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The great “oceanic heat
pump”-
Will the Gulf Stream disappear
with global warming…?
Motivation
• Atmosphere produces its circulation the hard way,
through heating/cooling (heat engine): no external force
pushes air parcels from West to East to create the Jet
Stream…
Tropopause
Stefan-Boltzman
radiation (cooling)
Enthalpy flux
(heating)
• What about the Ocean?
Sea surface
(i) Surface winds can accelerate
water parcels
Annual mean 10m wind from ERA40 Atlas (2006)
(ii) Heating and cooling only occur at the surface
Cooling:
Enthalpy flux+surface
radiation exceed
solar+greenhouse
radiation
Heating:
Solar+greenhouse
radiation exceed
Enthalpy flux+surface
radiation
Surface heat flux (Wm-2, color) and temperature (contours, °C) -NOC climatology
Outline
• The thermohaline circulation (THC)
• The “Great oceanic heat pump”
• Gulf Stream and Global Warming
Captain Ellis and the Earl of Halifax
“measurements” (1751)
• “Upon the passage, I made several trials with the bucket
sea-gage. I charged it and let it down to different depths,
from 360 feet to 5346 feet; when I discovered, by a small
thermometer of Fahrenheit’s, made by Mr. Bird, which
went down to it, that the cold increased regularly, in
proportion to the depths, till it descended to 3900 feet:
from whence the mercury in the thermometer came up to
53 degrees; and tho’ I afterwards sunk it to the depth of
5346 feet, it came up no lower. The warmth of the water
upon the surface, and that of the air, was at the time by
the thermometer 84 degrees.”
Captain Ellis and the Earl of Halifax
“measurements” (1751)
• “The experiment, which seem’d at first but
mere food for curiosity, became in the
interim very useful to us. By its means we
supplied our cold bath, and cooled our
wines or water at pleasure; which is vastly
agreeable to us in this burning climate”
NB: Captain Ellis was at 25N, 25W
Count Rumford explains the measurements…
But a still more striking, and I might, I believe, say,
an incontrovertible proof of the existence of currents
of cold water at the bottom of the Sea, setting from
the poles towards the equator, is the very remarkable
difference that has been found to subsist between
the temperature of the sea at the surface and at
great depth, at the tropic –though the temperature
of the atmosphere there is so constant that the
greatest changes produced in it by the seasons
seldom amounts to more than five or six degrees;
yet the difference between the heat of water at the
surface of the Sea, and that at the depth of 3600 feet,
has been found to amount to no less than 31 degrees;
the temperature above or at the surface being 84
degrees, and at the given depth below no more than
53 degrees.
born Benjamin Thompson
(1753-1814)
It appears to me to be extremely difficult, if not quite
impossible, to account for this degree of cold at the
bottom of the sea in the torrid zone, on any other
supposition than that of cold currents from the poles.
The Stommel-Aarons-Faller
experiments (1958)
Ω
Surface cooling
WARM
So
So
COLD
Volume of cold layer
increases at the expense
of the volume of warm water
Results
“North”
So
“East”
“West”
“South”
A “western boundary
current” is produced,
flowing towards the
“South” (equator)!
Salinity at 50m depth (in g/kg of seawater)
40
Higher salinity
Through excess
evaporation
Lower salinity
through excess
precipitation
30
World Ocean Atlas 2001
The “great oceanic conveyor belt”
The “great oceanic conveyor belt”
Summary (THC part)
• Temperature at depth is much colder than at the
surface as a result of deep currents from the
poles.
• Salty water flows southward, deep in the Atlantic
Ocean.
• It upwells over the surface Southern Ocean
where it mixes with fresher surrounding waters,
before subsequently flowing northward at
subsurface as a fresh current.
• The Gulf Stream closes this “thermohaline cell”
by replenishing the volume of warm water in the
North Atlantic.
What drives this thermohaline cell?
Z
B
C
D
A
Temperature
Thermodynamic cycle of the THC
D
A
C
B
SH
Equator
NH
Entropy
Separation between the
surface (diabatic) layer
and the deep (adiabatic)
layer
The “Great Oceanic heat pump”
Temperature
Cold
Southern Ocean
Atlantic
Ocean
A
C
B
Mechanical
Energy input
Warm
North Atlantic
D
Entropy
Work done by the surface wind on
the ocean (≈2TW total)
Wunsch (1998)
In units of 10
3
Wm
2
The “great oceanic heat pump”
Z
B
C
D
A
Temperature
Southern Ocean
Jet stream
D
A
C
B
SH
Equator
NH
Entropy
Atlantic heat
transport
Wind forcing
over S. Ocean
Q
TAD
C  
 1
W TAD  TBC
The Thermohaline circulation and
Global Warming
Movie Scenario:
-CO2 increases
-Hydrological cycle strengthens
-Atlantic Ocean becomes much less salty
-The heat pump structure breaks down:
the mechanical energy input by the wind can
not be converted to a heat transport
-Sea-ice margin moves southward
-Sea-ice albedo kicks in… Global Freeze!
… irrealistic
The Gulf Stream and Global warming
The Gulf Stream would exist even if there was
no such thing as the thermohaline circulation
Jet Stream
Surface winds
VGulf Stream 
Water
Gulf Stream
Trade winds
 
o 
(spherical)
Earth Rotating
Conclusions
• The thermohaline circulation works like a
heat pump
• The Gulf Stream will not disappear with
Global Warming
…I leave it to Jenny…
But I maintain office hours until the end of term:
Tuesdays, 2-3pm in Huxley, Room 726
NB: Except on week 10