The Global Ocean Data Assimilation System GODAS

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

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

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Current S-I Prediction at NCEP
Climate Forecast System (CFS)
GODAS 3DVAR XBT TAO etc Argo Salinity
(syn.) TOPEX/Jason-1
Reanalysis-2 3DVAR T62L28 update of the NCEP-NCAR
R1
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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)

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CFSRR at NCEP
Climate Forecast System
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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.

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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)).
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The observing system
XBT
TAO
TP/J-1
Argo
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The changing number and characteristics of
observations
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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.

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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
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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
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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.
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Assimilating Argo Salinity
Comparison with independent ADCP currents.
ADCP GODAS GODAS-A/S
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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.

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Appendix
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Sample annual distributions of T(z) as used by
GODAS
XBT-green TAO-red Argo-blue
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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.
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International Argo deployments in 2000
GODAS assimilates all Argo and proto-Argo
profiles.
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International Argo deployments as of October 31,
2007
Full Deployment
GODAS assimilates all Argo and proto-Argo
profiles.