The Global Ocean Data Assimilation System (GODAS)

1. The Global Ocean Data Assimilation System (GODAS) David W. Behringer NCEP 7 November 2007

2. Content • The new GODAS versus the operational GODAS. • The comparability of the old and new systems. • A description of the ocean observing system. • Summary.

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

4. Current Operational System • The current GODAS became operational in 2003. • - Based on MOMv3 (1o x 1o x 40L) and a 3DVAR assimilation scheme. • - Original assimilation data were temperature profiles (XBT, Argo, TAO, • TRITON, PIRATA) and synthetic salinity profiles derived from a • climatological T-S relation. 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. • - Reanalysis (1980-present) providing initial conditions for retrospective CFS • forecasts. • - Two versions: 14-day lag, 7-day lag. • Recent and pending upgrades. • - Jason-1 altimetry added to the operational data set (March 2007). • - Depth of the assimilation increased from 750m to 2200m (December 2007). • - The 7-day lag version replaced by 1-day lag version (December 2007)

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. GODAS for the CFSRR • Prototype GODAS to become operational in 2010. • - Based on MOMv4 (1/2o x 1/2o x 40L) and the 3DVAR assimilation • scheme. • - Assimilation data are temperature profiles (XBT, Argo, TAO, TRITON, • PIRATA), synthetic salinity profiles derived from a seasonal T-S relation, • Reynolds’ daily OI SST, Levitas seasonal SSS, TOPEX/Jason-1 Altimetry. • Data window extends from 10-days before to 10-days after the analysis • date. Data window for SST is 1-day. • - The analysis system is quasi-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. • - No external atmospheric forcing fields are needed. No relaxation of surface • temperature or salinity is used.

7. Comparability of MOM4 and MOM3 versions of GODAS • Differences in how physical parameterizations are implemented. • Assimilation code has been rewritten using FORTRAN 90 structures and MOM4 coding conventions. Comparison of 2 long runs with same resolution (1ox1o), using the same forcing (Reanalysis 2), and the same assimilation data set (T(z), S(z)).

10. 1980 1990 2000 The observing system XBT TAO TP/J-1 Argo

11. The changing number and characteristics of observations

12. Some consequences of the evolving observing system. • Shallow data in the 1980s and 1990s cannot control drift in mid-ocean. • Lack of salinity observations has consequences for tropical circulation.

13. 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. Shallow 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

14. 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

15. 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.

16. 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

17. Summary • The new version of GODAS is based on MOM4, is global and has increased resolution (1/2ox1/2o). • When run in a similar configuration, the new GODAS is comparable to the current operational GODAS. • The new GODAS incorporates SST and SSS into the assimilation in place of surface relaxation. • The new GODAS assimilation extends to 2200m, taking better advantage of the Argo data and limiting mid-depth temperature drift. • As part of the CFSRR, the GODAS will not be forced by an external analysis (e.g. R2), but instead will be integrated into the CFS.

18. Appendix

19. Sample annual distributions of T(z) as used by GODAS XBT-greenTAO-redArgo-blue

20. 30K/mo TOPEX/Jason-1 in GODAS. A consistent set of altimetry data has been available since late 1992. 12 / 28 / 1997 - 1 / 07 / 1998 N.B. GODAS assimilates altimetry data only within outlined box.

21. International Argo deployments in 2000 GODAS assimilates all Argo and proto-Argo profiles.

22. International Argo deployments as of October 31, 2007 Full Deployment GODAS assimilates all Argo and proto-Argo profiles.