Quick review of surface winds
90°N
(North Pole)
H
L
Polar Easterlies
60 °N
Westerlies
30 °N
H
Surface Ocean Circulation
Trade Winds
NE Trade Winds
ITCZ
L
0° (Equator)
Trade Winds
What Goes Around Comes Around
SE Trade Winds
H
30 °S
Westerlies
L
60 °S
Polar Easterlies
H
90°S
(South Pole)
WindWind-driven ocean circulation
• circulation of the upper water
am
tre
t
North Atlantic
Subtropical
Gyre
Caribbean
Sea
Ca
na
ry
lf S
Gu
rre
n
N orth
Mediterranean
Sea
30°N
Subtropical
Gyre
South Atlantic
West Wind Drift
East Wind 60°S
Drift
Benguela Curr
ent
30°S
Eq. Countercurrent l C urrent
ria
u ato
S . Eq
masses is set in motion by the
energy of the prevailing winds
• as wind blows across ocean
surface, air molecules collide
with water molecules
• winds
• currents
– inefficiently transfers energy from
air to water
– water moves at only about 33-4%
of the wind speed
N. Equatorial Current
0° (Equator)
Bra
zil C
urr
e nt
Gulf of
Mexico
rent
ic C ur
A tlant
Cu
60°N
Global ocean surface circulation
• gyres are large, circular,
horizontal current systems
– caused by west
west--moving ocean
currents in the trade winds, easteastmoving ocean currents in the
westerlies and deflection of ocean
currents by the continents
– the subtropical gyres transport
warm waters poleward along the
western edges of the ocean basins,
and cool waters equatorward along
the eastern sides
• Notice how the currents roughly correspond to
the wind directions, especially in the regions of
the subtropical gyres.
1
Ekman Spiral
change in direction
and current velocity with
increasing depth
Ekman theory
prevailing wind
• The prevailing winds create a drag
(wind stress) on the ocean surface.
• The momentum gained at the surface
•
•
•
•
is transferred deeper, but energy is
lost with increasing depth.
depth.
The Coriolis effect deflects the moving
water at each level.
A decrease in current speed coupled
with continuous deflection with
increasing depth creates a theoretical
spiral of moving water called the
Ekman Spiral.
Spiral.
Adding all the vectors (magnitude and
direction) of the Ekman Spiral yields a
net current direction that is ~90o to
the prevailing wind.
This composite current is called
Ekman Transport.
surface current
( ~45° to the prevailing wind)
Ekman Transport
= composite of Ekman Spiral;
o
~90 to the prevailing wind
• The surface current is ~45o to
the prevailing wind, due to
Coriolis deflection.
• Flow velocity decreases
exponentially with depth, all
the while being deflected by
Coriolis
– The depth where the direction of
flow is opposite to that of the
surface, and the velocity is only
about 4% of the surface flow, is
called the “Ekman
“Ekman depth.”
depth.”
– Ekman transport affects a surface
ocean layer with a typical depth
between 5050-150 m
• Ekman transport controls the
motion of the surface ocean.
– divergence: upwelling
– convergence: downwelling
Ekman transport on Earth
Ekman Transport
~90o to the
prevailing winds
Ekman transport
Ekman convergence & divergence
90°N
90°N
H
L
(North Pole)
(North Pole)
• Pressure gradients
Polar Easterlies
60 °N
60 °N
divergence
Westerlies
30 °N
30 °N
H
Trade Winds
convergence
•
NE Trade Winds
ITCZ
L
divergence
0° (Equator)
0° (Equator)
Trade Winds
SE Trade Winds
convergence
•
H
30 °S
30 °S
Westerlies
L
Polar
Easterlies
60 °S
H
90°S
(South Pole)
to the right of the prevailing
winds in the N. Hemisphere,
to the left of the prevailing
winds in the S. Hemisphere
divergence
60 °S
90°S
develop in the ocean
because the sea surface
is warped into broad
mounds and depressions
with a relief of about
one meter.
meter.
Mounds on the ocean’s
surface are caused by
converging currents,
currents,
places where water
sinks (or downwells).
Depressions on the
ocean’s surface are
caused by diverging
currents,
currents, places from
where water rises
(or upwells).
(South Pole)
2
phytoplankton
pigment
divergence of water masses
results in upwelling and high
productivity in subpolar waters
convergence of water masses
causes near-surface waters
to pile-up in the subtropics
divergence of water masses
results in upwelling and high
productivity at the equator
convergence of water masses
causes near-surface waters
to pile-up in the subtropics
divergence of water masses
results in upwelling and high
productivity in subpolar waters
3