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…

• What about the Ocean?

Enthalpy flux(heating)

Sea surface

TropopauseStefan-Boltzmanradiation (cooling)

(i) Surface winds can accelerate water parcels

Annual mean 10m wind from ERA40 Atlas (2006)

(ii) Heating and cooling only occur at the surface

Surface heat flux (Wm-2, color) and temperature (contours, °C) -NOC climatology

Cooling: Enthalpy flux+surface radiation exceed solar+greenhouseradiation

Heating:Solar+greenhouse radiation exceedEnthalpy flux+surfaceradiation

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…

born Benjamin Thompson (1753-1814)

But a still more striking, and I might, I believe, say, an incontrovertible proof of the existence of currentsof cold water at the bottom of the Sea, setting fromthe poles towards the equator, is the very remarkabledifference that has been found to subsist betweenthe temperature of the sea at the surface and atgreat depth, at the tropic –though the temperatureof the atmosphere there is so constant that the greatest changes produced in it by the seasonsseldom amounts to more than five or six degrees;yet the difference between the heat of water at thesurface 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.

It appears to me to be extremely difficult, if not quiteimpossible, 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)

WARM

COLD

Surface cooling

Volume of cold layerincreases at the expenseof the volume of warm water

WaterSo

So

Ω

Results

So

A “western boundary current” is produced,flowing towards the “South” (equator)!

“West” “East”

“South”

“North”

Salinity at 50m depth (in g/kg of seawater)

World Ocean Atlas 2001

30

40

Lower salinitythrough excessprecipitation

Higher salinityThrough excessevaporation

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?

Thermodynamic cycle of the THC

Z

AB C DA

B

C

D

EntropyT

empe

ratu

re

Equator NHSH

Cold and fresh

Warm and salty

Separation between thesurface (diabatic) layerand the deep (adiabatic)layer

The “Great Oceanic heat pump”

A

B

C

D

EntropyT

empe

ratu

re

Cold and fresh

Warm and salty

AtlanticOcean

ColdSouthern Ocean

WarmNorth Atlantic

MechanicalEnergy input

Work done by the surface wind on the ocean (≈2TW total)

Wunsch (1998)2310 WmIn units of

The “great oceanic heat pump”

1

BCAD

ADC TT

T

W

Q

Z

AB C DA

B

C

D

EntropyT

empe

ratu

re

Wind forcingover S. Ocean

Atlantic heat transport

Equator NHSH

Cold and fresh

Warm and salty

Southern Ocean Jet stream

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

Trade winds

Jet Stream

Gulf Stream

oStreamGulfV

The Gulf Stream would exist even if there was no such thing as the thermohaline circulation

Water

Surface winds

(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