Environmental Change 1 - Lecture 6
Ocean Circulation
Ron Kahana
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Ocean Circulation
• What drives the Ocean circulation?
• Two kinds of circulation (?)
– Wind-driven (surface) circulation.
– Thermohaline circulation (deep), driven
by fluxes of heat and freshwater
across the sea surface.
• Role of Oceans in climate
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Atmosphere and Ocean circulations are all about
the redistribution of heat on a rotating planet
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Transporting the heat to the poles
• Atmospheric and oceanic circulation
is about transporting heat from the
equator to the poles.
Northward heat transport across each latitude
(1PW=1015W)
• The maximum energy transport is
similar. Oceans peak at 200N
• A direct effect: The atmosphere is
heated from the bottom, air column
becomes unstable and rises.
• However, the oceans are heated from
the top therefore become more stable.
So how the energy received from the
sun keeps the ocean circulation going?
the Atmosphere holds the key for that.
Figure from: The Earth System. Kump,Casting and Crane, 2004
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Radiation from the
Sun
What drives the Ocean circulation?
Atmosphere
Oceans
Wind stress
momentum
Heating and cooling
Temp.
density
Evaporation and
precipitation
Wind driven
(Surface)
circulation
Salinity
Thermohaline
(deep)
circulation
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Two kinds of circulation
Surface ocean: Wind-driven
circulation, at the mixed
area between the surface and
the thermocline (300-1000 m
depth).
Ocean currents also subject to
Coriolis force, which deflects
them 20-25 degrees from
wind direction
Deep ocean: Thermohaline
circulation driven by
differences in density
(temperature and salinity).
The pycnocline separates the surface zone (aka mixed
layer) from the deep ocean
Figure from: The Earth System. Kump,Casting and Crane, 2004
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The Wind Driven Ocean Circulation
Subtropical Gyre
Equatorial currents
Antarctic
Circumpolar
Current
(ACC)
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Ekman Transport – How winds
effect currents?
•
Considered the effect of wind
and friction on multiple levels in
the ocean.
•
Coriolis effects all layers
because they’re moving
•
current velocity decreased
exponentially with depth
•
Surface current moved at 450 to
the wind.
•
Deviation is increased with
depth (Ekman Spiral)
• The mean current moves at
right angle to the wind
direction (to the right in the
NH, and to the left in the SH)
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How to build a subtropical gyre
1. Start from the
winds:
prevailing winds move
surface waters toward
subtropical regions
both from tropical
areas and from highlatitudes.
2. Add continent boundaries
the subtropical gyres are
zones of convergence where
surface water piles up and
forming slopes.
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Gyre Dynamics (how to build II)
4. Consider changes in
planetary vorticity to get
an asymmetric gyre.
(Find more about that in Ch. 5 of
‘The Earth System’, or in section
4.2 of the Open University ‘Ocean
Circulation’).
3. get geostrophic:
Water responds to the slopes of the ocean
surface as it would on land -- it runs
downhill, but because of earth's rotation it
is deflect and flows parallel to the slope.
•The balance between gravitational
forces and the Coriolis effect, gives
rise to geostrophic currents. Their
velocity is proportional to the pressure
gradient.
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The subtropical gyres
The asymmetrical subtropical gyres are the
main mechanism that carry heat poleward and
cold water towards the equator.
western boundary currents are narrow, fast and
deep
Gulf Stream (western North Atlantic): 1.5-2.5 m/s
(5-9 km/hr)
Kuroshio Current (western North Pacific)
eastern boundary currents
are wide, shallow, and sluggish.
Canary Current (eastern North
Atlantic), California Current
(eastern North Pacific):
0.03-0.07 m/s (< 2 km/hr)
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Equatorial Currents
•Best developed in the Pacific
•Consists of:
• currents flowing east-to-west
(e.g. north equatorial current,
south equatorial current),
directly driven by the prevailing
Trade winds.
•The equatorial countercurrent
flows down the slope (caused by
the trade winds) towards the
east, in zone of small westward
wind stress (the Doldrums).
Costal upwelling
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The Antarctic Circumpolar Current
(ACC)
• The only current that flows around the globe
(~24,000 km).
• “mighty Current” – transports more water than any
other current, est. at 100-150 Sverdrups (1 Sv=106
m3 s-1).
• extend from the sea surface to depths of 20004000m deep and can be as wide as 2000 km.
• typical velocity ~0.1m/s (Much slower than the Gulf
Stream ~1-2.5 m/s).
•The ACC is in approximately geostrophic
equilibrium (PGF=CF)
Try to figure out:
If the ACC is in geostrophic balance and flows from west to east.
Drake Passage
• In which direction the Coriolis force is acting?
• Could you say will the sea surface be higher to the north or
south of the ACC? Would you expect the prevailing winds over
the Southern Ocean to be westerly (flowing at the same direction
as the ACC) or easterly? (see slide #4 in lec. 3)
Figure from: The Earth System. Kump,Casting and Crane, 2004
13 of
For more information about the ACC (and other currents) see the ’Ocean Surface Currents’ page from the University
Miami at: http://oceancurrents.rsmas.miami.edu/index.html.
Thermohaline circulation
Another mechanism to drive ocean circulation is
through fluxes of heat and freshwater
• Seawater are made denser by cooling and/or
increasing salinity.
•Deep water is formed in localised areas.
when the water column then becomes unstable,
leading to large-scale deep overturning of the
oceans.
It is estimated that the turnover time required to
replace the deep water through deep water
formation is on the order of 1000 -5000 years.
