Ocean Data Assimilation for SI Prediction at NCEP David Behringer, NCEP/EMC Diane Stokes, NCEP/EMC Sudhir Nadiga, NCEP/EMC Wanqiu Wang, NCEP/EMC US GODAE Workshop Ocean.US June 17, 2003 Ftp:ftpprd.ncep.noaa.gov/pub/cmb/Global/Presentations

Outline of the presentation  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  New experiments with coupled global ocean - global atmosphere model

OGCM MOM Pacific v.1 Global v.3 Data Assimilation 3D VAR Observations: XBTs TAO P-Floats Altimetry Analyzed Fields: Temperature Salinity Ocean Data Assimilation System (ODAS) Oceanic I.C.for Coupled Model Statistical Models CCA, Markov ENSO Monitoring Surface Fluxes: Momentum Heat E - P

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

OGCM MOM Pacific v.1 Global v.3 Data Assimilation 3D VAR Observations: XBTs TAO P-Floats Altimetry Analyzed Fields: Temperature Salinity Ocean Data Assimilation System (ODAS) Oceanic I.C.for Coupled Model Statistical Models CCA, Markov ENSO Monitoring Surface Fluxes: Momentum Heat E - P

GODAS (MOM V.3) Grid: Quasi-global extending from 75 o S to 65 o N, zonal resolution is 1 o, meridional resolution is 1 o poleward of 30 o increasing smoothly to 1/3 o within 10 o 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: Pacific basin from 45 o S to 55 o N and 120 o E to 70 o W, zonal resolution is 1.5 o, meridional resolution is 1 o poleward of 20 o increasing smoothly to 1/3 o within 10 o 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 by a winds constructed by combining anomalies from the Florida State University wind product with an attenuated Hellerman 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.

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.

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 1 o 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.

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

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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.

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.

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.  Operational analyses will be run daily, forced by GDAS wind stress, heat flux and E-P. The ocean state data will be saved as 1-day averages and as “restart” files at 1-day intervals. It will be run retrospectively, beginning in January  Long ODASI reanalyses ( ) saved monthly. These will also include analyses alternately omitting western and eastern TAO mooring data.

Coupled Model Experiments Started in March 2003 GFS02 + MOM3 Direct coupling GFS02 with 28 layers and 64 layers Initial conditions taken from GDAS and GODAS for 1 January 2002

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