref name = "Davis">{{cite journal | last1 = Davis | first1 = R.E. | first2 = R. | last2 = de Szoeke | first3 = P. | last3 = Niiler. | title = Part II: Modelling the mixed layer response | journal = Deep-Sea Res. | volume = 28 | pages = 1453–1475 | year = 1981 | doi = 10.1016/0198-0149(81)90092-3 | issue = 12|bibcode = 1981DSRI...28.1453D }}</ref>
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| * Within the Sargasso Sea during the 1982 Long Term Upper Ocean Study <ref name = "Price">{{cite journal | last1 = Price | first1 = J.F. | last2= Weller | first2= R.A.| last3= Schudlich|first3=R.R.| title = Wind-Driven Ocean Currents and Ekman Transport | journal = Science | volume = 238 | pages = 1534–1538 | year = 1987 |bibcode = 1987Sci...238.1534P |doi = 10.1126/science.238.4833.1534 }}</ref>
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| * Within the California Current during the 1993 Eastern Boundary Current experiment <ref name = "Chereskin">{{cite journal|last=Chereskin|first=T.K.|title=Direct evidence for an Ekman balance in the California Current|journal=Journal of Geophysical Research|year=1995|volume=100|pages=18261–18269|bibcode = 1995JGR...10018261C |doi = 10.1029/95JC02182 }}</ref>
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| * Within the Drake Passage region of the Southern Ocean <ref name = "Lenn">{{cite journal|last1=Lenn|first1=Y|last2=Chereskin|first2=T.K.|title=Observation of Ekman Currents in the Southern Ocean|journal=Journal Of Physical Oceanograph|year=2009|volume=39|pages=768–779}}</ref>
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| *North of the Kerguelan Plateau during the 2008 SOFINE experiment <ref name = "Roach">{{cite journal|last=Roach|first=C.J.|last2= Phillips|first2=H.E.|last3= Bindoff|first3=N.L.|last4= Rintoul|first4=S.R.|title=Anomalous Ekman Transport Near Kerguelen Island|journal=Proceedings of the 18th Australasian Fluid Mechanics Conference|date=2012|year=2012}}</ref>
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| 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.<ref name = "Price" /><ref name = "Chereskin" /><ref name = "Lenn" /><ref name = "Roach" />
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| ==See also==
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| *[[Ekman spiral]]
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| *[[Ekman transport]]
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| *[[Tea leaf paradox]]
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| ==References==
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| {{reflist}}
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| == External links ==
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| *[http://paoc.mit.edu/labguide/ekman.html Bottom Ekman layer lab demonstration]
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| *[http://dennou-k.gaia.h.kyoto-u.ac.jp/library/gfd_exp/exp_e/exp/ek/1/app.htm Surface Ekman layer lab demonstration]
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| {{physical oceanography}}
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| [[Category:Boundary layer meteorology]]
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| [[Category:Oceanography]]
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