Modeling the MOC Ronald J Stouffer Geophysical Fluid Dynamics Laboratory NOAA The views described...
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![Page 1: Modeling the MOC Ronald J Stouffer Geophysical Fluid Dynamics Laboratory NOAA The views described here are solely those of the presenter and not of GFDL/NOAA/DOC.](https://reader035.fdocuments.net/reader035/viewer/2022081506/56649cab5503460f9496c904/html5/thumbnails/1.jpg)
Modeling the MOC
Ronald J StoufferGeophysical Fluid Dynamics Laboratory
NOAAThe views described here are solely those of the presenter and not of GFDL/NOAA/DOC or any other agency or institution.
http://www.andrill.org/iceberg/blogs/julian/images/greatoceanconveyor.jpg
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Modeling the MOCOutline
• Role of MOC in maintaining mean climate– Northward Heat Transport – Northward Salt transport
• Role of MOC in Abrupt Climate Change– Unforced– Forced
• Predictability
![Page 3: Modeling the MOC Ronald J Stouffer Geophysical Fluid Dynamics Laboratory NOAA The views described here are solely those of the presenter and not of GFDL/NOAA/DOC.](https://reader035.fdocuments.net/reader035/viewer/2022081506/56649cab5503460f9496c904/html5/thumbnails/3.jpg)
How well do models simulate the T, S structure in the ocean?
AR4 ensemble mean error Temperature
IPCC WGI Chapter 8
-2.5 +2.5
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AR4 ensemble mean errorSalinity
90N 90S
-0.2 0.2-0.4 0.4
IPCC WGI Chapter 8 Suppl. Material
EQ
PSU-1.0 +1.0
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Role of MOC in Heat Transport
![Page 6: Modeling the MOC Ronald J Stouffer Geophysical Fluid Dynamics Laboratory NOAA The views described here are solely those of the presenter and not of GFDL/NOAA/DOC.](https://reader035.fdocuments.net/reader035/viewer/2022081506/56649cab5503460f9496c904/html5/thumbnails/6.jpg)
Impact of MOC on Climate(SAT)
AOGCMs
EMICs
MOC “on” minus “off” oC
Conclusion – MOC warms NH, locally large values, MOC cools SH
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Impact of MOC on Climate
Zonally averaged Precipitation differences(mm/day)
Red lines controlBlue lines difference
Dashed – EMICsSolid - AOGCMs
Conclusion – MOC on => ITCZ toward north
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Impact of MOC on ClimateSalinity MOC “on” minus “off”
(PSU)
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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
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Ocean MOC Role in Abrupt Climate Changes
• Unforced – Hall and Stouffer Nature
• Forced– Idealized (Hosing ) - Stouffer et al. JoC 2006
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TransientAn Anomalous Event (Unforced)
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Maximum Negative Anomaly
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Maximum Positive Anomaly
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Surface Air TemperatureDecadal Mean Difference
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Surface variables/THC
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Summary of Physical
Mechanism
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Unforced MOC variability
• If model if “realistic”– Can we predict this event?– Complicating GHG changes– Possible explanation of some parts of paleo-
record
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Experimental DesignManabe 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
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Simulated Global MOC
SH Index box NH Index box
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Atlantic THC Response
SV
Years
Atlantic THCdoes notrespond in aseesaw-likemanner
NH Hosing –NH THCshuts down
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SH THC Response
SV
Years
SH Hosing –
SH THCweakens.
SH THCdoes notshut down
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SAT Difference mapSH hosing
K
Years 51-100 hosing minus 1-200 control
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Surface Salinity Response
0 100 200 Years
0 100 200 Years
NH
SH
NH SSS anomaly –Intense and confined
SH SSS anomaly –Weaker and spreads
PSU
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Differences in Sea Surface Salinity (PSU)
Southern Freshwater Escape
25 years 100 years
Hosing minus Control
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Sea Surface Temperature Response
0 100 200 Years
0 100 200 Years
NH
SH
Response more symmetrical than SSSMagnitude also becoming more similar
K
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Surface Air Temperature Response
0 100 200 Years
0 100 200 Years
NH
SH
Response remarkably symmetrical (first 100 yrs)Magnitude very similar
K
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Precipitation Response
0 100 200 Years
0 100 200 Years
NH
SH
Response very symmetricMagnitude very similarITCZ shifts toward warmer hemisphere
Cm/day
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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
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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?
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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
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Predictability ofAtlantic MeridionalOverturning Circulation(AMOC) in GFDLCM2.1 Climate Model
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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
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Can Observations constrain the MOC?
Need ARGOand atmosphericdata to constrain MOC
Other research suggests this may be too pessimistic.
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A
AR4 WG1 Assessment: MOC very likely to weaken MOC shutdown very unlikely
MOC and Forced Climate Change
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Why does MOC slow down as GHG increase?
• Role of surface fluxes– Heat fluxes– Water fluxes
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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
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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
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Warming greatest over land and at most high northern latitudes and least over Southern
Ocean and parts of the North Atlantic Ocean
• Weakening of MOC contributes to minimum in cooling in N Atlantic => smaller climate change => a positive impact?
Surface Warming Pattern
A1B, 2090-2099 relative to 1980-1999
IPCC WGI SPM
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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
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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?
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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
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Thank you