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The Global Ocean Data Assimilation System (GODAS) at NCEP

The Global Ocean Data Assimilation System (GODAS) at NCEP. David W. Behringer, NCEP/EMC Yan Xue, NCEP/CPC Boyin Huang , NCEP/CPC NOAA Climate Observation Division 5 th Annual System Review Silver Spring, Maryland 5-7 June 2007. Content. The impact of satellite altimetry data on GODAS

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The Global Ocean Data Assimilation System (GODAS) at NCEP

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  1. The Global Ocean Data Assimilation System (GODAS) at NCEP David W. Behringer, NCEP/EMC Yan Xue, NCEP/CPC Boyin Huang , NCEP/CPC NOAA Climate Observation Division 5th Annual System Review Silver Spring, Maryland 5-7 June 2007

  2. Content • The impact of satellite altimetry data on GODAS • The impact of deep Argo temperature data on GODAS • The impact of Argo salinity data on GODAS • Accessing GODAS data and products • Future plans

  3. Ocean Model MOMv3 quasi-global 1ox1o (1/3o in tropics) 40 levels Climate Forecast System (CFS) Atmospheric Model GFS (2003) T62 64 levels Reanalysis-2 3DVAR T62L28 update of the NCEP-NCAR R1 GODAS 3DVAR XBT TAO etc Argo Salinity (syn.) TOPEX/Jason-1 Seasonal to Interannual Prediction at NCEP

  4. Operational GODAS performance The standard GODAS reproduces the interannual variability well, notably the El Nino / La Nina signal.

  5. Argo profiles offer the possibility of a deeper assimilation. Altimetry can improve SSH beyond the tropical Pacific Argo provides observed salinity profiles to replace synthetic profiles. • GODAS (MOM V.3) • Operational since September 2003 • Grid: • Quasi-global, 1x1 degree horizontal resolution, enhanced to 1/3 degree in the tropics, 40 vertical levels. • Physics: • KPP boundary layer mixing scheme, GM isopycnal tracer mixing, Smagorinsky velocity mixing, free surface. • Forcing: • Wind stress, heat flux, E-P from Reanalysis 2 (R2), SST relaxed to weekly Reynolds’ analysis, SSS relaxed to Levitus monthly climatology. • Assimilation method: • 3D VAR, limited to upper 750m, univariate in temperature and salinity, error covariance varies geographically and temporally. • Assimilation data: • Temperature profile data from XBTs, profiling floats (Argo), moorings (TAO), synthetic salinity constructed from temperature and local Levitus T-S climatology. Satellite altimetry from TOPEX / Jason-1 (beginning in March 2007).

  6. Original operational GODAS • Temperature profiles from Argo, XBTs, TAO moorings • Depth of assimilation is 750 m. Altimetry in GODAS-T/J • Temperature profiles from Argo, XBTs, TAO moorings • Altimetry from TOPEX and Jason-1 • Depth of assimilation is 750 m. SSH innovations contribute to temperature and salinity corrections. Altimetry Assimilation 1993 - 2005

  7. Original operational GODAS • Temperature profiles from Argo, XBTs, TAO moorings • Depth of assimilation is 750 m. Argo salinity in GODAS-X • Temperature profiles from Argo, XBTs, TAO moorings • Depth of assimilation is 2200 m. Shallow profiles are augmented with climatology. Deep Assimilation 1980 - 2005

  8. The changing number and characteristics of observations

  9. Deep assimilation eliminates drift in the Indian and Pacific Oceans and allows true positive trend in the Atlantic Shallow assimilation can’t control temperature drift at 1200 m. Standard vs. Deep assimilation The standard assimilation extends down to 750 m. The deep assimilation extends down to 2200 m. IND PAC ATL IND PAC ATL

  10. Standard vs. Deep assimilation Independent WOCE CTD section completed in 1988 & 1989 … …and repeated in 2003 & 2005 by PMEL. Shallow assimilation has a strong cold bias of 1-3oC below 750 m. Standard Deep assimilation eliminates the cold bias. Deep

