Surface Currents, Gyres and Upwelling

1
Surface Currents, Gyres and Upwelling
Lecture 13
OEAS-306
March 5, 2009

• Outline
• Review from Last Lecture
• Global Wind Patterns
• The influence of rotation on ocean currentsEkman
  spiral
• Gyres
• Western Boundary CurrentsThe Gulf Stream
• Upwellling (Equatorial and Coastal)

2
Uneven Surface Heating Leads to Global
Circulation Cells
Global air circulation as described in the
six-cell circulation model. Air rises at the
equator and falls at the poles, but instead of
one great circuit in each hemisphere from equator
to pole, there are three in each hemisphere. Note
the influence of the Coriolis effect on wind
direction. The circulation show here is idea
that is, a long-term average of wind flow.
3
The Coriolis Effect Influences the Movement of
Air in Atmospheric Circulation Cells
The westerlies and the trade winds are two of the
winds that drive the oceans surface
currents. About 10 of the water in the world
ocean is involved in surface currents, water
flowing horizontally in the uppermost 400 meters
(1,300 feet) of the oceans surface, driven
mainly by wind friction. (left) Winds, driven by
uneven solar heating and Earths spin, drive the
movement of the oceans surface currents. The
prime movers are the powerful westerlies and the
persistent trade winds (easterlies).
4
Wind blowing over the surface of the ocean
transfers momentum from the wind to the ocean,
driving surface currents.
Wind 10 m/s
fast
friction
Momentum is transported down-gradient
slow
Current lt 1 m/s
slower
slowest
Direct Influence of Friction is limited to the
region very near the surface (lt 50 m)
5
Wind-induced currents are also influenced by
Earths rotation.
Coriolis force deflects surface currents to the
right of the direction of the wind in northern
hemisphere and to the left in the southern
Hemisphere. So, Northeast trade winds drive a
current towards the west near the equator in the
northern hemisphere. Similarly, the Westerlies
drive a current towards the east in the northern
hemisphere.
6
Wind-driven surface currents lead to Gyres.

• (below) A combination of four forces surface
  winds, the suns heat, the Coriolis effect, and
  gravity circulates the ocean surface clockwise
  in the Northern Hemisphere and counterclockwise
  in the Southern Hemisphere, forming gyres.

(above) The North Atlantic gyre, a series of four
interconnecting currents with different flow
characteristics and temperatures.
7
Influence of the Earths rotation and friction
lead to the Ekman Spiral
The Ekman spiral and the mechanism by which it
operates. The length of the arrows in the
diagrams is proportional to the speed of the
current in each layer. (a) The Ekman spiral
model. (b) A body of water can be thought of as a
set of layers. The top layer is driven forward by
the wind, and each layer below is moved by
friction. Each succeeding layer moves with a
slower speed and at an angle to the layer
immediately above it to the right in the
Northern Hemisphere, to the left in the Southern
Hemisphere until water motion becomes
negligible. (c) Though the direction of movement
varies for each layer in the stack, the
theoretical net flow of water in the Northern
Hemisphere is 90 to the right of the prevailing
wind force.
8
The net direction of transport is 90 degrees to
the right of the wind (in the northern
Hemisphere).
9
Net Ekman transport, piles up water in the middle
of the Gyre
x
y
Flow around a gyre is approximately in
Geostrophic Balance
Friction is important near the surface (lt50m),
but not deeper.
Pressure Gradient in x-direction is balanced by
rotation acting on the flow in the y-direction (v)
10
Major Gyre Systems and Their Currents
Geostrophic gyres are gyres in balance between
the pressure gradient and the Coriolis effect. Of
the six great currents in the worlds ocean, five
are geostrophic gyres. (above) A chart showing
the names and usual direction of the world
oceans major surface currents. The powerful
western boundary currents flow along the western
boundaries of ocean basins in both hemispheres.
11
Boundary Currents Have Different Characteristics

• Western boundary currents These are narrow,
  deep, fast currents found at the western
  boundaries of ocean basins.
• The Gulf Stream
• The Japan Current
• The Brazil Current
• The Agulhas Current
• The Eastern Australian Current
• Eastern boundary currents These currents are
  cold, shallow and broad, and their boundaries are
  not well defined.
• The Canary Current
• The Benguela Current
• The California Current
• The West Australian Current
• The Peru Current

12
Why are Western Boundary Currents Intensified?
Coriolis big
Coriolis small
Uniform Coriolis
Coriolis increases with latitude
PG gtgt PG
Sea surface Height
PG PG
west
east
west
east

• Coriolis force varies with latitude. It is
  stronger near the poles and weaker near the
  Equator.
• As a result, the northern portion of the gyre
  turns south much sooner than the southern portion
  turns north.
• As a result, the center of the pile of water in
  the middle of the gyre is offset to the west.
  This results in a stronger pressure gradient on
  the west side of basin.
• The strength of a geostrophic current is directly
  proportional to the pressure gradient.
• So, Western Boundary currents are intensified.

