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Ocean Current Switching: Can changes in ocean circulation cause rapid climate change?

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Title: Ocean Current Switching: Can changes in ocean circulation cause rapid climate change?


1
Ocean Current Switching Can changes in ocean
circulation cause rapid climate change?
  • Richard Karsten
  • Department of Mathematics and Statistics
  • rkarsten_at_acadiau.ca

IB Camp, Wolfville August 20, 2008
2
Climate change, Hollywood style
3
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.
4
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.
5
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)
6
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?

7
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?

8
Ocean and weather Nova Scotia
Australia
19o
15o
www.nsac.ns.ca
9
Ocean and climate El Nino
Australia
El Niño
La Niña
earthobservatory.nasa.gov
10
The villain the Gulf Stream
British Geomorphological Research
Group www.bgrg.org
11
Hopedale, 55º27 N
Stornoway, 58º54 N
12
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).
Bordeaux
Halifax
Depth 5000 m
13
The ocean stores the heat created by global
warming.
S. Levitus, J. Antonov, and T. Boyer, GRL 2005
14
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?

15
Ocean currents
The strongest currents in the ocean are a result
of the wind blowing over the surface of the ocean.
16
Average surface winds
17
A rotating earth changes dynamics Coriolis force
To observers in the Northern Hemisphere, rotation
of the earth deflects motion to the right.
18
Wind forcing Flow is 90o to the right!
50 m
19
Wind-driven circulation
H
20
Gyre flow
H
21
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).

22
Sea water density
Warm water low density (light) Cold Water high
density (heavy) Fresh Water low density Salty
Water high density
23
Water properties
Temperature
Salinity
24
Water properties Salinity
Temperature
Salinity
North
South
meridional
depth
25
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.
26
  • 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).
27
Gulf Stream and climate
www.travelblog.org/Bloggers/Dan--NZ/
www.theworldwidegourmet.com
Australia
28
Thermohaline circulation (Meridional overturning
circulation)
29
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?

30
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

31
Melting Ice
32
Water properties Salinity
zonal
Global warming is melting polar ice caps, which
should make the northern waters fresher.
meridional
North
South
meridional
depth
33
Fresh water shuts off convection
Fresh water from glacial melt makes surface water
fresher.
Heat loss
X
If the water is fresh enough, it doesnt become
heavy so it doesnt sink.
34
Thermohaline circulation (Meridional overturning
circulation)
x
35
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?

36
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
37
Evidence of freshening
 http//www.whoi.edu/mr/pr.do?id5098
38
Evidence of freshening
 (B. Dickson, et. al., in Nature, April 2002)
39
  • Northern waters are fresher
  • North Atlantic water has become fresher,
    possibly reducing the thermohaline circulation.

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

(after W. Broecker, modified by E. Maier-Reimer).
40
  • Are there changes in the circulation?

41
Shallow return flow has increased by almost 50.
Deep return flow has reduced by about 30.
Measurement line
Surface warm flow has decreased slightly
42
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.

43
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.

44
Ocean Current Switching Can changes in ocean
circulation cause rapid climate change?
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