1. Towards a new reanalysis with the IPSL climate model
Juliette Mignot, Didier Swingedouw, Eric Guilyardi, Pablo Ortega

2. The large-scale ocean and the Arctic climate
Motivation
The large-scale ocean and the Arctic climate
Zhang et al 2015
Quadfasel 2005

3. Recent measurements: decadal variability or decreasing trend?
Motivation
Recent measurements: decadal variability or decreasing trend?
Ovide section
Quadfasel 2005
Smeed et al. 2014
Mercier et al. (2015)

4. Longer reconstructions of the N. Atl ocean circulation:
Motivation
Longer reconstructions of the N. Atl ocean circulation:
oceanic reanalysis
Poorly consistent in terms of oceanic circulation
Western subpolar gyre intensity
Anomalous intensity of the AMOC circulation at 45°N from 6 reanalysis data sets
Mean values (Sv)
Karspeck et al 2015
Born et al 2015

5. Longer reconstructions of the N. Atl ocean circulation:
Motivation
Longer reconstructions of the N. Atl ocean circulation:
oceanic reanalysis
Which data to constrain?
Correlation of the zonally averaged temperature between SODA and ORAS4
Even on temperature, ocean reanalysis hardly agree below 300m
Ray et al. 2014
How are reanalysis constructed?
Very complex (sometimes ad-hoc) systems.
May alter the physics of the model and of the reconstruction
Difficult to trace the origins of the biases
Zhang et al 2010

6. First approach to reconstruct the ocean circulation at IPSL
Constrain (nudge) towards SST only
(and only towards anomalies)
1963-Agung
1982-El Chichon
1991-Pinatubo
Global SST
nudged simulations (4 members)
Free historical simulations (4 members)
HadISST
SODA 2.2.4
Ray et al. 2014
Close agreement with observations and reanalysis (independent from the nudging data set)
Spread among members of nudged runs strongly reduced as compared to free historical runs

7. First approach to reconstruct the ocean circulation at IPSL
Convincing agreement with independent reconstructions
Robson et al. 2014
Density in the Labrador Sea
EN3, Smith and Murphy 2007 6 4 2 -2 -4
Density m (1013kg)
Independent reconstructions
Obs. (Huck et
Huck et al 2008
Latif et al. 2004
IPSL climate model + SST nudging
Swingedouw et al. 2013

8. Second step to reconstruct the ocean circulation at IPSL:
Improving the surface nudging
AMOC maximum at 48N (No Ekman)
Perfect model framework
Nudging SST and SSS + imposing the wind stress
Reconstruction sensitive to the initial conditions and fails reproducing the extreme AMOC peak.
Ortega et al. 2017
To improve the representation of AMOC extreme events
To test the sensitivity to the initial AMOC state

9. Second step to reconstruct the ocean circulation at IPSL:
Improving the surface nudging
Causes of the AMOC extreme and its misrepresentation
AMOC maximum at 48N (No Ekman)
m density – South of Iceland
The extreme is related to the formation of deep dense waters by convection
Ortega et al. 2017

10. Second step to reconstruct the ocean circulation at IPSL:
Improving the surface nudging
Need to reconstruct the precise convection details
g0 = - 40 W/m2/K is equivalent to
a relaxation time of 6 weeks in a layer of h0=40m
However, convection happens down to 2000 m & lasts for 2-3 months
g0 = - 40 W/m2/K is too weak to enforce this
We introduce a variable gT, proportional to the mixed layer depth, but keeping the same relaxation time
gT = (g0 / h0) • mixed layer depth
Ortega et al. 2017

11. Second step to reconstruct the ocean circulation at IPSL:
Improving the surface nudging
AMOC48N in a new set of SurfaceTSWind experiments
15 yrs
AMOC maximum
Adverse initial conditions (15 yrs before peak)
Constant g
AMOC extreme
Any initial state
Ortega et al. 2017

12. Second step to reconstruct the ocean circulation at IPSL:
Improving the surface nudging
AMOC48N in a new set of SurfaceTSWind experiments
15 yrs
AMOC maximum
Exact initial conditions (15 yrs before peak)
Constant g
AMOC extreme
Any initial state
Ortega et al. 2017

13. Second step to reconstruct the ocean circulation at IPSL:
Improving the surface nudging
AMOC48N in a new set of SurfaceTSWind experiments
15 yrs
AMOC maximum
Adverse initial conditions (15 yrs before peak)
Variable g
AMOC extreme
Any initial state
Ortega et al. 2017

14. Second step to reconstruct the ocean circulation at IPSL:
Improving the surface nudging
AMOC48N in a new set of SurfaceTSWind experiments
15 yrs
AMOC maximum
Exact initial conditions (15 yrs before peak)
Variable g
AMOC extreme
Any initial state
Ortega et al. 2017

15. Second step to reconstruct the ocean circulation at IPSL:
Improving the surface nudging
AMOC48N in a new set of SurfaceTSWind experiments
25 yrs
AMOC maximum
Any initial conditions (25 yrs before peak)
Variable g
AMOC extreme
Any initial state
Ortega et al. 2017

16. Second step to reconstruct the ocean circulation at IPSL:
Improving the surface nudging
The use of a relaxation term proportional to the mixed layer depth contributes decisively to characterize the extreme AMOC events, irrespective of the initial conditions considered.

17. Future steps to reconstruct the ocean circulation at IPSL:
BLUEACTION
Apply the « variable restoring » configuration in historical conditions
-> requires salinity data, still very uncertain
Subpolar Gyre [56N-64N]
Atlantic [30N-50N]
SSS_ORAS4
SSS_ORAS4
SSS_SODA224
SSS_SODA224
SSS_Reverdin2010
SSS_Reverdin2009

18. Future steps to reconstruct the ocean circulation at IPSL:
BLUEACTION
Apply the « variable restoring » configuration in historical conditions
-> requires salinity data, still very uncertain
Test constrains of the coupled model by atmospheric winds
Global 2m-temperature
HadCRUT
historical free runs
SST-nudged runs
Wind-nudged runs
SST+Wind nudged runs

19. Future steps to reconstruct the ocean circulation at IPSL:
BLUEACTION
Apply the « variable restoring » configuration in historical conditions
-> requires salinity data, still very uncertain
Test constrains of the coupled model by atmospheric winds
Compare to reconstructions performed with 3D-oceanic nudging on the ARGO period
-> necessarily much shorter.
Relevance for Blue Action
WP2: Develop and analyse the reanalyse
WP3: Test the sensitivity of the reanalysis to additionnal Greenland Ice Sheet melting
WP4: Use this reanalysis as initial conditions and/or validation data set for decadal predictions