13
Gulf Stream is a very strong, deep, warm western
boundary current.
Velocity is approximately 2 m/s.
2000 m
14
Gulf Stream Eddies
Eddy formation The western boundary of the Gulf
Stream is usually distinct, marked by abrupt
changes in water temperature, speed, and
direction. (a) Meanders (eddies) form at this
boundary as the Gulf Stream leaves the U.S. coast
at Cape Hatteras. The meanders can pinch off (b)
and eventually become isolated cells of warm
water between the Gulf Stream and the coast (c).
Likewise, cold cells can pinch off and become
entrained in the Gulf Stream itself (d). (C
cold water, W warm water blue cold, red
warm.)
15
Surface Currents Affect Weather and Climate
General summer air circulation patterns of the
east and west coasts of the United States. Warm
ocean currents are shown in red cold currents,
in blue. Air is chilled as it approaches the west
coast and warmed as it approaches the east coast.
16
Satellite Image of Gulf Stream

• Transports a lot of heat from South to North
  (helps keep earth is thermal equilibrium).
• Keeps climate relatively mild in British Isles
  (London is over 300 miles north of Caribou Maine).

17
Upwelling and Downwelling

• Upwelling is the upward motion of water caused by
  surface divergence. This motion brings cold,
  nutrient rich water towards the surface.
• Downwelling is downward motion of water caused by
  surface convergence. It supplies the deeper ocean
  with dissolved gases.

Upwelling
Downwelling
Rotation, acting on the wind driven circulation
(momentum equation) leads to either surface
convergence or divergence, which results in a
vertical velocity (continuity equation).
18
Equatorial Upwelling
NORTH
View from above
Ekman Transport
N.E. Trade Winds
Equator
S.E. Trade Winds
Ekman Transport
SOUTH
Equator
Warm nutrient poor waters
NORTH
SOUTH
Cold nutrient rich waters
19
Upwelling Velocity
20
Equatorial Upwelling Leads to High Biological
Productivity
Increased Biology
Colder upwelled water
21
Coastal Upwelling
Coastal upwelling. In the Northern Hemisphere,
coastal upwelling can be caused by winds from the
north blowing along the west coast of a
ccontinent. Water moved offshore by Ekman
transport is replaced by cold, deep,
nutriend-laden water. In this diagram,
temperature of the ocean surface is shown in
degrees Celsius.
22
Coastal Downwelling
Coastal downwelling. Wind blowing from the south
along a Northern Hemisphere west coast for a
prolonged period can result in downwelling. Areas
of downwelling are often low in nutrients and
therefore relatively low in biological
productivity.
23
Coastal Upwelling Also Leads to High Biological
Productivity
Red Colors indicate high chlorophyll
concentrations (more biology).
24
Upwelling with Gap Winds
Winds get funneled through gaps in the mountains,
leading to very strong wind jets.

• Strong Ekman transport associated with the wind
  jet leads to convergence to the right of the wind
  and divergence to the left.
• So, there is strong upwelling to the left of
  these gaps wind (colder water).

25
Upwelling with Gap Winds
Sea Surface Temperature
Cold
26

• Summary
• Ocean water circulates in currents caused mainly
  by wind friction at the surface
• Water near the ocean surface moves to the right
  of the wind direction in the Northern Hemisphere,
  and to the left in the Southern Hemisphere
  because of the Coriolis force.
• The largest surface currents are organized into
  huge circuits known as gyres.
• Gyres are largely in geostrophic balance (the
  balance between Pressure Gradient and Coriolis
  force).
• The pressure gradient results from water that
  piles up in the center of the gyre due to
  convergence by Ekman transport.
• Currents on the western boundaries of gyres are
  deeper and faster. This occurs because of the
  variations in the strength of the Coriolis force
  with latitude.
• Upwelling and downwelling describe the vertical
  movements of water masses. Upwelling is due to
  the divergence of surface currents downwelling
  is caused by surface current convergence.
• Upwelling regions are often very biologically
  productive because of the constant flux of
  nutrients into the surface waters.