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MOC Sensitivity to AnthropogenicMOC Sensitivity to AnthropogenicClimate Forcing in the CCSM:Climate Forcing in the CCSM:
Are There Any Abrupt SurprisesAre There Any Abrupt Surprises??
Peter R. GentPeter R. GentNCARNCAR
Blue: T85, ×1 Red: T42, ×1 Black: T31, ×3
CCSM3: Present Day Control Runs Maximum MOC in North Atlantic
T31, ×3
T42, ×1
T85, ×1
Control: Black
1% CO2 Run: Red, Cyan
2 × CO2: Green
4 × CO2: Blue
THC Response toIncreasing CO2
Year
1900 22002000 2100
CO
2 con
cent
ratio
n
20 century run
B1 stabilization(nominal 550 ppm)
A1B stabilization(nominal 750 ppm)
2300
Overshootscenario
Committed climate change
A1B
B1
1870
1870 control run
Future Scenario Runs Using CCSM3
Nominal 550 ppm stabilization
Nominal 750 ppm stabilization
A1B
B1
Year
Max
. mer
idio
nal o
vertu
rnin
g (S
v)in
Nor
th A
tlant
ic
Overshoot scenario
MOC Response to Future Scenarios
Sea
leve
l ris
e by
the
rmal
exp
ansi
on (
cm)
Year
Nominal 750 ppm stabilization
A1B
B1
Nominal 550 ppm stabilization
Overshoot scenario
Sea
leve
l ris
e by
the
rmal
exp
ansi
on (
cm)
Year
Nominal 750 ppm stabilization
A1B
B1
Nominal 550 ppm stabilization
Overshoot scenario
Sea
leve
l ris
e by
the
rmal
exp
ansi
on (
cm)
Year
Nominal 750 ppm stabilization
A1B
B1
Nominal 550 ppm stabilization
Overshoot scenario
Sea Level Response to Future Scenarios
SummarySummary
•• Gradual changes in anthropogenic forcingGradual changes in anthropogenic forcing⇒⇒ Gradual changes in North Atlantic MOCGradual changes in North Atlantic MOC
•• THC reductions, but no collapsesTHC reductions, but no collapses•• THC strength appears to be simply relatedTHC strength appears to be simply related
to the stability of the water columnto the stability of the water column•• Both in deep water formation regions andBoth in deep water formation regions and
in the overall North Atlantic MOCin the overall North Atlantic MOC
By end of 21st century SAT consistently warms in all scenarios
Abrupt transitions in Sea Ice10
6 km
2
Although forcing is gradual, abrupt transitions in sea ice do occurin some simulations.
September ice extent timeseries
Sea Ice Conditions
Simulated abrupt transitions in sea iceabrupt forcing (freshwater hosing) can result in abrupt ice changes
•Sea ice changes amplify climate response•Global teleconnections can result•Longevity of these changes are an issue
Sea ice change SAT Change
(From Vellinga and Wood, 2002;Vellinga et al, 2002)
N. Atlantic Fresh Water Input Expts. (CCSM2)
ConclusionsConclusions•• THC reductions due to anthropogenicTHC reductions due to anthropogenic
forcingforcing•• Europe always warms in future scenariosEurope always warms in future scenarios•• Abrupt sea ice changes in one A1BAbrupt sea ice changes in one A1B
ensemble memberensemble member•• Abrupt MOC changes are caused byAbrupt MOC changes are caused by
abrupt fresh water additions in the Northabrupt fresh water additions in the NorthAtlanticAtlantic
•• Europe most often cools in these expts.Europe most often cools in these expts.
Abrupt transitions in future climatescenarios?
Meehl et al, 2005Wigley, 2000
• Relatively gradual forcing.• Relatively gradual response in global air temperature