1 / 34

400 likes | 891 Vues

Ekman Transport. Ekman transport is the direct wind driven transport of seawater Boundary layer process Steady balance among the wind stress, vertical eddy viscosity & Coriolis forces Story starts with Fridtjof Nansen [1898]. Fridtjof Nansen. One of the first scientist-explorers

Télécharger la présentation
## Ekman Transport

**An Image/Link below is provided (as is) to download presentation**
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.
Content is provided to you AS IS for your information and personal use only.
Download presentation by click this link.
While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

**Ekman Transport**• Ekman transport is the direct wind driven transport of seawater • Boundary layer process • Steady balance among the wind stress, vertical eddy viscosity & Coriolis forces • Story starts with Fridtjof Nansen [1898]**Fridtjof Nansen**• One of the first scientist-explorers • A true pioneer in oceanography • Later, dedicated life to refugee issues • Won Nobel Peace Prize in 1922**Nansen’s Fram**• Nansen built the Fram to reach North Pole • Unique design to be locked in the ice • Idea was to lock ship in the ice & wait • Once close, dog team set out to NP**Ekman Transport**• Nansen noticed that movement of the ice-locked ship was 20-40o to right of the wind • Nansen figured this was due to a steady balance of friction, wind stress & Coriolis forces • Ekman did the math**Ekman Transport**Motion is to the right of the wind**Ekman Transport**• The ocean is more like a layer cake • A layer is accelerated by the one above it & slowed by the one beneath it • Top layer is driven by tw • Transport of momentum into interior is inefficient**Ekman Spiral**• Top layer balance of tw, friction & Coriolis • Layer 2 dragged forward by layer 1 & behind by layer 3 • Etc.**Ekman Spirals**• Ekman found an exact solution to the structure of an Ekman Spiral • Holds for a frictionally controlled upper layer called the Ekman layer • The details of the spiral do not turn out to be important**Ekman Layer**• Depth of frictional influence defines the Ekman layer • Typically 20 to 80 m thick • depends on Az, latitude, tw • Boundary layer process • Typical 1% of ocean depth (a 50 m Ekman layer over a 5000 m ocean)**Ekman Transport**• Balance between wind stress & Coriolis force for an Ekman layer • Coriolis force per unit mass = f u • u = velocity • f = Coriolis parameter = 2 W sin f W = 7.29x10-5 s-1 & f = latitude • Coriolis force acts to right of motion**Ekman Transport**Coriolis = wind stress f ue = tw / (r D) Ekman velocity = ue ue = tw / (r f D) Ekman transport = Qe Qe = tw / (r f) = [m2 s] = [m3 s-1 m-1] (Volume transport per length of fetch)**Ekman Transport**• Ekman transport describes the direct wind-driven circulation • Only need to know tw & f (latitude) • Ekman current will be right (left) of wind in the northern (southern) hemisphere • Simple & robust diagnostic calculation**Current Meters**Vector Measuring Vector Averaging Current Meter Current Meter**Ekman Transport Works!!**• Averaged the velocity profile in the downwind coordinates • Subtracted off the “deep” currents (50 m) • Compared with a model that takes into account changes in upper layer stratification • Price et al. [1987] Science**Ekman Transport Works!!**theory observerd**Ekman Transport Works!!**• LOTUS data reproduces Ekman spiral & quantitatively predicts transport • Details are somewhat different due to diurnal changes of stratification near the sea surface**Inertia Currents**• Ekman dynamics are for steady-state conditions • What happens if the wind stops? • Ekman dynamics balance wind stress, vertical friction & Coriolis • Then only force will be Coriolis force...**Inertial Currents**• Motions in rotating frame will veer to right • Make an inertial circle • August 1933, Baltic Sea, (f = 57oN) • Period of oscillation is ~14 hours**Inertial Currents**• Inertial motions will rotate CW in NH & CCW in the SH • These “motions” are not really in motion • No real forces only the Coriolis force**Inertial Currents**• Balance between two “fake” forces • Coriolis & • Centripetal forces**Inertial Currents**• Balance between centripetal & Coriolis force • Coriolis force per unit mass = f u • u = velocity • f = Coriolis parameter = 2 W sin f W = 7.29x10-5 s-1 & f = latitude • Centripetal force per unit mass = u2 / r • fu = u2 / r -> u/r = f**Inertial Currents**• Inertial currents have u/r = f • For f = constant • The larger the u, the larger the r • Know size of an inertial circle, can estimate u • Period of oscillation, T = 2pr/u (circumference of circle / speed going around it) • T = 2pr/u = 2p (r/u) = 2p (1/f) = 2p /f**Inertial Period**• f = 2 W sin(f) • For f = 57oN, f = 1.2x10-4 s-1 • T = 2 W / f = 51,400 sec = 14.3 hours • Matches guess of 14 h**Inertial Oscillations**D’Asaro et al. [1995] JPO**Inertial Currents**• Balance between Coriolis & centripetal forces • Size & speed are related by value of f - U/R = f • Big inertial current (U) -> big radius (R) • Vice versa… • Example from previous slide - r = 8 km & f = 47oN • f = 2 W sin(47o) = 1.07x10-5 s-1 • U = f R ~ 0.8 m/s • Inertial will dominate observed currents in the mixed layer**Inertial Currents**• Period of oscillations = 2 p / f • NP = 12 h; SP = 12 h; SB = 21.4 h; EQ = Infinity • Important in open ocean as source of shear at base of mixed layer • A major driver of upper ocean mixing • Dominant current in the upper ocean

More Related