GODAS

1. GODAS David Behringer, EMC Diane Stokes, EMC Sudhir Nadiga, EMC Yan Xue, CPC Briefing May 9, 2003 Ftp:ftpprd.ncep.noaa.gov/pub/cmb/Global/Presentations

2. Outline of the GODAS Briefing • Description of the Global Ocean Data Assimilation System (GODAS) and comparison with the Pacific Ocean system (RA6) • Description of the data used for assimilation • Comparison of GODAS and RA6 analyses with observations • Summary • Products

3. Seasonal to Interannual Forecasting at NCEP 1 2 Ocean Data Assimilation System (ODAS) OGCM 3D VAR Coupled Ocean Atmosphere General Circulation Model (CGCM) Atmosphere General Circulation Model (AGCM) Ocean Initial Conditions SST Forecast US Forecasts Surface Temp Precip

4. Ocean Data Assimilation System (ODAS) Data Assimilation 3D VAR OGCM MOM Pacific v.1 Global v.3 Observations: XBTs TAO P-Floats Altimetry Analyzed Fields: Temperature Salinity

6. GODAS (MOM V.3) • Grid: • Quasi-global extending from 75oS to • 65oN, zonal resolution is 1o, • meridional resolution is 1o poleward • of 30oincreasing smoothly to 1/3o • within 10o of the equator, 40 vertical • levels, 10 meter resolution in the • top 200 meters. • Physics: • Nonlinear horizontal viscosity, KPP • boundary layer mixing scheme, • free surface and realistic topography. • Forcing: • Forced by wind stress, heat flux, • precipitation-evaporation from • Reanalysis 2, incoming short-wave • radiation penetration, SST is relaxed • to weekly NCEP SST analysis, • surface salinity is relaxed to • Levitus monthly SSS climatology. • RA6 (MOM V.1) • Grid: • Pacificbasin from 45oS to 55oNand • 120oE to 70oW, zonal resolution is • 1.5o,meridional resolution is 1o • poleward of 20oincreasing smoothly • to 1/3o within 10o of the equator, 27 • vertical levels, 10 meter resolution • in the top 100 meters. • Physics: • Nonlinear horizontal viscosity, • Richardson number based scheme, • rigid lid and realistic topography. • Forcing: • Forced bya winds constructed by • combining anomalies fromthe Florida • State University wind product with an • attenuatedHellerman and Rosenstein • (1983)wind climatology (pre-1995), or • GDAS winds (post-1995), GDAS heat • flux, no surface fresh-water flux, SST • is relaxed to weekly NCEP SSTanalysis, • no constraint on surface salinity.

7. GODAS (MOM V.3) • Assimilation method: • 3D variational scheme (Derber and • Rosati, 1989; Behringer et al., 1998), • analyzes temperature and salinity, • background error variance varies • geographically and temporally. • Assimilation data: • Temperature profile data from XBTs, • profiling floats (Argo), moorings • (TAO), synthetic salinity profiles • constructed from temperature profiles • and local Levitus T-S climatology. • RA6 (MOM V.1) • Assimilation method: • 3D variational scheme (Derber and • Rosati, 1989; Behringer et al., 1998), • analyzes temperature only, fixed • background error covariance prior to • TOPEX era. • Assimilation data: • Temperature profile data from XBTs, • profiling floats (Argo), moorings • (TAO), TOPEX sea level since • 1993.

8. Data for Assimilation • Temperature profiles: XBTs (Volunteer Observing Ships), moorings (TAO, TRITON, PIRATA), profiling floats (Argo). There are approximately 5000 profiles available globally per month via satellite (GTS). • Satellite altimetry (TOPEX/Poseiden, JASON). Corrected data averaged in 1o bins along the ground track. There are approximately 20,000 data points per month. These data, originally provided by NESDIS, are now available from the U.S. Navy. The variable part of the altimetry is assimilated by RA6, but not by this version of GODAS.

12. Comparisons of GODAS and RA6 with observations • Equatorial Salinity compared with Levitus • Temperature profiles compared with TAO mooring data • Sea level compared with TOPEX and with Pacific tide gauges • Currents compared with TAO mooring data and with the Ocean Surface Current Analysis – Real-time (OSCAR)

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28. Summary of GODAS vs RA6 in the tropical Pacific • The temperature field in GODAS is generally closer to observations than is the temperature field in RA6.GODAS is most improved below the thermocline and in the western Pacific. GODAS does not do as well as RA6 above the thermocline in the extreme eastern Pacific. • The poor representation of salinity in RA6 has been corrected in GODAS. • GODAS does as well as RA6 in comparisons with TOPEX altimetry, even though this version of GODAS does not assimilate TOPEX while RA6 does. • GODAS sea level is generally closer than RA6 sea level to the tide gauge records in the equatorial zone. The largest improvements are in the west.

29. Summary of GODAS vs RA6 in the tropical Pacific • The mean Equatorial Undercurrent is similar in GODAS and RA6. Compared with TAO data, both do well at 170W, 140W, and 110W, getting the core depth right, but being slightly weak at 110W. Neither does well at 165E, where GODAS has a better defined core depth, but has worse amplitude than RA6 at the surface. • The rms error in the Equatorial Undercurrent is smaller in GODAS than in RA6 at all depths at all 4 TAO sites (165E, 170W, 140W, and 110W) with the exception of the near surface at 165E. • Surface currents in both GODAS and RA6 remain a problem. Both have large discrepancies (30 cm/s) when compared the OSCAR data. Mean currents off the equator are larger in GODAS and RA6 than in OSCAR, while differences between the two model analyses and OSCAR show changes of sign along the equator.

30. GODAS Products • A long reanalysis beginning in 1979 and continuing through the present. It will be forced by daily Reanalysis 2 wind stress, heat flux and E-P. The ocean state will be saved both as 5-day averages and as “restart” files at 5-day intervals. • It is proposed that the operational analyses will be run daily, forced by GDAS wind stress, heat flux and E-P. The ocean state data will be post-processed and saved as 5-day averages and as “restart” files at 5-day intervals.

31. The End