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Key features of the Thermohaline
circulation
1) Deep water
formation:
in a few localized
areas:
North Atlantic: in the
Greenland-IcelandNorwegian (GIN)
Seas, the Labrador
Sea
Soutern Ocean: in
the Weddell Sea, the
Ross Sea.
The Mediterranean
Sea.
3) Upwelling of deep waters not as localized as convection
and difficult to observe (ACC?).
2) Spreading of
deep waters
4) Near-surface currents: these are required to close the flow
( ~20% of the Gulf stream, ~20Sv).
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Spreading of deep waters
Water Masses basics:
Most of the heat and salt exchange
between the atmosphere and the
oceans occurs in the upper 150m .
Once a parcel of water is removed from
the surface, its properties (T,S) will not
change until it rises again, many years
later.
Spreading of deep waters
North Atlantic Deep Water (NADW),
Antarctic Bottom Water (AABW)
Movement of water masses
is slowly – and it is unrealistic to measure directly. We
deduce it by measuring the age of the water (through carbon14 dating, for example). or from the distribution of the water
properties themselves.
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Role of Oceans in Climate variations
On time-scale of months or years Oceans are vast
reservoir of heat and will regulate the climate by
heating or cooling the atmosphere (hurricanes / El
Niño).
On longer time-scale it
is the large heat
transport (1PW) of the
deep circulation that
could change the
climate.
3-D Schematic of Thermohaline Circulation by William Schmitz
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The THC and Climate change
One way to estimate the
effect of the THC is to switch
it off in coupled climate
models (by adding a lot of
freshwater to the northern
Atlantic), and compare the
surface climate before and
after switching it off.
Change in surface air temperature during years 20-30 after
the collapse of the THC. (from Vellinga and Wood, 2002).
-60C
Unfortunately, the magnitude
and location of the cooling is
model dependent.
most tend to affect
temperatures over land
in north-western Europe
(Scandinavia, Britain) by
several degrees, others show
strong cooling further west
[This figure].
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But have the THC ever been shut?
Both Palaeo-data and model simulations
suggest that the THC have been weakened or
shut during the last glacial.
•Would future global warming effect the THC?
•Surface warming and surface freshening both
reduce density of high latitudes.
•Most models predict a significant weakening of the
NADW
These are the sort of model
experiments that we perform in
Schematic of three circulation modes of the glacial
Atlantic: the prevailing cold mode (center), the warm mode
associated with Dansgaard-Oeschger events (lower) and
the ‘off’ mode occurring after Heinrich events (upper
globe).
From: S. Rahmstorf: Thermohaline Ocean Circulation. In:
Encyclopedia of Quaternary
Sciences, Edited by S. A. Elias. Elsevier, Amsterdam 2006.
The role of weakening the Gulf Stream is
debateable, see: realclimate.org
http://www.realclimate.org/index.php/archives/2005/05/gulfstream-slowdown/
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Summary
• The circulations of the oceans and atmosphere are driven by energy
from the sun and modified by the earth rotation. Both help to reduce
equator-to-pole temperature gradients.
• Oceans and atmosphere are a single system of two interacting
components, coupled by the air-sea interactions. Oceans can absorb
heat in one region and restore it to the atmosphere (perhaps decades
or centuries later) at a quite different place.
• Thermohaline and wind-driven currents cannot be separated by
oceanographic measurements.
• two distinct physical forcing
• very different time scales
• but not two uniquely separable circulations.
• A THC collapse is seen to have happen in the past. It is discussed as
one of "low probability - high impact“ risks associated with global
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warming.
Reading List
Most of the material covered here is in:
Kump, L.R., Kasting, J.F and Robert, G. Crane: The Earth System (Ch. 5)
Other sources:
Open University: Ocean Circulation
Tomczak, M., Godfrey, S.J: Regional Oceanography (Ch. 1-5). This is an online book that can
be downloaded from: http://www.es.flinders.edu.au/~mattom/regoc/pdfversion.html
Bigg, GR.: The Oceans and Climate
Dennis Hartmann: Global Physical Climatology
Good online sources for the thermohaline circulation:
Rahmstorf, S: A brief fact sheet
http://www.pik-potsdam.de/~stefan/thc_fact_sheet.html
Rahmstorf, S: Thermohaline Ocean Circulation. In: Encyclopedia of Quaternary Sciences,
http://www.pik-potsdam.de/~stefan/Publications/Book_chapters/Rahmstorf_EQS_2006.pdf
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Extra Slides
• Why there’s no deep water formation in the
Pacific.
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Inferring the age of deep water masses
Age of bottom water can be
inferred by the rate of
decay of carbon-14 to 12C.
older
younger
~300 yrs
~1000 yrs
Oxygen at 4000 m gets
depleted because of
biological consumption
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70C
120C
Source: Most Recent Sea Surface Temperature Field
(Polar Orbiting Satellite SST Experimental Products)
http://coralreefwatch.noaa.gov/satellite/current/key_sst_50km_field.html
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Characteristics of Ocean Gyres
• Downwelling in
the center of the
gyre (low
productivity.
• Bringing warm
tropical waters
north.
Transphers heat.
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Fact Sheet
Typical salinity: 34.7 Practical Salinity Units (PSU)
Density of surface seawater: kg m3
Water density anomaly: freshwater max. density is at
40C. lakes and rivers will freeze from top to
bottom. For salty water, the density increases right
down to the freezing point (~ -1.80C) so the whole
surface layer of the ocean (down to 100-150m) has
to be cooled to the freezing point before freezing
can begin at the surface.
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