CT2 : Assessing sources of uncertainty in ocean analysis and forecasts We consider the structural sources of uncertainty generic to all practical forecasting system – CT4 (predictability) will deal specifically with parameter uncertainty and initial conditions using ensemble forecasts. WP 2.1 – Limitations of available observations and analysis systems in estimating the current state of the ocean. WP 2.2 – The lack of modelling of the melt-water input from the Greenland ice sheet WP 2.3 – Poor resolution of key small scale components of the THC and in particular their role in ocean freshwater redistribution modulating the THC Thor annual meeting 2009, Paris

CT2: Status and plans Thor annual meeting 2009, Paris Approach WP 2.1 Exploits a number of existing ocean analysis and hindcasts simulations: compare different independent state estimates (CT3) -The ‘analysis’ are constrained by ocean observations but generally of coarse resolution. Unclear how well the THC is constrained. -The ocean ‘simulations’ are run using surface fluxes only without assimilation of ocean observations. In addition, a set of focussed sensitivity experiments will be performed with the DePreSys analysis system complementing the data impact studies of WP 4.2 Time-series of state variables have been agreed upon. An initial evaluation against observations along the GSR mainly will be performed and reported by the end of the year (D05 months 12).

Thor annual meeting 2009, Paris The Faroe Bank Channel overflow We can reproduce a number of features of the observed transport time-series from seasonality to inter-annual variability: Identical seasonal amplitude and phasing High correlation (de-seasoned) Monthly r=0.7 (p<10 -7 ) Interannual r=0.9 (p<10 -4 ) Updated from Olsen et al CT2: Status and plans

Thor annual meeting 2009, Paris Model uncertainty Simple model inter-comparison document uncertainty on different scale and indicate a potential for identifying key processes of variability. CT2: Status and plans

Thor annual meeting 2009, Paris WP Our approach is to perform coordinated experiments and analysis. Baseline experiments using HadCM3 and ECHAM5-MPIOM simulations with coupled atmosphere ocean ice-sheet climate projections: Runs analysed for possible freshwater fluxes A number of simulations will be carried out next year with 0.1 Sv freshwater perturbation along the coast of Greenland Plans for 2010 CT2 Workshop in Hamburg early November - will be announced.

Thor annual meeting 2009, Paris Links with other CT’s We need updated time-series of transports from CT3, now in particular across the GSR Monthly mean time-series - Volume - Heat - Salt - Core watermass characteristics (NISE+new data) We deliver an assessment of the ocean state (CT4) We supply a suite of model time-series of ocean state from ’analysis’ and ’simulations’ of various complexity.