Post on 26-Jul-2022
Surface Es+mates of the Atlan+c Overturning in Density Space in an Eddy-‐Permi=ng Ocean Model
Jeremy Grist1, Simon Josey1
Robert Marsh2
1Na+onal Oceanography Centre, UK 2University of Southampton, UK
Funded by Natural Environment Research Council, UK
Context
Marsh (2000) described (but did not test) a method that might allow ‘the meridional stream function to be largely inferred from surface fluxes alone”. We’ve examined this possibility using output from:
1) Three IPCC coupled climate models. (100-400 years of GFDL2.1, BCM, HadCM3) (Grist et al. 2009; Josey et al. 2009).
2) Eddy-permitting (1/4 º) ocean only model (78 years of ORCA-025, ‘NEMO’) (Grist et al., 2012).
Method
Net diapycnal volume flux, G (Θ,ρ) and Diapycnal density fluxes D (Θ,ρ) in an idealized North Atlantic.
Marsh (2000), Walin (1982)
F (θ,ρ)
Net diapycnal volume flux, G (Θ,ρ) and Diapycnal density fluxes D (Θ,ρ) in an idealized North Atlantic.
Marsh (2000), Walin (1982)
F (θ,ρ)
!
G(", #) = F(", #) - $Ddiff (",# )$# + C(", #)
!
F(", #) = $Din (",# )$#
Method
Net diapycnal volume flux, G (Θ,ρ) and Diapycnal density fluxes D (Θ,ρ) in an idealized North Atlantic.
Assuming incompressibility and steady state of water masses, the meridional streamfunc+on then: ψ (Θ,ρ) = G (Θ,ρ)
Marsh (2000), Walin (1982)
F (θ,ρ)
!
F(", #) = $Din (",# )$#
Method
!
G(", #) = F(", #) - $Ddiff (",# )$# + C(", #)
Net diapycnal volume flux, G (Θ,ρ) and Diapycnal density fluxes D (Θ,ρ) in an idealized North Atlantic.
Assuming incompressibility and steady state of water masses, the meridional streamfunc+on then: ψ (Θ,ρ) = G (Θ,ρ)
Marsh (2000), Walin (1982)
F (θ,ρ)
!
G(", #) = F(", #) - $Ddiff (",# )$# + C(", #)
!
F(", #) = $Din (",# )$#
Method
!
G(", #) $ F(", #) = %Din (",# )%#
‘Surface-‐Forced’ Streamfunc+on (Sv)
Latitude (ºN) 20 40 60 80
20 40 60 80 Latitude (ºN)
Latitude (ºN) 20 40 60 80
GFDL2.1 25.5 26.0 26.5 27.0 27.5 28.0
25.5 26.0 26.5 27.0 27.5 28.0
HadCM3
25.5 26.0 26.5 27.0 27.5 28.0
BCM2.0
σ0
σ0
σ0
Grist et al. (2009)
Marsh (2000)
Correla+on of AMOC (48N) vs Surface Forced Index (SFI)
GFDL2.1 HadCM3
BCM2.0
• Year on year SFI ≠ AMOC
• But significant correlation when SFI leads by a few years in all models.
• Averaging the SFI over preceding years may give a useful estimate of AMOC variability.
Correla+ons Maximum ‘Surface-‐Forced’ (SFI) & Overturning Stream func+ons
GFDL2.1
HadCM3
BCM2.0
AMOC (48N) solid lines and past average of SFI (dashed) 3 yrs 15 yrs 10 yrs 6 yrs
0.38
0.37 0.38 0.06
0.59 0.66 0.60
0.19
0.31 0.44 0.51 0.56
15-44% of interannual variability
SFI & AMOC in the Coupled Models
We compared our estimate to AMOC (z,θ)
The theory suggests compare With MOC (σ or ρ,θ). (Density as a vertical coordinate!)
Our surface forced streamfunction shows similarities to MOC (σ,θ).
