Transcript OceanCurrentSwitchin..

Ocean Current Switching:
Can changes in ocean circulation
cause rapid climate change?
Richard Karsten
Department of Mathematics and Statistics
[email protected]
IB Camp, Wolfville
August 20, 2008
Climate change, Hollywood style
Premise: Climate change might
occur rapidly, causing untold
destruction
This year,
sweater
won'tdidn't
do. check the weather.
Whoever said "Tomorrow
Nature
is aanother
has spoken.
day"...
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
19
o
15
www.nsac.ns.ca
o
Ocean and climate: El Nino
Australia
El
Niño
La Niña
earthobservatory.nasa.gov
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).
Depth
5000 m
Halifax
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.
o
Wind forcing: Flow is 90 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
Temperature
meridional
North
South
depth
depth
South
Salinity
meridional
North
Convection and overturning
Cooling at the surface
can create heavy
water at the surface,
which then “sinks.”
The net effect is an
overturning circulation.
Heat loss
Gulf Stream transports heat north
Warm water pulled north by
cooling and sinking.
Gulf Stream
Warm water “pushed”
north by wind.
(after W. Broecker, modified by E. Maier-Reimer).
Gulf Stream and climate
www.theworldwidegourmet.com
Australia
www.travelblog.org/Bloggers/Dan--NZ/
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?
Melting Ice
• when ice melts in turns into COLD,
FRESH water
• If this water enters the north Atlantic
it can prevent the sinking of the salty
surface waters
Melting Ice
Water properties: Salinity
Global warming is melting
polar ice caps, which should
make the northern waters
fresher.
South
depth
meridional
zonal
meridional
North
Fresh water shuts off convection
Fresh water from
glacial melt
makes surface
water fresher.
If the water is
fresh enough, it
doesn’t become
heavy so it
doesn’t sink.
Heat loss
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?
Is the Gulf Stream
changing?
Weaker Gulf Stream threatens
Britain's climate
By Mark Henderson, Science Correspondent
London Times, November 30, 2005
An ominous current event
Matthew Hart
Globe and Mail, January 14, 2006
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
• 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
(after W. Broecker, modified by E. Maier-Reimer).
Are there changes in the circulation?
Deep return
flow has
reduced by
about 30%.
Shallow return
flow has
increased by
almost 50%.
Measurement
line
Surface warm flow has
decreased slightly
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?