North Atlantic Climate variability - Hamburg 24-26 September 2012 1/15

Impact of initial conditions with respect to

external forcing in the decadal

predictions: a sensitivity experiment

Susanna CortiECMWF

With contributions fromAntje Weisheimer Tim Palmer Linus Magnusson and Magdalena Balmaseda

North Atlantic Climate variability - Hamburg 24-26 September 2012 2/15

10-year integrations from:

A 1965 initial conditions, observed forcing (GHG & aerosols) from 1965 (control1)

B 1995 initial conditions, observed forcing (GHG & aerosols) from 1995 (control2)

C 1965 initial conditions, observed forcing from 1995

D 1995 initial conditions, observed forcing from 1965

By comparing A with D, and B with C, we have two estimates of decadal predictability (arising from having different initial conditions and the same GHG forcing).

By comparing A with C, and B with D, we have two estimates of the impact of GHG forcings (since initial conditions are the same).

SWAP Experiment - 1965-1995

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Models

ECMWF coupled system (SYS4+LIM2) (5 ensemble meembers)

KNMI EC-EARTH (3 ensemble members)

MPI-M (5 ensemble members)

Met Office HadCM3 (3 ensemble members)

1965 1995

1965 A C

1995 D B

B minus A= signal

B minus C = Initial conditions

Boundary conditions (forcing)

Init

ial con

dit

ion

s

D minus A = Initial conditionsC minus A = ForcingB minus D = Forcing

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1965 1995

1965 A C

1995 D B

Boundary conditions (forcing)

Init

ial con

dit

ion

s

Impact of boundary (forcing) versus initial conditions in decadal prediction experiments with the ECMWF system by swapping ICs and BCs for two different

decades

B minus A1995to1996 minus 1965to1966

First yearB minus D B minus C

C minus A D minus A

Impact of boundary (forcing) versus initial conditions

ECMWF MODEL

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1965 1995

1965 A C

1995 D B

Boundary conditions (forcing)

Init

ial con

dit

ion

s

Impact of boundary (forcing) versus initial conditions in decadal prediction experiments with the ECMWF system by swapping ICs and BCs for two different

decades

B minus A1995to2000 minus 1965to1970

5-year meansB minus D B minus C

C minus A D minus A

Impact of boundary (forcing) versus initial conditions

ECMWF MODEL

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KNMI5-year means

MPI-M5y-year means

HadCM35yrs

Impact of boundary (forcing) versus initial conditions

1965 1995

1965 A C

1995 D B

Boundary conditions (forcing)

Init

ial con

dit

ion

s

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AMOC 2 decades climatology & anomaly 1995to2005 minus 1965to1975 All models

ECMWF

KNMI

MPI-M

HadCM3

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1995to2005minus

1965to1975

1995to2000minus

1965to1970

ECMWF KNMI MPI-MHadCM3

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D

time

d(F)

Distance D(t)=|A-B|

D(t)=|B-C| or |D-A|Impact of Initialisation

D(t)=|B-D| or |C-A|Impact of Forcing

1965 1995

1965 A C

1995 D B

d(I)

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d

time

Impact of Initialisation

Impact of Forcing1965 1995

1965 A C

1995 D B

d(F)

d(I)

T-cross

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AMOC –Forcing vs. Initial Conditions all models

KNMI HadCM3

MPI-MECMWF

1965 HadCM3

ECMWF

KNMI

MPI-M

1995

1965 1995

1965 A C

1995 D B

Boundary conditions (forcing)

Init

ial con

dit

ion

s

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Global SSTs

KNMI HadCM3

MPI-MECMWF

1965 HadCM3

ECMWF

KNMI

MPI-M

1995

1965 1995

1965 A C

1995 D B

Boundary conditions (forcing)

Init

ial con

dit

ion

s

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NORTH ATLANTIC SSTs

KNMI HadCM3

MPI-MECMWF

1965 HadCM3

ECMWF

KNMI

MPI-M

1995

1965 1995

1965 A C

1995 D B

Boundary conditions (forcing)

Init

ial con

dit

ion

s

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Southern Indian Ocean SSTs

KNMI HadCM3

MPI-MECMWF

1965HadCM3

ECMWF

KNMI

MPI-M

1995

1965 1995

1965 A C

1995 D B

Boundary conditions (forcing)

Init

ial con

dit

ion

s

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Summary

SSTs:

Results from the 1965-1995 SWAP experiment indicate that over time scales longer than about 1 year predictability of SSTs arises mainly from the forcing.

The correct initialisation has a strong impact up to about 1 year on a global domain, but it seems to affect the predictability over the North Atlantic up to 3/4 years .

The high sensitivity to initial condition over the North Atlantic is common to all models investigated.

AMOC:

In all model considered the impact of forcing is negligible when compared with that of initial conditions.

Caveat: The band of uncertainty associated with the natural variability is (at least in the case of SSTs) big.