Transcript Modeling MOC

Modeling the MOC
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Ronald J Stouffer
Geophysical Fluid Dynamics Laboratory
NOAA
The views described here are solely those of the presenter and
not of GFDL/NOAA/DOC or any other agency or institution.
Modeling the MOC
Outline
• Role of MOC in maintaining mean climate
– Northward Heat Transport
– Northward Salt transport
• Role of MOC in Abrupt Climate Change
– Unforced
– Forced
• Predictability
How well do models simulate the T,
S structure in the ocean?
AR4 ensemble mean error Temperature
-2.5
+2.5
IPCC WGI Chapter 8
AR4 ensemble mean error
Salinity
EQ
90N
-1.0
-0.4
-0.2
PSU
90S
0.2
0.4
+1.0
IPCC WGI Chapter 8 Suppl. Material
Role of MOC in Heat Transport
AOGCMs
EMICs
Impact of
MOC on
Climate
(SAT)
MOC “on” minus “off”
oC
Conclusion – MOC warms
NH, locally large values,
MOC cools SH
Impact of MOC on Climate
Red lines control
Blue lines difference
Dashed – EMICs
Solid - AOGCMs
Zonally
averaged
Precipitation
differences
(mm/day)
Conclusion – MOC on =>
ITCZ toward north
Impact of MOC on Climate
Salinity MOC “on” minus “off”
(PSU)
Role of MOC in maintaining mean
climate
• North Atlantic saltier than without MOC
– Rest of world ocean surface more fresh
• Northern Hemisphere warmer than without
– Particularly the N Atlantic
– Role of atmosphere mixing heat
Ocean MOC Role in Abrupt Climate
Changes
• Unforced
– Hall and Stouffer Nature
• Forced
– Idealized (Hosing ) - Stouffer et al. JoC 2006
Transient
An Anomalous Event (Unforced)
Maximum Negative Anomaly
Maximum Positive Anomaly
Surface Air Temperature
Decadal Mean Difference
Surface variables/THC
Summary of
Physical
Mechanism
Unforced MOC variability
• If model if “realistic”
– Can we predict this event?
– Complicating GHG changes
– Possible explanation of some parts of paleorecord
Experimental Design
Manabe Climate Model “MCM”
• R30 AOGCM coupled model
• Idealized Water hosing
– 1 Sv for 100 years
– After 100 years, stop hosing - allow recovery
• Case 1: Hosing 50N to 70N in Atlantic
• Case 2: Hosing south of 60S in Southern
Ocean
• Compare to long control integration
Simulated Global MOC
SH Index box
NH Index box
Atlantic THC Response
Atlantic THC
does not
respond in a
seesaw-like
manner
SV
NH Hosing –
NH THC
shuts down
Years
SH THC Response
SH Hosing –
SH THC
weakens.
SV
SH THC
does not
shut down
Years
SAT Difference map
SH hosing
K
Years 51-100 hosing minus 1-200 control
Surface Salinity Response
NH
P
S
U
0
100
Years
NH SSS anomaly –
Intense and confined
200 0
100
Years
SH SSS anomaly –
Weaker and spreads
200
SH
Differences in Sea Surface Salinity
(PSU)
Southern Freshwater Escape
25 years
100 years
Hosing minus Control
Sea Surface Temperature Response
NH
K
SH
0
100
Years
200
0
100
Years
Response more symmetrical than SSS
Magnitude also becoming more similar
200
Surface Air Temperature
Response
NH
K
0
100
Years
200
0
100
Years
200
Response remarkably symmetrical (first 100 yrs)
Magnitude very similar
SH
Precipitation Response
NH
Cm/
day
SH
0
100
Years
200
0
100
Years
Response very symmetric
Magnitude very similar
ITCZ shifts toward warmer hemisphere
200
Hosing Experiment Summary
• Symmetrical Atmospheric Response
• Much less symmetry in ocean
• Why?
– Strong Circum-Antarctica winds
– Northward flowing surface waters
– Freshwater “escapes” into other basins
• Far a field impacts
• Less local impacts
MOC Predictability
• Unforced Changes
– Are MOC predictable?
– How long into future
– Manifest in surface changes?
• Forced - GHG increase
– Does MOC weaken?
– How much?
– Likelihood of complete shutdown?
Predictability Investigations just
starting
• Perfect model experiments
• Use ICs from long control
– Ocean ICs unchanged from control
– Atmosphere ICs shifted in time by day or 2
• Probably “best case” for predictability
– No model errors
– Ocean ICs perfectly known
Predictability of
Atlantic
Meridional
Overturning
Circulation
(AMOC) in GFDL
CM2.1 Climate
Model
Are past ocean observations good
enough to constrain MOC?
• Past ocean observations mainly XBTs
– Temperature only
– Upper 700 m or so
• Since 2003 or so – ARGO
– T, S
– Upper 2 km
Can Observations constrain the
MOC?
Need ARGO
and atmospheric
data to constrain
MOC
Other research
suggests this may
be too
pessimistic.
MOC and Forced Climate Change
A
AR4 WG1 Assessment:
 MOC very likely to weaken
 MOC shutdown very unlikely
Why does MOC slow down as
GHG increase?
• Role of surface fluxes
– Heat fluxes
– Water fluxes
Design of Partially Coupled
Experiments
Gregory et al. 2006 GRL
• Run control and 1% per year CO2 increase
experiment
– Save out surface fluxes
• Use water fluxes from control in 1% run
– TRAD_CH2O
• Use water fluxes from 1% in control
– CRAD_TH2O
• Isolates role of heat and water fluxes
Summary of Partially Coupled
Experiment
Conclusions:
1. Heat fluxes
changes always
weaken MOC
2. Water fluxes
changes mixed,
but usually
weaken MOC
3. Response to
heat flux changes
fairly uniform
Warming greatest over land and at most high
northern latitudes and least over Southern
Ocean and parts of the North Atlantic Ocean
Surface Warming
Pattern
A1B, 2090-2099
relative to 1980-1999
• Weakening of MOC contributes to minimum in
cooling in N Atlantic => smaller climate
change => a positive impact?
IPCC WGI SPM
Summary:
Ocean’s Role in Abrupt Climate
Change
• Unforced
– Possible to have large abrupt climate changes in
AOGCMs
• Forced
– Idealized
• Allows easy study of climate response
• Application to paleo-data and future climate changes
– GHG increase
• Predictability of MOC changes
Questions
• Basic Issues
– Why does the MOC exist?
– How much mixing is there in the ocean?
• Do models have too much/too little?
• Impacts of mixing on the MOC
• What are the physical processes?
– What is the role of ocean eddies?
– How does MOC changes impact biology and
associated changes in atmospheric pCO2?
Questions
• Variability
– Are observations good enough to constrain MOC?
– Are the MOC changes predictable?
• Time scale?
• Does it matter for where people live?
• Paleo-data tests?
– What is role of MOC variability/changes in tropical Atlantic
SST/hurricane changes?
• Future
– Is future weakening of the MOC “bad”?
– Interactions with Greenland/Antarctic ice melting
Thank you