AMOC(z,θ)
MOC(σ,θ)
F(σ,θ)
1000
3000
5000 Dep
th (m
)
21 23 25 27
Pot
. Den
sity
, σ (k
g m
-3)
21 23 25
27 P
ot. D
ensi
ty, σ
(kg
m-3
)
20 30 40 50 60 70
20 30 40 50 60 70
20 30 40 50 60 70 Latitude
Latitude
Latitude
AMOC in ¼º NEMO Model
We compared our estimate to AMOC (z,θ)
The theory suggests compare With AMOC (σ or ρ,θ). (Density as a vertical coordinate)
Our surface forced streamfunction shows similarities to MOC (σ,θ).
AMOC(z,θ)
AMOC(σ,θ)
F(σ,θ)
1000
3000
5000 Dep
th (m
)
21 23 25 27
Pot
. Den
sity
, σ (k
g m
-3)
21 23 25
27 P
ot. D
ensi
ty, σ
(kg
m-3
)
20 30 40 50 60 70
20 30 40 50 60 70
20 30 40 50 60 70 Latitude
Latitude
Latitude
AMOC in ¼º NEMO Model
We compared our estimate to AMOC (z,θ)
The theory suggests compare With AMOC (σ or ρ,θ). (Density as a vertical coordinate)
Our surface forced streamfunction shows similarities to AMOC (σ,θ).
AMOC in ¼º NEMO Model AMOC(z,θ)
AMOC(σ,θ)
F(σ,θ)
1000
3000
5000 Dep
th (m
)
21 23 25 27
Pot
. Den
sity
, σ (k
g m
-3)
21 23 25
27 P
ot. D
ensi
ty, σ
(kg
m-3
)
20 30 40 50 60 70
20 30 40 50 60 70
20 30 40 50 60 70 Latitude
Latitude
Latitude
20 30 40 50 60 700
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
r2
Latitude
0.9
0.7
0.5
0.3
0.1
r2
20 30 40 50 60 70 Latitude
0.9
0.7
0.5
0.3
0.1
r2
Fraction AMOC explained By Surface Fluxes
AMOC (Z) HadCM3 Climate Model Josey et al. 2009
20 30 40 50 60 700
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
r2
Latitude20 30 40 50 60 70 Latitude
0.9
0.7
0.5
0.3
0.1
r2
AMOC (z) NEMO Ocean Model
Fraction AMOC explained By Surface Fluxes
AMOC (Z) HadCM3 Climate Model Josey et al. 2009
20 30 40 50 60 700
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
r2
Latitude20 30 40 50 60 70 Latitude
0.9
0.7
0.5
0.3
0.1
AMOC (σ) NEMO Ocean Model
r2
AMOC (z) NEMO Ocean Model
Fraction AMOC Explained By Surface Fluxes
AMOC (Z) HadCM3 Climate Model Josey et al. 2009
20 30 40 50 60 700
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
r2
Latitude20 30 40 50 60 70 Latitude
0.9
0.7
0.5
0.3
0.1
AMOC (σ) NEMO Ocean Model
r2
AMOC (z) NEMO Ocean Model
Fraction AMOC Explained By Surface Fluxes
20 30 40 50 60 70 800
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
r2
MHT vs AMOC (σ)
MHT vs AMOC (z)
AMOC (Z) HadCM3 Climate Model Josey et al. 2009
20 30 40 50 60 700
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
r2
Latitude20 30 40 50 60 70 Latitude
0.9
0.7
0.5
0.3
0.1
AMOC (σ) NEMO Ocean Model
r2
AMOC (z) NEMO Ocean Model
Fraction AMOC Explained By Surface Fluxes
AMOC (Z) HadCM3 Climate Model Josey et al. 2009
20 30 40 50 60 700
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
r2
Latitude20 30 40 50 60 70 Latitude
0.9
0.7
0.5
0.3
0.1 AMOC (Z) HadCM3 Climate Model Josey et al. 2009
AMOC (σ) NEMO Ocean Model
AMOC (σ) Ocean Model + Ekman wind variability
r2
AMOC (z) NEMO Ocean Model
Fraction AMOC Explained By Surface Fluxes
Theory
Theory + Surface Observations
Theory
-100 -50 0-60
-40
-20
0
20
40
60
80
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
Theory + Surface Observations SSS SST Wind Stress
Net Heat E-‐P
Updated from Grist et al. (2009)
1960 1970 1980 1990 2000 2010
-4
-2
0
2
4
