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Jon Robson (Uni. Reading) (J.i.robson@reading.ac.uk)

Analysis of a decadal prediction system: issues for anomaly initialisation. Jon Robson (Uni. Reading) (J.i.robson@reading.ac.uk) Rowan Sutton (Uni. Reading) and Doug Smith (UK Met Office). Introduction and motivation. Global mean Ts RMSE.

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Jon Robson (Uni. Reading) (J.i.robson@reading.ac.uk)

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  1. Analysis of a decadal prediction system: issues for anomaly initialisation Jon Robson (Uni. Reading) (J.i.robson@reading.ac.uk) Rowan Sutton (Uni. Reading) and Doug Smith (UK Met Office)

  2. Introduction and motivation Global mean Ts RMSE • Smith et al, 2007. showed that assimilating in the observed state of the climate improved forecasts of surface temperature relative to forecasts that do not assimilate information. • Mean skill scores provide little insight • What are the mechanisms giving rise to the improved predictability? • Evaluating the climate models against observations at the process level

  3. DePreSys • Fully coupled decadal forecast system, based on HadCM3 • Atmosphere = 2.5° x 3.75°, 19 levels, Ocean = 1.25° x 1.25°, 20 levels • Initialised from the observed climate but also forced by anthropogenic emissions (SRES B2 scenario), previous 11 year solar cycle and volcanic aerosol (which is decayed in the forecast). • There are no future volcanoes in the forecasts • Assimilates observed anomalies onto the model climate to avoid drift Hindcast Set • 4 member ensemble DePreSyshindcasts initialised seasonally (March, June, Sept and December) over the years 1982-2001 Assimilation Run DePreSys Transient Run’s Obs anomaly 1941 1996 1960 2000 Global Temp

  4. What actually happened over the hindcast period (eg 1982 - 2001) June 1995 Subpolar gyre 500m heat content DePreSys observations Sept 1995 anomalies relative to 1941-1996 climatology

  5. However it doesn’t get it right all the time…. • After 1990 DePreSys hindcasts become very eager to warm rapidly in the subpolar gyre region. • Are these early warmers caused by changes in the Atlantic Meridional Overturning Circualtion (AMOC)? • Look at the initialised density anomalies Correlation of 0-1000m density anomaly leading the AMOC index by 5 years from HadCM3 control run

  6. Density anomalies 150-1000m density anomalies normalised by observed interannual standard deviation • Hypothesis A:- The early warming hindcasts are caused by the presence of errors in the assimilated density anomalies, which arise due to the non-linear equation of state, and cause an increase in the AMOC that is too early or too large

  7. Drifts present in DePreSys AMOC strength at 50°N 800-3000m mean hindcast density anomalies 1980 1990 2000 2010 Hypothesis B:- Imbalances in the model climatology lead to drifts that corrupt the hindcasts in the North Atlantic

  8. Testing the hypotheses A. The early warming hindcasts are caused by the presence of errors in the assimilated density anomalies that arise due to the non-linear equation of state, and cause an increase in the AMOC that is too early or too large • Perturb the assimilated density so that the density anomalies are the same as observed, by perturbing salinity anomalies B. Imbalances in the climatology lead to drifts that corrupt the hindcasts in the North Atlantic • Use a different climatology to test the sensitivity to the climatology • Re-run the December 1991 hindcast

  9. The effect of density errors Control – Perturbed Salinity overturning stream function as a fn of Latitude and time Subpolar gyre 0-500m Temp Unperturbed Observations Perturbed salinity 2nd year SST forecast difference control – perturbed Salinity.

  10. The effect of a new climatology Subpolar gyre 0-500m Temp Unperturbed Unperturbed Observations New Clim New Clim 2nd year SST forecast difference control – new clim.

  11. Conclusions and implications • Moving past mean skill scores to looking at individual hindcasts for case studies is an important route for improving decadal prediction systems • Issues for anomaly assimilation • Hindcasts can be very sensitive to the choice of climatology used • The non-linear equation of state means that some imbalance may be inevitable when climatologies are derived from time mean temperature and salinity • Non-linearities also lead to errors in the assimilated density anomalies that can have a significant effect upon the hindcasts • More work needed on balanced initialisation for decadal climate prediction • Validating the models at the process level to understand model errors and missing processes

  12. Thank you j.i.robson@reading.ac.uk www.met.rdg.ac.uk/~swr06jir

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