The Global Ocean Data Assimilation System (GODAS) at NCEP

1. The Global Ocean Data Assimilation System (GODAS) at NCEP D W Behringer (EMC), A Vinzileos (EMC), I Fukumori (JPL), R Miller (OSU) 4th Annual CTB SAB Meeting September 11-12, 2008 ESSIC University of Maryland College Park, MD

2. Current Operational GODAS • Operational in 2003. • Based on MOMv3 (1o x 1o, 1/3o in the tropics, 40 lvls) and a 3DVAR assimilation scheme. • The assimilation data are temperature profiles (XBT, Argo, TAO, TRITON, PIRATA), synthetic salinity profiles derived from a climatological T-S relation and Jason-1 altimetry (March 2007). The data window extends from 2-weeks before to 2-weeks after the analysis date. • Atmospheric forcing from the NCEP Reanalysis-2, surface relaxation to Reynolds weekly OIv2 SST, Levitas climatological SSS. • Ocean reanalysis (1980-present) providing initial conditions for retrospective CFS forecasts used for calibration. • Two versions associated with the CFS: 14-day lag, 1-day lag. • A third stand-alone version with a 14-day lag.

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. GODAS in the CFSRR • To become operational in 2010. • Based on MOMv4 (1/2o x 1/2o, 1/4o in the tropics, 40 lvls) and an updated 3DVAR assimilation scheme. • Assimilation data are temperature profiles (XBT, Argo, TAO, TRITON, PIRATA), synthetic salinity profiles derived from a seasonal T-S relation and TOPEX/Jason-1 Altimetry. The data window is asymmetrical extending from 10-days before the analysis date. • Surface temperature relaxation to (or assimilation of) Reynolds new daily, 1/4o OIv2 SST, surface salinity relaxation Levitas climatological SSS. • The analysis system is coupled to the CFS in the sense that the first guess for the assimilation is provided by the CFS. After each analysis cycle the ocean model is stepped forward as a fully coupled component of the CFS. • The current stand-alone operational GODAS will be upgraded in 2009 to the higher resolution MOMv4 and be available for comparison with the coupled version.

5. 6hr Atmospheric Model GFS (2007) T382 64 levels GDAS GSI 24hr 6hr Land Model Ice Mdl SIS LDAS Ice Ext Ocean Model MOMv4 fully global 1/2ox1/2o (1/4o in tropics) 40 levels 6hr GODAS 3DVAR CFSRR at NCEP Climate Forecast System

6. Nino 3.4 Nino 3.0 ave: 0.05 rms: 0.13 ave: 0.05 rms: 0.20 GODAS-M4 ½o x ½o resolution assimilating Reynolds ¼o daily vs GODAS-M3 1o x 1o resolution relaxing to Reynolds ½o weekly

7. ave: 0.08 rms: 0.10 ave: 0.04 rms: 0.10 GODAS-M4 ½o x ½o resolution assimilating Reynolds ¼o daily vs GODAS-M3 1o x 1o resolution relaxing to Reynolds ½o weekly Warm Pool Cold Tongue

8. Comparison of GODAS/M4 and GODAS/M3 with TAO temperature and zonal velocity In the thermocline both GM4 and GM3 are warm at 140w, while GM4 is warm and GM3 is cold at 110w. The undercurrent is stronger than observed in GM4 and weaker in GM3. The vertical structure at 165e is better in GM4 than in GM3.

9. Collaborations • Impact of the GODAS mixed layer analysis on subseasonal forecasting with the CFS. Augustin Vintsileos (EMC) • Improved estimates of representation errors for observations. Bob Miller (OSU) • Alternative assimilation method. An adaption of the ECCO Kalman Filter (a partitioned, reduced-state, time-asymptotic filter) to MOM4. Ichiro Fukumori (JPL)

10. SST 20oC DynHt U KF 3DVAR - A 20oC DynHt U SST KF 3DVAR - B Comparison of GODAS/KF and GODAS/3DVAR with TAO temperature and zonal velocity anomalies Re = [model explained variance] / [data variance] For points toward the top (GKF) and toward the right (G3DV) the models are closer to the data. For points above (below) the diagonal GKF (G3DV) is closer to the data. in collaboration with I Fukumori (JPL)