Ocean Circulation
Chapter 5
Ocean 101/ATMS 211
Surface Winds Recap
n  Trade
winds
converge at the
equator
n  Winds are
named by the
direction they
are coming
FROM
Wind-Driven Surface Currents
n
n
Wind movement
causes friction,
dragging top 50-100 m
of seawater
Coriolis deflects water
about 25° to the right
of wind direction (in
northern hemisphere)
Wat
er
dire
ct
ion
Wind direction
Atlantic Gyres
n
n
Note circular motion
of ocean currents in
both hemispheres
Can you see the
Coriolis Effect in
action?
North Atlantic Gyre
n
Divided into four
interconnected
currents:
•
•
•
•
Gulf Stream
North Atlantic
Current
Canary Current
North equatorial
Current
Sea Surface Height
2 meters higher on western side of Atlantic
n
Colors represent sea surface heights
What Causes the Bulge?
n
Westerly Winds
n
Trade Winds
In theory,
water is
transported to
the RIGHT of
the wind
direction in the
northern
hemisphere.
But why??
Ekman Spiral
n  Topmost
layer of
water flows at
45° to the right of
wind direction
due to Coriolis
Effect in northern
hemisphere
Ekman Spiral
n
n
n
Next layer down
doesn’t “feel” the
wind, it “feels” the
motion of the layer
above it
Motion of this layer is
45° to the right of the
layer above
This continues to
about 100 m depth
Ekman Transport
n
n
Due to friction, each
layer moves more
slowly than layer
above, and 45° to the
right of the layer
above
Note that at some
depth, water is
flowing opposite to
direction of surface
current
Ekman Transport
n
n
In theory, the net
effect of Ekman
transport is that water
is transported 90° to
the right of wind
direction
In reality, angle is
less than 90°
Geostrophic Currents
n
n
Water flows down the hill due to gravity, then turns to the
RIGHT in northern hemisphere
Gravity/pressure gradient & Coriolis force are in balance
The Bottom Line
n  The
interaction between these forces keep
ocean currents flowing in gyres:
¨ Wind
energy
¨ Coriolis Effect
¨ Gravity (downhill pressure gradient)
¨ Friction between water molecules
Surface Currents
SST View of Geostrophic Gyres
Surface currents carry warm water away from the equator
and cold water away from the poles
Western Boundary Currents
n
n
Warm, fast-moving, narrow and deep
Gulf Stream, Kuroshio, Brazil ,Agulhas, E.
Australia
Westward Intensification
n  Water
piles up on the western side of
ocean basins due to Earth’s rotation and
trade winds
n  Piled up water “pushes” western
boundary currents along the continent
towards the poles
n  Result: Gulf Stream moves at 5 miles/hr
to depth of 1500 ft.
Gulf Stream Schematic
Text Gulf animation
Warm & Cold Core Eddies
n
Meanders become cold and warm core eddies
Gulf Stream from Space
n
Warm Core Eddy
n
n
Meander
Cold Core Eddy
Gulf Stream
moves north
along east coast
Moves offshore
near Cape
Hatteras, NC and
starts to meander
Cold core and
warm core eddies
form when
meanders pinch
off
Eastern Boundary Currents
n  Cold,
weak, broad, shallow currents
n  Canary, Benguela, Peru, California
Equatorial Currents &
Countercurrents
n
n
Equatorial currents driven west by the trade
winds
Countercurrents caused by backflow of water
along the equator
West Wind Drift
n
n
West Wind Drift travels around the Earth without
interruption
Very useful for round the world sailing but BEWARE of
extratropical cyclones and rogue waves!
Upwelling, Downwelling
& El Niño
Upwelling
n
n
n
Found in areas of
surface divergence
Brings nutrient-rich,
cold water up to the
surface to replace
surface water
Associated with
areas of high
biological productivity
Where Does Upwelling Occur?
Equatorial
upwelling
Coastal
upwelling
Why Does Equatorial Upwelling
Occur?
n
Divergence of currents at equator due to Coriolis
Effect & Ekman transport produces upwelling
Coastal Upwelling
n
In northern hemisphere, wind blowing from north
causes Ekman transport to the west
Upwelling & Primary Productivity
n
n
Upwelling off US west
coast brings up cold,
nutrient-rich water
Image shows high
amounts of
phytoplankton due to
coastal upwelling
Downwelling
n
n
n
Found in areas of
surface
convergence
Downward
movement of
nutrient-poor, warm
surface water
Associated with
areas of low
biological
productivity
Downwelling
n
Wind from the south leads to Ekman transport of
surface water towards shore in northern
hemisphere
Let’s Practice!
