Ocean Current Switching:Can changes in ocean circulation

cause rapid climate change?

Richard KarstenDepartment of Mathematics and Statistics

rkarsten@acadiau.ca

IB Camp, WolfvilleAugust 20, 2008

Climate change, Hollywood style

Premise: Climate change might occur rapidly, causing untold

destruction

This year, a sweater won't do.Whoever said "Tomorrow is another day"... didn't check the weather. Nature has spoken.

The villain: the Gulf Stream

Wikipedia Synopsis:The movie is based on the idea that the Gulf Stream (or North Atlantic drift), an ocean current which circulates warm water from the tropics to the Northern Hemisphere, is disrupted by the melting of the polar ice caps. This leads to catastrophic changes in the Earth's climate, as the temperature of the Earth's atmosphere stabilizes into a new pattern.

A little more science on the big screen …

Melt water from Greenland, because of its lower salinity, could then halt the currents that keep northern Europe warm and quickly trigger dramatic local cooling there.(source: Wikipedia)

Is any of this realistic?

• Can the ocean affect climate?

• What is the Gulf Stream and how does it affect climate?

• Can melting icecaps move ocean currents?

• Is the effect of the Gulf Stream changing due to global warming?

Is any of this realistic?

• Can the ocean affect climate?

• What is the Gulf Stream and how does it affect climate?

• Can melting icecaps move ocean currents?

• Is the effect of the Gulf Stream changing due to global warming?

Ocean and weather: Nova Scotia

Australia

15o

19o

www.nsac.ns.ca

Ocean and climate: El Nino

Australia

earthobservatory.nasa.gov

El Niño

La Niña

The villain: the Gulf Stream

British Geomorphological Research Group

www.bgrg.org

Hopedale, 55º27’ N

Stornoway, 58º54’ N

The ocean stores a lot of heat.A column of ocean water only 3 m thick contains as much heat capacity as the full atmosphere above

(Gill, 1982). D

epth

5000

mHalifax Bordeaux

The ocean stores the heat created by global warming.

S. Levitus, J. Antonov, and T. Boyer, GRL 2005

Is any of this realistic?

• Can the ocean affect climate?

• What is the Gulf Stream and how does it affect climate?

• Can melting icecaps move ocean currents?

• Is the effect of the Gulf Stream changing due to global warming?

Ocean currents

The strongest currents in the ocean are a result of the wind blowing over the surface of the ocean.

Average surface winds

A rotating earth changes dynamics: Coriolis force

To observers in the Northern Hemisphere, rotation of the earth deflects motion to the right.

Wind forcing: Flow is 90o to the right!

50 m

Wind-driven circulation

H

Gyre flow

H

Thermohaline circulation

• Changes in the density of sea water can also drive ocean circulation.

• Vertical Motion: Light water rises to the top and heavy water sinks.

• Density changes are due to changes in temperature (thermo) and salinity (haline).

Sea water density

Warm water low density (light)

Cold Water high density (heavy)

Fresh Water low density

Salty Water high density

Water properties

Temperature Salinity

Water properties: Salinity

South Northmeridional

dept

h

South Northmeridional

dept

h

Temperature Salinity

Convection and overturning

Cooling at the surface can create heavy water at the surface, which then “sinks.”

Heat loss

The net effect is an overturning circulation.

Gulf Stream transports heat north

(after W. Broecker, modified by E. Maier-Reimer).

Gulf Stream

Warm water “pushed” north by wind.

Warm water pulled north by cooling and sinking.

Gulf Stream and climate

Australia

www.travelblog.org/Bloggers/Dan--NZ/www.theworldwidegourmet.com

Thermohaline circulation (Meridional overturning circulation)

Is any of this realistic?

• Can the ocean affect climate?

• What is the Gulf Stream and how does it affect climate?

• Can melting icecaps move ocean currents?

• Is the effect of the Gulf Stream changing due to global warming?

• when ice melts in turns into COLD, FRESH water

Melting Ice

• If this water enters the north Atlantic it can prevent the sinking of the salty surface waters

Melting Ice

Water properties: Salinityzonal

mer

idio

nal

South Northmeridional

dept

h

Global warming is melting polar ice caps, which should make the northern waters fresher.

Fresh water shuts off convection

Heat lossFresh water from glacial melt makes surface water fresher.

If the water is fresh enough, it doesn’t become heavy so it doesn’t sink.

X

Thermohaline circulation (Meridional overturning circulation)

x

Is any of this realistic?

• Can the ocean affect climate?

• What is the Gulf Stream and how does it affect climate?

• Can melting icecaps move ocean currents?

• Is the effect of the Gulf Stream changing due to global warming?

An ominous current eventMatthew Hart

Globe and Mail, January 14, 2006

Weaker Gulf Stream threatens Britain's climateBy Mark Henderson, Science Correspondent

London Times, November 30, 2005

Is the Gulf Stream changing?

Evidence of freshening

 http://www.whoi.edu/mr/pr.do?id=5098

Evidence of freshening

 (B. Dickson, et. al., in Nature, April 2002)

Northern waters are fresher

(after W. Broecker, modified by E. Maier-Reimer).

• North Atlantic water has become fresher, possibly reducing the thermohaline circulation.

• If the warm water is no longer pulled into the North Atlantic, it should return south at shallower depths.

X

Are there changes in the circulation?

Surface warm flow has decreased slightly

Shallow return flow has

increased by almost 50%.

Deep return flow has

reduced by about 30%.

Measurement line

Reaction:Climate change: A sea change

by Quirin Schiermeier in Nature 2006 • The results are a surprise to scientists in the field.

• Modelling suggests that increase of fresh water flows large enough to shut down the thermohaline circulation would be an order of magnitude greater than currently estimated to be occurring.

• The Bryden results could be caused by natural variation, or "noise", that is, coincidence.

• If the results are correct, perhaps thermohaline circulation reductions will not have the drastic effects that have been predicted on European cooling.

More … Ocean circulation noisy, not stalling

by Quirin Schiermeier in Nature 2007 • “…the seemingly dramatic reduction discovered two

years ago in the strength of the Atlantic meridional overturning circulation (MOC) is easily within the range of huge seasonal variability.”

Ocean Current Switching:Can changes in ocean

circulation cause rapid climate change?