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Forecasting North Atlantic Decadal Variability and its Impacts

Noel Keenlyside, Mojib Latif, Leibniz Institute of Marine Sciences Johann Jungclaus, Luis Kornblueh, and Erich Roeckner Max Planck Institute for Meteorology. Forecasting North Atlantic Decadal Variability and its Impacts. Outline. Motivation for and Status of Decadal Prediction

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Forecasting North Atlantic Decadal Variability and its Impacts

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  1. Noel Keenlyside, Mojib Latif, Leibniz Institute of Marine Sciences Johann Jungclaus, Luis Kornblueh, and Erich Roeckner Max Planck Institute for Meteorology Forecasting North Atlantic Decadal Variability and its Impacts

  2. Outline Motivation for and Status of Decadal Prediction Initialisation of Meridional Overturning Circulation Hindcast and Forecast Results Comments on forecast drift Summary

  3. AMO : Multi-Decadal variability in hurricane activity and Atlantic SST Above normal tropical Atlantic SSTs are associated with increased hurricane activity (ACE)

  4. Status of Decadal Prediction Atlantic MOC predictable in model world (e.g., Collins et al., 2006) Observed atmospheric impacts of North Atlantic SST reproducible (e.g., Sutton and Hodson 2005) Boundary Value Problem External radiative forcing provides skill on large regional scales (Lee et al. 2007) Initial + Boundary Value Problem Smith et al. 2007: Ocean Initialisation enhances global mean temperature prediction. Limitted skill over the North Atlantic

  5. Mechanisms for the AMO/AMV Due to limited observations key characterisitcs and mechanisms for AMO (AMV) are unclear: Periodicity? Ocean only or Ocean-Atmosphere Coupled? Models show a wide spectrum of variability and mechanisms Thus, how can we expect to make useful predictions? Inertia of MOC Ability to capture atmospheric response However, lack of understanding is a clear limiting factor to extending predictions

  6. Outline Motivation for and Status of Decadal Prediction Initialisation of Meridional Overturning Circulation Hindcast and Forecast Results Comments on forecast drift Summary

  7. Initialising the Atlantic MOC Problems We do not have good observations of the MOC. We do not have much data to initializepredictions. Strategy : Coupled SST assimilation, as applied to Seasonal Forecasting (e.g., Keenlyside et al. 2005) SST anomalies nudged into model 30S-30N uniform & strong (0.25 day) Decreasing linearly to 0 at 60N-60S Fully coupled 60N/S to pole

  8. The MPI-M IPCC AR4 Model ECHam5: MPI atmosphere model (Roeckner et al., 2003), interactive runoff and glacier calving scheme. Resolutions: T63L31 (IPCC) OASIS 3.0 PRISM coupler MPI-OM (Marsland et al., 2003) C-Grid, z-level, partial cells, BBL parameterization Hibler-type sea ice model incl. snow and fractional ice cover Conformal mapping: 1.5° with refinement in grid pole regions NO FLUX ADJUSTMENT ECHam5 OASIS MPI-OM

  9. Experiments (1) 20C full forcing simulation 1860-2000, 3-members known greenhouse gases and sulfate aerosols, solar cycle, major volcanic eruptions (2) Analysis 1950-2005, 3-member Initial Conditions from (1) Radiative forcing as (1) SST from NCEP

  10. Experiments (3) Hindcasts/Forecasts 3-members, 10 years long 11 start dates between 1955-2005 Initial conditions from (2) Radiative forcing greenhouse gases and sulfate Aerosols as (1) or A1B scenario Solar cycle repeated from last 11 years No Volcanoes, Impact of prior eruptions decays with 1-year e-folding timescale (4) Forecasts : GHG stabalised at 2000

  11. Coupled SST assimilation Atlantic Meridional Overturning Circulation at 30N Gulf Stream Index provided by T. Joyce

  12. Labrador Sea Convection forces MOC LSW Thickness provided by R. Curry

  13. NAO, SST, & SSS force Lab. Sea Convection

  14. Outline Motivation for and Status of Decadal Prediction Initialisation of Meridional Overturning Circulation Hindcast and Forecast Results Comments on forecast drift Summary

  15. Correlation Skill for SAT (years 6-10) 9 hindcasts, 1960-2005 DYNAMICAL HINDCAST PERSISTENCE RADIATIVE FORCING ONLY ANALYSIS

  16. Hindcasts Time Series (Year 6-10 mean)Renormalised to observed Std. Dev.

  17. Hindcasts Time Series (Year 6-10 mean)Renormalised to observed Std. Dev.

  18. Outline • Motivation for and Status of Decadal Prediction • Initialisation of Meridional Overturning Circulation • Hindcast and Forecast Results • Comments on forecast drift • Summary

  19. Forecast DriftAverage Linear Trend in Surface Temperature

  20. Forecast Drift in MOC

  21. Forecast Drift in VariabilityStand. Dev. of yr 6-10 mean Surf. Temp. (1960-2005) Observed Hindcast K

  22. Conclusions MOC initialisation can lead to predictability despite uncertainties in mechanisms Simple technique implemented that initialises well low frequency variability NA climate variability Translates into skill, greater than that due to external radiative forcing alone Systematic error limits skill of the forecasts

  23. Hindcast Correlation Skill SAT at Different Lead Time Years 1-5 Years 6-10

  24. Correlation Skill for SAT (years 6-10)

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