THOR CT 4 Predictability of the THC

1. THOR CT 4 Predictability of the THC

2. GOALS of CT4 • Predict the Atlantic Meridional Overturning Circulation (and associated climate state) at decadal time scales • Separate forced (anthropogenic) component from natural variations • Make suggestions for the ocean observing system

3. Observations Time series Overflows Storage (re)analyses Gridded data GECCO NEMOVAR Predictions Models Probabilistic IPCC Verifications Metrics Windows of Opportunity

4. Decadal Predictions of the AMOC • Initialize atmosphere-ocean-sea ice models from observed/analyzed ocean state • Perturb initialized models to generate ensembles • Perform decadal hindcasts and forecasts • Verify the results against own analyses and independent observations • In general: CT4 starts with CMIP5 (IPCC 5AR). Delivery first model results summer 2010. Sensitivity runs in 2011.

5. Initialization in THOR • Full initialization (KNMI, ECMWF) • - Drift • - No spinup needed • Anomaly initialization (UKMO, MPI-M, IFM-GEOMAR) • - Need spinup • - Choice for nudging (how strong, long, which variables) • Sea ice is a challenge: short records, hardly thickness information • Analyses used: GECCO (MPI-M), NEMOVAR (ECMWF, KNMI), DePreSys (UKMO)

6. Anomaly initialisation (ECHAM5/OM1) Full fields Mean bias removed Full field initialisation (ARPEGE4/OPA) Full fields Mean bias removed Courtesy: Francisco Doblas-Reyes (ECMWF, now IC3)

7. Perturbing in THOR • Atmosphere • But, seems not to produce enough spread • Ocean perturbations • Use forecast error • Use analyses error • Lagged analysis • Perturbed parameters • Stochastic physics • Optimal perturbations (e.g. Singular Vectors) • UREAD activity in THOR

8. Optimal perturbations Models considered so far Models planned Only requirement is a long (>500 year) control integration See talk Ed Hawkins

9. Verification • Use ‘own’ analyses AND independent observations: yr 1, yr 2, yr 2-5, yr 6-10. • Always verify against simple statistical model (e.g. damped persistence) • Verify trend and deviations from trend [deal with ‘forced’ trend, ideally with a control run without initialization (no-assim) or subtract global mean signal (presuming that is unrelated to AMOC)] • Metrics based on list of Atlantic-panel of WCRP-CLIVAR  CT 2/3 may have suggestions (integrated, long time series, monthly means)

10. Oldenborgh, Doblas Reyes, Wouters, Hazeleger, in prep See talk Bert Wouters

11. THOR CT4.2: impact ocean observations on THC predictions Initialisation with sub-surface temperature and salinity (idealized experiments) Initialisation with sea surface temperatures (idealized experiments Dunstone and Smith, 2009, submitted

12. THOR CT 4.2 Forecast skill of top 360m ocean temperature (5-yr mean; idealized experiments) Dunstone and Smith, 2009, submitted

13. Milestones & deliverables THOR • Paper: Multi-model decadal predictions of the AMOC. Start writing late 2010 when data of CMIP5 becomes available (if possible earlier), to be in time for deadline IPCC. KNMI lead. • Paper: Relative impact of initial conditions and GHG in different coupled models . Start writing late 2011, lead MPI-M • Paper: Assessment of ocean observations on predictability UKMO lead

14. Summary/planning • Experimental multi-model set up for CT 4.1 clear (CMIP5/IPCC) • Perform runs and deliver results in July 2010 (MPI-M, UKMO, ECMWF, IFM-GEOMAR, KNMI) ; first multi-model THOR-AMOC review paper fall 2010 • Decided on verifications directions. May need workshop in summer 2010 (CT 2?) • Data management at World Climate Data Centre, compliant with CMIP5/IPCC. • To work out in more detail: • Protocol for observing system experiments (first tests done). Experiments in 2011. • Protocol for experiments separating initial state and radiative forcing. Experiments in 2011.

15. Reporting month 18 • Optimal ocean perturbations from long ocean runs • Decadal prediction runs prepared and ongoing • Metrics verifying decadal predictions (ensemble means and spread) • Comparison of ocean (re)analysis in assimilation and hindcast modes • Idealized observing system simulation experiments

16. CT interaction • CT1: provide long coupled runs for generating optimal perturbations • CT2/3: Metrics (robust, integrated, long term) for verifying decadal predictions • CT2/3: Suggestions for large events (GSA, 90s warming) to use for verification (windows of opportunity) • CT2: Ocean analyses without specific ocean observing system (e.g. without ARGO), to use for observing system simulation experiments • CT 5.2 ????? • Offer: predictions in WCDC

18. Metrics for THOR (see contribution Geert Jan) • Directions: • For verification of climatology and forecast skill • Trend • Fluctuations around trend: yr 1, yr 2 – 5 averaged, yr 6-10 averaged • Skill against simple model (trend only or with damped persistence) • Against independent observations and own analyses

19. Data management • Why? • Share data for multi-model studies with common data-standards • Not all experiments will be in CMIP5-database (or not on time) • Where? DKRZ could host an Opendap-like system • Follow CMIP5 data standards • FP7 Combine would like to ‘join’

20. Potential Predictability of DEN overflowNCEP-forced (Poster: Daniela Matei) • Denmark Strait (DEN) overflow transport potentially predictable up to 6 years in advance

21. Initialization strategies at MPI-M based on anomaly nudging of different types of data • Approach 1: Nudging ocean reanalysis (eg GECCO, ECMWF ORA-S3, SODA, ...)‏ • Approach 2: Nudging atmosphere reanalysis (eg ERA40) • Approach 3: Nudging both ocean and atmosphere reanalysis (eg ERA40 and GECCO/ORA-S3) • Approach 4: Nudging to a NCEP-driven ocean simulation

22. Sensitivity of Assimilation to applied Ocean State Estimate 5yr Mean North Atlantic SST [x=60W:0E,y=20N:80N] HadCRUT3 ECMWF ORA-S3 IFM GECCO NCEP-MPIOM