Transcript g104_class17_wind_driven_currents

Geography 104 - “Physical Geography of the World’s Oceans”
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
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)
- t (Greek letter tau) represents wind stress
- t acDu2
 a is air density (1 kg m-3)
cD is a drag coefficient (~1.4x10-3)
U2 is wind velocity (m2 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)
drag coefficient
wind speed
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
(u2)
wind stress acts on surface layer, surface
layer acts on underlying layer, underlying
layer acts on layer below that,………..
Coupling between two ocean layers is larger under
greater winds. Transfer of momentum is due to turbulent
water parcels, rather than just individual molecules
winds contribute to upper ocean mixed
layer (along with heat and salt)
Ekman Flow – theory for direct wind driven
currents published by Ekman in 1905
Vagn Walfrid Ekman (1874-1954)
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 86o 14’ N
Ekman Flow
• Nansen noticed that movement of the icelocked ship was 20-40o 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
time = 0
water parcel
wind force
surface flow development
time = 1
surface current
surface flow development
time = 2
friction force
Coriolis force
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 (force balance)
vector addition
(decomposition)
surface flow development
time = 4 (force balance)
components
of friction &
Coriolis
forces opposite
wind force
surface flow development
time = 4 (force balance)
surface current
at 45° to right
of wind
surface flow development
time = 4 (force balance)
force balance where:
friction + Coriolis = wind
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 t
• Transport of momentum into
interior is turbulent and
inefficient
Ekman spiral
Ekman spiral, another view
Ekman spiral, plan view (looking down)
DE 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
Ekman layer balance: wind force = Coriolis force
Coriolis
force
Ekman layer movement
Coriolis
force
Coriolis force is 90°
to right of Ekman
transport in NH
Ekman spiral in observations
Price et al. 1987, Science
Ekman transport in observations
theory
observerd
Price et al. 1987, Science
Readings (Surface and Deep Circulation):
Text Chapter 9 (pgs 165 – 170)
Reader pgs. 63 – 72