Operational NCEP Global Ocean Data Assimilation System at NCEP: the impact of TOPEXJason1 altimetry

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Operational NCEP Global Ocean Data Assimilation
System at NCEP the impact of TOPEX/Jason-1
altimetry

• D W Behringer NOAA/NCEP
• 4th JCSDA Science Workshop
• Greenbelt Marriott Hotel, Greenbelt, MD
• May 31 - June 1, 2006

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Seasonal to Interannual Prediction at NCEP
Climate Forecast System (CFS)
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Seasonal to Interannual Prediction at NCEP
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Ocean observations used in GODAS

• Requirements
• A data set that spans 20 years.
• Global coverage with emphasis on the tropics.
• Emphasis on the surface and upper ocean.

• Consequently
• Primary reliance on ocean profiles (XBT, TAO,
  Argo) and on SST (Reynolds analysis).
• Stability of TAO and growth of Argo mean in situ
  profiles retain central importance.
• Satellite altimetry, available since 1992,
  arrived at a time when ocean models were already
  well constrained by in situ data.

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GODAS Experiments

• GODAS - operational version, assimilates
  temperature from XBTs (xT), Argo/Argo-like (aT)
  and TAO/Triton/PIRATA (tT) and synthetic salinity
  (sS)
• GODAS-T/J - assimilates xT, aT, tT, sS and
  TOPEX/Jason1 (TJ)
• RA6 - Previous operational system, Pacific Ocean
  only, assimilates xT, aT, and tT, but NOT
  salinity
• CONTROL - control, model is configured as in
  GODAS, but there is no data assimilation

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12 / 28 / 1997 - 1 / 07 / 1998
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Guam RMS COR CON 9.62
0.53 GDS 4.42 0.88 GDS-T/J1 3.13
0.93 Majuro RMS COR CON
6.43 0.56 GDS 4.63
0.81 GDS-T/J1 3.22 0.90 Pago Pago
RMS COR CON 4.71 0.80 GDS
3.06 0.92 GDS-T/J1 2.27 0.96
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Nauru RMS COR CON 5.05
0.85 GDS 5.11 0.82 GDS-T/J1 4.38
0.88 Kapingamarangi RMS
COR CON 4.97 0.82 GDS 3.73
0.90 GDS-T/J1 3.14 0.93 Santa Cruz
RMS COR CON 4.39 0.88 GDS
2.54 0.97 GDS-T/J1 2.10 0.97
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Atlantic Ocean - Temperature Analyses vs CTD - A16
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Atlantic Ocean - Salinity Analyses vs CTD - A16
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Concluding Remarks

• A consistent, uninterrupted altimetry data set
  (TOPEX/Jason-1) has been available since late
  1992, providing a good basis for testing its
  impact on GODAS.
• In the equatorial Pacific the assimilation of the
  TAO mooring data leads to a good representation
  of anomalous SSH in the operational GODAS (and
  RA6). Assimilation of T/J-1 improves GODAS SSH
  beyond the bounds of the TAO array and well into
  the subtropics.
• In the Atlantic and Indian Oceans the operational
  GODAS does a poor job representing the SSH
  anomaly field and appears to do no better than
  the Control experiment. In these two oceans the
  assimilation of T/J1greatly improves GODAS SSH.
• Comparison of the Control and GODAS experiments
  with an Atlantic CTD section shows that the
  assimilation of subsurface data (XBTs, Argo)
  largely serves to correct model bias. Comparison
  with the GODAS-T/J experiment shows that only
  small additional subsurface changes are needed to
  improve anomalous SSH.

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Concluding Remarks - continued

• The question remains whether adding new data sets
  to the operational GODAS will have an impact on
  the NCEP S/I forecasts. An answer to this
  question is only possible after a data set is
  available for a period of many years (20)
  spanning several ENSO events. Any conclusions
  drawn from retrospective forecasts over a shorter
  period of time would lack statistical
  reliability.
• In the interim, the most sensible strategy may be
  to work to improve both the model and the
  assimilation method so as to make the best
  possible use of the available data.
• Thus, from the perspective of S/I prediction, it
  is imperative to support not only the acquisition
  of new data, but also the development of improved
  techniques to make use of them.

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The End
cfs.ncep.noaa.gov/cfs/godas www.cpc.ncep.noaa.gov/
products/GODAS