Upwelling or Downwelling?
Equator
Wind
Upwelling
Upwelling or Downwelling?
Upwelling
Wind
Downwelling
Equator
Upwelling or Downwelling?
Equator
Wind
Downwelling
Upwelling or Downwelling?
Upwelling
Wind
Equator
Normal Tropical Pacific Circulation
Normal SST
Note
upwelling of
cold water
off the coast
of South
America
Normal Ocean Cross Section
Note
upwelling of
cold water
off the coast
of South
America
El Niño (ENSO)
El Niño, SST
Note warm
water off the
coast of
South
America
El Niño (ENSO)
Note warm
water off the
coast of
South
America
Which is ENSO SST Map?
A
B
El Niño (ENSO) Events
n  FIRST,
trade winds weaken near
equator
n  SECOND, equatorial currents grind to a
halt
n  THIRD, warm water flows backwards
towards South America
n  FOURTH, coastal upwelling system
shuts down as warm water blocks Peru
Current
El Niño Effects
n
n
n
n
Warm, dry winters in
Pacific Northwest
Drought and fires in
Australia, Indonesia
Heavy rains along
western South
America coast
Fisheries in Peru
collapse
A Pacific Event, Felt Worldwide
El Niño Frequency
82-83
2009-2010
97-98
Are ENSO events happening more frequently?
Thermohaline
Circulation
Could This Really Happen??
Vertical Stratification
n
Stratification is the vertical
density layering of the water
column
¨  Waters
of different densities are
found at different depths
¨  Less dense water floats on top of
more dense water
¨  Warmer water floats & cooler
water sinks
¨  Fresher water floats &
saltier water sinks
Example: Thermal Stability
n  Lake
Washington
¨ Surface
warms in summer
¨ Warm (lower-density) water
lies on top of cool (higherdensity) water
¨ Thermocline: boundary to
mixing between layers of
different temperatures
Temperature à
Instability & Overturning
n  IF
the water column becomes unstable…
¨ More
dense water on top of less dense
¨ Dense water sinks, less dense water rises
Thermohaline Circulation:
Density Driven Currents
n  Water
moves
vertically due to
density differences
n  Changes in
temperature and
salinity in surface
water can cause
water to sink
What Causes Instability?
n  Anything
That Increases
Surface Density or
Decreases Deep Water
Density
¨ Surface
Cooling
¨ Sea Ice Formation
¨ Surface Evaporation
¨ Sea-floor Heating at
Hydrothermal Vents
Instability in Polar Oceans
n  Rapid
Cooling in Winter
¨ Dense,
cold water at the surface sinks below
warmer water beneath
¨ Major factor in global density-driven ocean
currents
n  Sea
Ice Formation in Winter
¨ Sea
ice freezing à excludes salt
¨ Salt remains in sea water, raising salinity &
density
¨ Salty water sinks below less-salty water
beneath
In A Test Tube Ocean…
Fresh water ice
Salty, slushy and dense
Where Is Deep Water Forming?
North Atlantic, near Greenland
Around Antarctica
Mediterranean Geography…
Mediterranean Sea
Evaporation in
desert climate
creates highsalinity, high
density surface
layer
n  Very little fresh
water inflow
from rivers
n  High salinity
water sinks
n
Evaporation of Fresh Water
Surface water getting saltier
Water sinks until
it finds its density
level
Cross Section View…
German U-boat
Global Thermohaline Circulation
Ocean Circulation & Climate
n  The
ocean stores
and releases huge
quantities of heat
n  Bottom water is a
CO2 holding tank…
until it returns to the
surface every 1000
years or so
Climate Change and Thermohaline
Circulation
Increased rain at the poles could dilute surface
water, preventing sinking
n  Melting ice dilutes water, preventing sinking
n  Warming surface water is not dense enough to
sink
n  It’s possible that increasing temperatures
could shut down the global conveyor
system…there are signs in the North
Atlantic that it’s already starting
n