1. 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 Corti ECMWF With contributions from Antje Weisheimer Tim Palmer Linus Magnusson and Magdalena Balmaseda

2. 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

3. North Atlantic Climate variability - Hamburg 24-26 September 2012 3/15 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) 19651995 1965AC 1995DB B minus A= signal B minus C = Initial conditions Boundary conditions (forcing) Initial conditions D minus A = Initial conditions C minus A = Forcing B minus D = Forcing

4. North Atlantic Climate variability - Hamburg 24-26 September 2012 4/15 19651995 1965AC 1995DB Boundary conditions (forcing) Initial conditions 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 A 1995to1996 minus 1965to1966 First year B minus DB minus C C minus AD minus A Impact of boundary (forcing) versus initial conditions ECMWF MODEL

5. North Atlantic Climate variability - Hamburg 24-26 September 2012 5/15 19651995 1965AC 1995DB Boundary conditions (forcing) Initial conditions 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 A 1995to2000 minus 1965to1970 5-year means B minus DB minus C C minus AD minus A Impact of boundary (forcing) versus initial conditions ECMWF MODEL

6. North Atlantic Climate variability - Hamburg 24-26 September 2012 6/15 KNMI 5-year means MPI-M 5y-year means HadCM3 5yrs Impact of boundary (forcing) versus initial conditions 19651995 1965AC 1995DB Boundary conditions (forcing) Initial conditions

7. North Atlantic Climate variability - Hamburg 24-26 September 2012 7/15 AMOC 2 decades climatology & anomaly 1995to2005 minus 1965to1975 All models ECMWF KNMI MPI-M HadCM3

8. North Atlantic Climate variability - Hamburg 24-26 September 2012 8/15 1995to2005 minus 1965to1975 1995to2000 minus 1965to1970 ECMWFKNMIMPI-MHadCM3

9. North Atlantic Climate variability - Hamburg 24-26 September 2012 9/15 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 19651995 1965AC 1995DB d(I)

10. North Atlantic Climate variability - Hamburg 24-26 September 2012 10/15 d time Impact of Initialisation Impact of Forcing 19651995 1965AC 1995DB d(F) d(I) T-cross

11. North Atlantic Climate variability - Hamburg 24-26 September 2012 11/15 AMOC –Forcing vs. Initial Conditions all models KNM I HadCM3 MPI-MECMWF 1965 HadCM3 ECMWF KNMI MPI-M 1995 19651995 1965AC 1995DB Boundary conditions (forcing) Initial conditions

12. North Atlantic Climate variability - Hamburg 24-26 September 2012 12/15 Global SSTs KNM I HadCM3 MPI-MECMWF 1965 HadCM3 ECMWF KNMI MPI-M 1995 19651995 1965AC 1995DB Boundary conditions (forcing) Initial conditions

13. North Atlantic Climate variability - Hamburg 24-26 September 2012 13/15 NORTH ATLANTIC SSTs KNM I HadCM3 MPI-MECMWF 1965 HadCM3 ECMWF KNMI MPI-M 1995 19651995 1965AC 1995DB Boundary conditions (forcing) Initial conditions

14. North Atlantic Climate variability - Hamburg 24-26 September 2012 14/15 Southern Indian Ocean SSTs KNM I HadCM3 MPI-MECMWF 1965 HadCM3 ECMWF KNMI MPI-M 1995 19651995 1965AC 1995DB Boundary conditions (forcing) Initial conditions

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