Surface-Forced Overturning Anomaly (Sv) at 48ºN (solid)
with Ekman Contribution (dashed)
Updated from Grist et al. (2009)
Theory
-100 -50 0-60
-40
-20
0
20
40
60
80
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
AM
OC
Ano
mal
y (S
v)
420-2 -4
Year
Theory + Surface Observations SSS SST Wind Stress
Net Heat E-‐P
1960 1970 1980 1990 2000 2010
Surface-Forced Overturning Anomaly (Sv) at 48ºN (solid)
with Ekman Contribution (dashed)
Updated from Grist et al. (2009)
Theory
-100 -50 0-60
-40
-20
0
20
40
60
80
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
1960 1970 1980 1990 2000 2010
-4
-2
0
2
4
1960 1970 1980 1990 2000 2010
-4
-2
0
2
4
AM
OC
Ano
mal
y (S
v)
420-2 -4
1960 1970 1980 1990 2000 2010 Year
Theory + Surface Observations SSS SST Wind Stress
Net Heat E-‐P
GECCO
Balmaseda et al (2007)
Bingham & Hughes (2009)
Hobbs & Willis(2012)
Summary • In ¼º NEMO ocean model, the water mass transformation method
can be used to estimate AMOC variability. • In sub-polar regions the method explains much more variability in
AMOC (σ0) than AMOC (z). • The surface density fluxes capture much of the decadal signal
while the additional calculation of Ekman transport allows the higher frequency variability to be captured
• The method shows greatest potential between 33ºN and 54ºN where 70-84% of the AMOC (σ0) variance is explained.
• As the method relies only on surface observations, estimate of AMOC variability can be made for the reanalysis era.
• We seek to determine the spread in time series resulting from the different reanalysis / salinity products & reconcile the surface forced signal with other mid-latitude AMOC estimates.
100yr Mean MOC (Sv) GFDL2.1
HadCM3
MOC (Sv) at 48ºN MOC vs SFOC in Coupled Climate Models
• BCM2.0
100yr Mean SFOC (Sv)
SFOC Index (Sv)
Dep
th (m
) D
epth
(m)
Dep
th (m
)
σ0
σ0
σ0
Latitude
Latitude
Latitude
Latitude
20N
20N
20N
20N
20N
20N
70N
70N
70N
70N
70N 70N
A
r2 – Surface Forced Estimate vs AMOC (σ0)
20 30 40 50 60 70 La+tude
0.2 0.4 0.6 0.8
0.0
1.0
Grist, Josey, Marsh (JGR-‐Oceans, under review, 2012)
Model AMOC (σ0) (black) & Surface Forced Estimates (red) at 50ºN
AMOC (σ0) Surface Fluxes Surface Fluxes (with Ekman)
Surface Fluxes Surface Fluxes (with Ekman)
0
19 10 20 30 40 50 60 70 Model Year
r2
0 2 4
-‐2 -‐4 A
MO
C A
nom
aly
(Sv)
4. Influence of Surface Fluxes on AMOC Variability
20 30 40 50 60 700
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
r2
Latitude20 30 40 50 60 70 Latitude
0.9
0.7
0.5
0.3
0.1 AMOC (Z) HadCM3 Climate Model
AMOC (σ) Ocean Model
AMOC (σ) Ocean Model + Ekman wind variability
r2
AMOC (z) NEMO Ocean Model
Fraction AMOC explained By Surface Fluxes
Updated from Grist et al. (2009)
1960 1970 1980 1990 2000 2010
-4
-2
0
2
4
1960 1970 1980 1990 2000 2010
-4
-2
0
2
4
Surface-Forced Overturning Anomaly (Sv) at 48ºN (solid)
with Ekman Contribution (dashed)
Updated from Grist et al. (2009)
Theory
-100 -50 0-60
-40
-20
0
20
40
60
80
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.25
AM
OC
Ano
mal
y (S
v)
420-2 -4
Year
Theory + Surface Observations SSS SST Wind Stress
Net Heat E-‐P
GECCO
Balmaseda et al (2007) 1960 1970 1980 1990 2000 2010