Upper ocean currents, Coriolis force, and Ekman Transport Gaspard-Gustave de Coriolis Walfrid Ekman.
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Transcript of Upper ocean currents, Coriolis force, and Ekman Transport Gaspard-Gustave de Coriolis Walfrid Ekman.
Upper ocean currents, Coriolis force, and Ekman Transport
Gaspard-Gustave de CoriolisWalfrid Ekman
Upper ocean currents, Coriolis force, and Ekman Transport
• In the open ocean mixed layer: vertical structure was the key to biological productivity– mixing, light, stratification, critical depth
• In coastal regions and the equator, wind-driven horizontal currents can cause upwelling of water with high nutrients leading to sustained production over a season, or longer– Wind stress + Coriolis force give…
Ekman currents and upwelling
Seasonal input o
f new
nutrients
from winter
convection
Time scales greater than
a day introduce earth’s
rotation in the dynamics –
Coriolis force
Fro
m L
alli
and
Par
son,
“B
iolo
gica
l Oce
anog
raph
y”
Some physics…
Coriolis force:
In a fixed frame of reference the ball travels in a straight line (Newton’s laws)
In a rotating frame of reference (on the table, or Earth), the ball appears to turn.
In the example, the merry-go-round is turning clockwise and the ball turns toward the left. This is the Southern Hemisphere effect.
In the Northern Hemisphere the local rotation is counter-clockwise, and Coriolis force deflects motion to the right.
N
S
http://marine.rutgers.edu/dmcs/ms320/coriolis.mov
Handout-Coastal-Upwelling.pdf
Figure 9.1 Inertial currents in the North Pacific in October 1987 (days 275-300) measured by holey-sock drifting buoys drogued at a depth of 15 meters. Positions were observed 10-12 times per day by the Argos system on NOAA polar-orbiting weather satellites and interpolated to positions every three hours. The largest currents were generated by a storm on day 277. Note: these are not individual eddies. The entire surface is rotating. A drogue placed anywhere in the region would have the same circular motion. From van Meurs (1998).
In the Northern hemisphere …
Earth’s rotation is counter clockwise …
and Coriolis force is to the right …
of the direction of movement
Coriolis force
dire
ctio
n of
flo
w
In the absence of any other forces, Coriolis drives clockwise (NH) rotating inertial oscillations
Suppose a balance of forces between wind stress and Coriolis
Coriolis force is to right of the directionof movement
So direction of movementis to the right of the wind (in the northern hemisphere)
Wind force Coriolis force
Cur
rent
Win
d fo
rce
Current
Win
d fo
rce
Current
Wind force
Current
Wind force
Current
DE
DE
• V0 is 45° to the right of the wind (in the northern hemisphere)• V0 decreases exponentially with depth as it turns clockwise (NH)• At depth z = -DE the flow speed falls to e-π = 0.04 times the surface current and is in the opposite direction (typical DE is 20 to 40 m)
Progressive vector diagram, using daily averaged currents relative to the flow at 48 m, at a subset of depths from a moored ADCP at 37.1°N, 127.6°W in the California Current, deployed as part of the Eastern Boundary Currents experiment. Daily averaged wind vectors are plotted at midnight UT along the 8-m relative to 48-m displacement curve. Wind velocity scale is shown at bottom left. (From: Chereskin, T. K., 1995: Evidence for an Ekman balance in the California Current. J. Geophys. Res., 100, 12727-12748.)
windwind
water
Eastern Boundary Current program Progressive vector diagram. Apr-Oct 1993
Equator-ward winds on ocean eastern boundaries
Pole-ward wind on ocean western boundaries
Pole-ward winds on ocean eastern boundaries
Equator-ward wind on oceanwestern boundaries
http://marine.rutgers.edu/cool/research/upwelling.html
Wind
forc
e
Current
The magnitude of the Ekman transport is
m2 s-1
τ = wind stress (Pascals or N m-2) = water density (1027 kg m-3) f = Coriolis parameter = 2 Ω sin φ Ω = 2π/(24 hours) = 2 x Earth rotation rate x sin(latitude)
Wind speed and along-shelf currents at various depths along the continental shelf off northwest Africa
W
ind
sp
ee
d m
s-1
Strong wind toward south
Weak or no wind
Wind-driven currents and upwelling
On timescales longer than a few days:
Earth’s rotation introduces Coriolis force
• flow turns to the right (northern hemisphere) or left (southern hemisphere)
• wind stress balances Coriolis force = Ekman transport
Oceanographer’s rule: Ekman transport is toward the right of the wind stress(in northern hemisphere)
Adjacent to a coast…Alongshore wind produces Ekman transport across-shore …causes upwelling or downwelling of a few meters per day
Estimating the upwelling velocity from NJ coast data
3 4 1/ 0.7 /(15 10 ) 0.5 10
x Ekman
Ekman x
w L U
w U L x x ms
Wind data show southerly of 6 m s-1 over a few days
To balance mass transport in a 2-dimensional (across-shelf/vertical) process, the average upwelling velocity (w) times the width of the upwelling zone must balance the Ekman transport.
Over 1 day (86400 sec) this is 4.32 m day-1
Dep
th (
z)
uE
Dep
th (
z)
Dep
th (
z)
fDE
2
uE
Dep
th (
z)
Typical τ= 0.1 Pa
f = 5x10-5 at 20N
typical DE ~ 30 m
uE ~ 0.1 / 1027 / 5x10-5 / 30 = 6.5 cm s-1
Northwest Africa values:
50/8 = 6.25 km day-1typical τ= 0.1 Pa
f = 5x10-5 at 20N, DE ~ 30 m
= 5.6 km/day
Alongshore flowshaded into page
i.e. poleward
Across-shore flowshaded to left (offshore)
Density(kg m-3)
6 cm/s
Upwelling favorable wind is out of the page
Wind-driven currents and upwellingOn timescales longer than a few days:
Earth’s rotation introduces Coriolis force
• flow turns to the right (northern hemisphere) or left (southern hemisphere)
• wind stress balances Coriolis force = Ekman transport
Oceanographer’s rule: Ekman transport is toward the right of the wind stress(in northern hemisphere)
Adjacent to a coast…Alongshore wind produces Ekman transport across-shore …causes upwelling or downwelling of a few meters per day