  11. Original operational GODAS • Temperature profiles from Argo, XBTs, TAO moorings • Salinity profiles are 100% synthetic (via TS-relationship) Argo salinity in GODAS-A/S • Temperature profiles from Argo and XBTs only • Salinity profiles are 75% observed (Argo) and 25% synthetic (XBTs) Assimilating Argo Salinity 2005

  12. Equatorial salinity section in the Pacific (vertical bars show positions of time-series below). Assimilating Argo Salinity GODAS Salinity variability due to correlation with temperature. GODAS-A/S Salinity variability introduced by observations.

  13. Along with the salinity changes there are changes of up to 0.3m/sec in the mean zonal equatorial currents. Assimilating Argo observations alters GODAS salinity by as much as 0.5 psu over broad regions. Assimilating Argo Salinity Changes in both S and U are largely confined in and above the thermocline.

  14. In the west, assimilating Argo salinity corrects the bias at the surface and the depth of the undercurrent core and captures the complex structure at 165oE. In the east, assimilating Argo salinity reduces the bias at the surface and sharpens the profile below the thermocline at 110oW. Assimilating Argo Salinity Comparison with independent ADCP currents. ADCP GODASGODAS-A/S

  15. Summary - The impact of observations in GODAS • Assimilating TOPEX/Jason-1 into an ocean analysis improves the SSH and the upper ocean heat content outside of the tropical Pacific. Those improvements, however, haven’t led to improved SI forecasts. In the tropical Pacific the GODAS is already well constrained by data from the TAO / TRITON array and it is difficult to improve on that. Further improvements in data assimilation and coupled modeling may be needed to take advantage of satellite altimetry. • Deep assimilation (2200m) of Argo data eliminates the deep temperature drift in the standard GODAS and produces an analysis in good agreement with independent CTD sections (WOCE A16). In the years prior to the availability of Argo profiles, deep climatology serves to prevent the temperature drift. • Accurate equatorial currents require accurate salinity. Assimilating Argo salinity produces the detailed structure in the equatorial mass field needed to accurately portray the equatorial currents. This has not been possible with synthetic salinity.

  16. Accessing GODAS productsand data Products http://www.cpc.ncep.noaa.gov/products/GODAS Data http://cfs.ncep.noaa.gov/cfs/godas/monthly http://cfs.ncep.noaa.gov/cfs/godas/pentad http://www.cgd.ucar.edu/cas/catalog/ocean/godas_variable.shtml

  17. Monthly Ocean Briefing - A New CPC Product • Background • Air-sea coupled modes such as ENSO, IOD, TAV, PDO and AMO have robust impacts on climate. • The coupled modes vary on interannual to decadal time scales due to both natural variability and anthropogenic forcings. It is imperative to monitor and assess those modes in real time to support operational climate predictions at CPC. • A real-time assessment of CFS’s performance in forecasting those coupled modes is desired. • A real-time assessment of GODAS biases and their potential impacts on the seasonal forecasts of CFS is needed. • The briefing • Conference call on the 6th-9th day of every month. • PowerPoint Presentation and Ocean Briefing web page • Outside colleagues are welcome to contribute to and participate in the Ocean Briefing.

  18. Future Plans • A new version of the CFS, operational by 2010. • Fully global with atmosphere, ocean, land and ice components • ODAS based on MOMv4 / 3DVAR (NCEP) • ODAS based on MOMv4 / KF (JPL) • ADAS based on GFS / GSI • Reanalysis (1980-present), 1st guess for each component from the CFS • Retrospective forecasts (1980-present) • A stand alone operational version of GODAS. • Frees ocean analysis from long update cycle of the CFS and thus • Serves as a useful platform for introducing new methods and data. • Serves as a tool for CPC “state of the ocean” products and discussions.

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