Will the Gulf Stream disappear with global warming…?

Strength of ocean overturning at 30N (A1B Scenario + constant after yr2100)

Our own EPcm

After a sudden 2XCO2

Control experiment

Strength of ocean overturning at 30N (A1B Scenario + constant after yr2100)

What is the ocean doing...??

Outline

• Background on the thermohaline circulation and the Gulf Stream

• Why is there such a large spread among climate models when it comes to the Gulf Stream?

• Will the Gulf Stream disappear with global warming?

Part 1: Background on the Gulf Stream and the thermohaline

circulation

Broecker, 2005NB: 1 Amazon River ≈ 0.2 Million m3/s

Broecker, 2005NB: 1 Amazon River ≈ 0.2 Million m3/s

The Gulf Stream refers to the North Atlantic portion of this global “thermohaline circulation”

When was the thermohaline circulation discovered?

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 “great oceanic conveyor belt”

The “great oceanic conveyor belt”

Theory of the thermohaline circulation also led to observational

discoveries

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”

Results

So

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

“West” “East”

“South”

“North”

This deep current was observed in the North Atlantic in 1960

More recent measurementsof Deep Western Boundary currents in the North Atlantic

Meinen et al. (2004)

Subtropical gyrecurrent (northward)

Deep westernBoundary current

(southward)

“Observations” of the Gulf Stream

and the thermohaline circulation

No such observations can exist…

• Impossible to have current meters everywhere in the ocean at any time!

• So one needs to be smart and interpolate the available data in the best possible way…

• To do this the solution is to constrain a numerical ocean model to be close to observations

Sea surface height measured from Space

Why is sea surface height useful?

… Newton’s law!

CORIOLIS FORCE = PRESSURE FORCE

Current Sea surface height

10-yr average sea surface height (deviation from geoid)

10-yr average sea surface height (deviation from geoid)

GULF STREAM

KUROSHIO GULF STREAM

AGULHAS

East Australian Current

Malvinas

Sv2010max

Atlantic ocean’s meridional overturning streamfunction simulated by MITgcm constrained to satellite altimetry

136101 smSv

Wunsch (2000)

Part 2: Mechanisms

• Why is there so much spread among climate models?

Mechanisms

• Why is there so much spread among climate models?

→ Feedback analysis!

Stommel’s model of the thermohaline circulation (1961)

1 2

Heating & Salinification

(low latitudes)

Cooling & Freshening(high latitudes)

)( 21 k

)1( STo

z

(Anticlockwise if box1 is denser)

(density increases with S &decreases with T)

temperature salinity

“sea surface”

“Gulf Stream”

Heating and cooling at the sea 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

Net Evaporation minus Precipitation

Two equilibrium states

WARM(salty)

COLD(fresh)

“Thermal”:-Density driven by T-Strong Gulf Stream

Ψ

High latitudesLow latitudes

SALTY(warm)

FRESH(cold)

High latitudesLow latitudes

Ψ“Haline”:

-Density driven by S -Weak (reverse) Gulf

Stream

Positive salt feedback

High S at high latitudes

Enhanced N/Sdensity gradient

Enhanced GulfStream

Positive salt feedback

High S at high latitudes

Enhanced N/Sdensity gradient

Enhanced GulfStream

UNSTABILITY, leads tolarge response to

perturbations

Low T at high latitudes

Negative temperature feedback

High T at high latitudes

Reduced N/Sdensity gradient

Reduced GulfStream

(reduced polewardheat transport)

Low T at high latitudes

Negative temperature feedback

High T at high latitudes

Reduced N/Sdensity gradient

Reduced GulfStream

(reduced polewardheat transport)

STABILITY, leads tosmall response to

perturbations

Different models have different temperature & salinity feedbacks

and this is what leads to the spread in models’ behaviour

Different models have different temperature & salinity feedbacks because these involve coupled ocean-atmosphere processes…

• Changes in sea surface temperature induced by changes in ocean circulation alter the atmospheric circulation

• This in turn modifies the cooling and freshening of the ocean at the sea surface…

• … which changes back the sea surface temperature and salinity!

Inter-model comparison is a very active area of research…

100yr 200yr 300yrOce

an c

ircul

atio

n in

tens

ity

Strong Sfeedback because ofweak A/Ocoupling…

Yin & Stouffer (2007)

Part 3: Will the Gulf Stream disappear with global warming?

Strength of ocean overturning at 30N (A1B Scenario + constant after yr2100)

…pushing the limit:a movie scenario…?

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!

The answer is no

• As a result of increased CO2, more freshening and more surface heating, which indeed reduces the high latitude density

The answer is no

• As a result of increased CO2, more freshening and more surface heating, which indeed reduces the high latitude density

• No model predicts an abrupt switch to an “off-state” within the 21st century

The answer is no

• As a result of increased CO2, more freshening and more surface heating, which indeed reduces the high latitude density

• No model predicts an abrupt switch to an “off-state” within the 21st century

• The models which predict a significant weakening are the least reliable (very elementary atmospheric feedbacks, “flux adjusted”)

The answer is no

• As a result of increased CO2, more freshening and more surface heating, which indeed reduces the high latitude density

• No model predicts an abrupt switch to an “off-state” within the 21st century

• The models which, predict a significant weakening are the least reliable (very elementary atmospheric feedbacks, “flux adjusted”)

• In general, the more complex the model, the more stable the Gulf Stream

Conclusions

• A lot of work needs to be done on modelling the ocean circulation and its interaction with the atmosphere

• 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, 1-2pm in HuxleyRoom 726