OCEAN
CIRCULATION
Title slide
I. Ocean Layers and
circulation types
1) Ocean Layers
• Ocean is strongly Stratified
• Consists of distinct LAYERS – controlled by
density
•takes huge amounts of energy to mix up the stable
layers!
• two very separate types of ocean circulation –
•1) surface -driven by wind
•2) deep driven by density
Temperature
slice in the central Atlantic
South
North
Thermocline – a sharp transition in temperature and hence DENSITY (marked by
many contour intervals)
The “main” thermocline divides WARM SURFACE from COLD DEEP water.
Ocean Layers
SURFACE OCEAN
THERMOCLINE
DEEP OCEAN
• Water above the thermocline can be moved by wind (via
friction with the atmosphere)
• Water below thermocline- much larger barrier, due to density
layers.
II. Wind Fields and
Main Ocean “Gyres”
Recall: Main WINDBELTS – these drive Surface circulation!
Rev:
Main-wind
Prevailing
Westerlies
belts
Trade Easterlies
Subtropical gyres and their Main Current Types
W
60°N
Prevailing Westerlies
E
High latitude
currentL
H
30°N
Eastern Boundary
Easterly
Trade
current
Equatorial/tropical
current
0°
PrevailingBoundary
Westerlies
Western
current
L
Easterly Trade
H
30°S
60°S
High
L latitude
current
5 Major subtropical gyres
H
30°N
H
30°S
Notice smaller Sub-polar Gyres as well
Global Surface Currents
Kaufman Fig. 4.12
How do we know they exist?
Ben Franklin’s map of the Atlantic deduced the Atl. Gyre from the way
ships would drift along their route to and from the New World
Had Just started:
The strange story of Nansen’s
icebergs…
Franklin
map
Summary: Ocean Gyres
• Large ~ circular current flows
• Set in motion by main wind patterns
• Bounded by continents on E and W
• “subtropical” gyres most pronounced
II. EKMAN & GEOSTROPHIC FLOW
II. ECKMAN TRANSPORT
(CORIOLIS strikes again)
Nansen noticed icebergs moving
20˚ - 40˚ to right of wind direction
Nansen’s funky icebergs
BACK TO BOARD- ECKMAN + GEOSTROPHIC
?
Ekman Spiral
Model of surface
response to wind
forcing
- When wind blows
over the ocean,
surface water moves
45o to the right of
the wind in the
northern hemisphere
- Current rotates and
weakens deeper in
water column
‘Ekman Transport’
Net flow of surface layer
when forced by wind
90o
Most important
point:
Add up all the
arrows, the AVERAGE
direction of flow is
90o to the right of
the wind!
Ekman Transport Causes
Convergence of Water in
Middle of Gyres
Prevailing Westerlies
30°N
CONVERGENCE
Northeasterly Trades
H
Convergence Zone
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Gyre Flow
into
screen
X
H
H
Gyre Flow
out of
screen
Surface Layer
Ekman Trans. CONVERGENCE
Ekman Trans.
Thickens
Geostrophic Flow
Para ver esta película, debe
disponer de QuickTime™ y de
un descompresor .
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PRESSURE
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•PGF
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•Plus hCoriolis
•Results in Flow Clockwise around Gyre (NH)
•Strength of Flow related to PG
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X
Pressure gradients in surface ocean
• Wind fields create areas of lower and higher water:
“humps” and “valleys” of water.
• Result: just like in atm = Pressure gradients
• And just like in atm, water tries to flow from Hight
to Low pressure zones
“Dynamic Height Maps” Maps of relative
height of surface ocean
from the
measurements of
the ocean density
• Can be used to
calculate current
strengths.
From Regional Oceanography © 2001 - 2003 M. Tomczak and J. S. Godfrey
Gyres are around
surface highs
SUMMARY:
WHAT DRIVES SURFACE CURRENTS?
What Drives Surface Currents?
• 1) Direct Effect of Wind
– Confined to upper 50-100 m
– Frictional surface layer flow is ~90˚ to right of wind
direction in NH, to left in SH -> Ekman Transport
• 2) Indirect Effects of Coriolis
– Eckman Transport sets up Pressure gradients drive
focused currents (jets)
– Can be 1 - 2 km below surface
-------- to board--- -
WEST INTENSIFICATION &
BOUNDARY CURRENTS
“WESTERN
INTENSIFICATION”
&
W. Boundary Currents
Gyre Circulation & W. Boundaries.
Summary circulation
• Geostrophic current is
around the sea-surface
high
• BUT Gyres are
“squeezed” up against
western boundaries of
ocean basins
Notice asymmetry in circulation
Reasons for western intensification
(asymmetric gyres)
• 1) Earth’s rotationcoriolis effect
asymmetries with latitude:
Westerlies have more
coriolis effect than
Easterlies
• 2) higher speed along the
western margin of the hill
compared to the eastern
side.
Notice asymmetry in circulation
Western boundary currents
• Found in all the ocean basins: NH and SH
• Fast (~1 m/s) and narrow (~100 km)
Figure 7-4
Gulf stream
• Classic W. Boundary
current.
• “Starts” as the Florida
current between Florida
and the Bahamas
• Leaves coast at Cape
Hatteras, NC, cross the
Atlantic.
• Transports HUGE
amounts of water and
HEAT – keeps Europe
warm..
Water transport
Water flow rates, units are in Sverdrups (= 1 million cubic meters/second)
100
~6
Moves so fast, that creates:
Gulf Stream “rings”
Temperature June 11, 1997
cold water surrounded by warm rings:unique physical
characteristics and biological habitats
Warm core rings
Currents shape
climate!
• Warm current - warms air (L) = high water vapor = humid coas
 East Coast USA, also E. Coast Asia (Japan in Aug..)
•
Cool current  cools air (H) =
low water vapor  dry coast
 W. Coast USA (Santa Cruz)
But coastlines also can shape
climate in another way…
Why is it
so much
warmer
out to sea?
(something
about land
makes it
cold…?>)
- UPWELLING -----------------
“UPWELLING”
A key phenomenon for coastal
ocean
-------- to board--- -
Coastal Upwelling
Example from Northern Hemisphere
W
middle of gyre
Sea level
E
Typical Northerly wind
Ekman Transport
i
l
a
C
1000 m
a
i
n
fo r
Coastal Upwelling
Example from Southern Hemisphere
W
middle of gyre
Sea level
1000 m
southerly wind
Ekman Transport
X
E
X
u
r
e
P
Peru and CAAmazingly productive
Upwelling-driven
Environments! -due to the
upwelling of nutrients to
the surface
Global chl map
Chlorophyl a: productive upwelling areas vs oceanic desserts
Equatorial Divergent Upwelling
cross-section
Trade Winds
X X X
2˚S
2˚N
Ekman Trans. Surface
Layer Ekman Trans.
DIVERGENCE
Thins
West
UPWELLING
{to left in SH}
{to right in NH}
East
summary cartoon