Modeling the moc
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Modeling the MOC. http://www.andrill.org/iceberg/blogs/julian/images/greatoceanconveyor.jpg. 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.

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Modeling the moc

Modeling the MOC

http://www.andrill.org/iceberg/blogs/julian/images/greatoceanconveyor.jpg

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

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


How well do models simulate the t s structure in the ocean ar4 ensemble mean error temperature

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

AR4 ensemble mean errorSalinity

EQ

90N

90S

+1.0

-1.0

-0.4

-0.2

0.2

0.4

PSU

IPCC WGI Chapter 8 Suppl. Material


Role of moc in heat transport

Role of MOC in Heat Transport


Impact of moc on climate sat

Impact of MOC on Climate(SAT)

AOGCMs

EMICs

MOC “on” minus “off”

oC

Conclusion – MOC warms NH, locally large values, MOC cools SH


Impact of moc on climate

Impact of MOC on Climate

Zonally averaged Precipitation differences

(mm/day)

Red lines control

Blue lines difference

Dashed – EMICs

Solid - AOGCMs

Conclusion – MOC on => ITCZ toward north


Impact of moc on climate salinity moc on minus off psu

Impact of MOC on ClimateSalinity MOC “on” minus “off” (PSU)


Role of moc in maintaining mean climate

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

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

TransientAn Anomalous Event (Unforced)


Maximum negative anomaly

Maximum Negative Anomaly


Maximum positive anomaly

Maximum Positive Anomaly


Surface air temperature decadal mean difference

Surface Air TemperatureDecadal Mean Difference


Surface variables thc

Surface variables/THC


Summary of physical mechanism

Summary of Physical Mechanism


Unforced moc variability

Unforced MOC variability

  • If model if “realistic”

    • Can we predict this event?

    • Complicating GHG changes

    • Possible explanation of some parts of paleo-record


Experimental design manabe climate model mcm

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


Simulated global moc

Simulated Global MOC

SH Index box

NH Index box


Atlantic thc response

Atlantic THC Response

Atlantic THC

does not

respond in a

seesaw-like

manner

NH Hosing –

NH THC

shuts down

SV

Years


Sh thc response

SH THC Response

SH Hosing –

SH THC

weakens.

SH THC

does not

shut down

SV

Years


Sat difference map sh hosing

SAT Difference mapSH hosing

K

Years 51-100 hosing minus 1-200 control


Surface salinity response

Surface Salinity Response

NH

P

S

U

SH

0 100 200

Years

0 100 200

Years

NH SSS anomaly –

Intense and confined

SH SSS anomaly –

Weaker and spreads


Differences in sea surface salinity psu southern freshwater escape

Differences in Sea Surface Salinity (PSU)Southern Freshwater Escape

25 years

100 years

Hosing minus Control


Sea surface temperature response

Sea Surface Temperature Response

NH

K

SH

0 100 200

Years

0 100 200

Years

Response more symmetrical than SSS

Magnitude also becoming more similar


Surface air temperature response

Surface Air Temperature Response

NH

K

SH

0 100 200

Years

0 100 200

Years

Response remarkably symmetrical (first 100 yrs)

Magnitude very similar


Precipitation response

Precipitation Response

NH

Cm/

day

SH

0 100 200

Years

0 100 200

Years

Response very symmetric

Magnitude very similar

ITCZ shifts toward warmer hemisphere


Hosing experiment summary

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

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

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


Modeling the moc

Predictability of

Atlantic Meridional

Overturning Circulation

(AMOC) in GFDL

CM2.1 Climate Model


Are past ocean observations good enough to constrain moc

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

Can Observations constrain the MOC?

Need ARGO

and atmospheric

data to constrain

MOC

Other research suggests this may be too pessimistic.


Modeling the moc

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

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

Design of Partially Coupled ExperimentsGregory 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

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


Modeling the moc

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


Summary ocean s role in abrupt climate change

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

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?


Questions1

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

Thank you


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