# Wind Driven Circulation - wind energy to the ocean is a “wind stress” - wind directly drives currents at ~2 - 3% of wind speed - wind directly drives currents.

## Presentation on theme: "Wind Driven Circulation - wind energy to the ocean is a “wind stress” - wind directly drives currents at ~2 - 3% of wind speed - wind directly drives currents."— Presentation transcript:

Wind Driven Circulation - wind energy to the ocean is a “wind stress” - wind directly drives currents at ~2 - 3% of wind speed - wind directly drives currents to right/left of wind in NH/SH - wind driven currents set up variations in sea level - surface waves transport wind energy over long distances - turbulent mixing dissipates energy from wind Geography 104 - “Physical Geography of the World’s Oceans”

Wind Stress (overview) - forces give rise to ocean circulation - wind stress is a frictional force parallel (tangential) to the sea surface per unit area - effect of wind stress is to accelerate ocean’s surface layer

Wind Stress (details) -  (Greek letter tau) represents wind stress -  a c D u 2  a is air density (1 kg m -3 ) c D is a drag coefficient (~1.4x10 -3 ) U 2 is wind velocity (m 2 s -2 ) - units are Newtons (force) per meter squared F = ma or 1 Newton = 1 N = 1 kg m s -2 N m -2 = kg m -1 s -2

Wind Stress (details) wind speed drag coefficient exact processes creating wind stress are complex

Wind Stress (summary) - many processes contribute to the transfer of momentum from the atmosphere to the ocean - turbulent friction - capillary waves - wind waves Wind Stress Bottom Line: process is turbulent; force proportional to wind speed squared (u 2 )

Coupling between two ocean layers is larger under greater winds. Transfer of momentum is due to turbulent water parcels, rather than just individual molecules wind stress acts on surface layer, surface layer acts on underlying layer, underlying layer acts on layer below that,………..

winds contribute to upper ocean mixed layer (along with heat and salt)

Vagn Walfrid Ekman (1874-1954) Ekman Flow – theory for direct wind driven currents published by Ekman in 1905 Wind driven flow named after Ekman because he knew how to do math!

Fridtjof Nansen – deserves much credit for Ekman flow Nansen was a scientist – explorer Later dedicated his 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

Fram locked in ice

1893 -1896 - Nansen got to 86 o 14’ N

Ekman Flow Nansen noticed that movement of the ice- locked ship was 20-40 o to right of the wind Nansen figured this was due to a steady balance of wind stress, friction & Coriolis forces Ekman later developed a mathematical framework based on Nansen’s observations

Ekman Flow motion at the surface is 45° to right (NH) of wind

Ekman surface flow development water parcel

surface flow development water parcel time = 0 wind force

surface flow development surface current time = 1

surface flow development Coriolis force friction force time = 2

surface flow development time = 3 surface current turns to right (NH) due to Coriolis

surface flow development time = 4 (force balance; wind, friction, Coriolis) surface current 45° to right (NH) of wind

surface flow development time = 4 time = 4 (force balance) vector addition (decomposition)

surface flow development time = 4 components of friction & Coriolis forces opposite wind force time = 4 (force balance)

surface flow development time = 4 surface current at 45° to right of wind time = 4 (force balance)

surface flow development time = 4 force balance where: friction + Coriolis = wind time = 4 (force balance)

Ekman Flow motion at the surface is 45° to right (NH) of wind

ice drift wind direction ice drift

Ekman Transport The ocean is like a layer cake A layer is accelerated by the one above it & slowed by the one beneath it Top layer is driven by  Transport of momentum into interior is turbulent and inefficient

Ekman spiral

Ekman spiral, another view

Ekman spiral, plan view (looking down) D E is depth of Ekman layer (top ~100 m)

Ekman flow, Ekman spiral, Ekman transport Ekman transport is 90° to right (NH) of wind

Ekman transport refers to movement of Ekman layer Friction force in Ekman layer No friction force for layer coupling beneath Ekman layer

Ekman layer movement Coriolis force Ekman layer balance: wind force = Coriolis force

Ekman layer movement Coriolis force is 90° to right of Ekman transport in NH Coriolis force

Ekman spiral in observations Price et al. 1987, Science

theory observerd Ekman transport in observations Price et al. 1987, Science

Readings (Surface and Deep Circulation): Text Chapter 9 (pgs 165 – 170) Reader pgs. 63 – 72

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