The Global Ocean Data Assimilation System (GODAS) at

NCEP

David W. Behringer, NCEP/EMCYan Xue, NCEP/CPC

Boyin Huang , NCEP/CPC

NOAA Climate Observation Division5th Annual System Review

Silver Spring, Maryland5-7 June 2007

Content

• The impact of satellite altimetry data on GODAS

• The impact of deep Argo temperature data on GODAS

• The impact of Argo salinity data on GODAS

• Accessing GODAS data and products

• Future plans

Seasonal to Interannual Prediction at NCEP

ClimateForecastSystem(CFS)

Ocean ModelMOMv3

quasi-global1ox1o (1/3o in tropics)

40 levels

Atmospheric ModelGFS (2003)

T6264 levels

GODAS3DVAR

XBTTAO etc

ArgoSalinity (syn.)

TOPEX/Jason-1

Reanalysis-23DVART62L28

update of theNCEP-NCAR R1

Operational GODAS performance

The standard GODAS reproduces the interannual variability well, notably the El Nino / La Nina signal.

GODAS (MOM V.3)Operational since September 2003

Grid: Quasi-global, 1x1 degree horizontal resolution, enhanced to 1/3 degree in the tropics, 40 vertical levels.

Physics: KPP boundary layer mixing scheme, GM isopycnal tracer mixing, Smagorinsky velocity mixing, free surface.

Forcing: Wind stress, heat flux, E-P from Reanalysis 2 (R2), SST relaxed to weekly Reynolds’ analysis, SSS relaxed to Levitus monthly climatology.

Assimilation method: 3D VAR, limited to upper 750m, univariate in temperature and salinity, error covariance varies geographically and temporally.

Assimilation data:Temperature profile data from XBTs, profiling floats (Argo), moorings (TAO), synthetic salinity constructed from temperature and local Levitus T-S climatology. Satellite altimetry from TOPEX / Jason-1 (beginning in March 2007).

Argo profiles offer the possibility of a deeper assimilation.

Argo provides observed salinity profiles to replace synthetic profiles.

Altimetry can improve SSH beyond the tropical Pacific

Original operational GODAS

• Temperature profiles from Argo, XBTs, TAO moorings • Depth of assimilation is 750 m.

• Temperature profiles from Argo, XBTs, TAO moorings• Altimetry from TOPEX and Jason-1• Depth of assimilation is 750 m. SSH innovations

contribute to temperature and salinity corrections.

Altimetry in GODAS-T/J

Altimetry Assimilation1993 - 2005

Original operational GODAS

• Temperature profiles from Argo, XBTs, TAO moorings • Depth of assimilation is 750 m.

• Temperature profiles from Argo, XBTs, TAO moorings• Depth of assimilation is 2200 m. Shallow profiles are

augmented with climatology.

Argo salinity in GODAS-X

Deep Assimilation1980 - 2005

The changing number and characteristics of observations

Standard vs. Deep assimilation

The standard assimilation extends down to 750 m.

The deep assimilation extends down to 2200 m.

Shallow assimilation can’t control temperature drift at1200 m.

Deep assimilation eliminates drift in the Indian and Pacific Oceans and allows true positive trend in the Atlantic

IND PAC ATL IND PAC ATL

Independent WOCE CTD section completed in 1988 & 1989 …

…and repeated in 2003 & 2005 by PMEL.

Standard

Deep

Standard vs. Deep assimilation

Shallow assimilation has a strong cold bias of1-3oC below 750 m.

Deep assimilation eliminates the cold bias.

Original operational GODAS

• Temperature profiles from Argo, XBTs, TAO moorings• Salinity profiles are 100% synthetic (via TS-relationship)

• Temperature profiles from Argo and XBTs only• Salinity profiles are 75% observed (Argo) and 25%

synthetic (XBTs)

Argo salinity in GODAS-A/S

Assimilating Argo Salinity2005

AssimilatingArgo Salinity

GODAS

GODAS-A/S

Salinity variability due to correlation with temperature.

Salinity variability introduced by observations.

Equatorial salinity section in the Pacific (vertical bars show positions of time-series below).

Assimilating Argo Salinity

Assimilating Argo observations alters GODAS salinity by as much as 0.5 psu over broad regions.

Changes in both S and U are largely confined in and above the thermocline.

Along with the salinity changes there are changes of up to 0.3m/sec in the mean zonal equatorial currents.

Assimilating Argo Salinity

ADCP GODAS GODAS-A/S

In the east, assimilating Argo salinity reduces the bias at the surface and sharpens the profile below the thermocline at 110oW.

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.

Comparison with independent ADCP currents.

Summary - The impact of observations in GODAS

• Deep assimilation (2200m) of Argo data eliminates the deep temperature drift in the standard GODAS and produces an analysis in good agreement with independent CTD sections (WOCE A16). In the years prior to the availability of Argo profiles, deep climatology serves to prevent the temperature drift.

• Accurate equatorial currents require accurate salinity. Assimilating Argo salinity produces the detailed structure in the equatorial mass field needed to accurately portray the equatorial currents. This has not been possible with synthetic salinity.

• Assimilating TOPEX/Jason-1 into an ocean analysis improves the SSH and the upper ocean heat content outside of the tropical Pacific. Those improvements, however, haven’t led to improved SI forecasts. In the tropical Pacific the GODAS is already well constrained by data from the TAO / TRITON array and it is difficult to improve on that. Further improvements in data assimilation and coupled modeling may be needed to take advantage of satellite altimetry.

Accessing GODAS productsAccessing GODAS products and dataand data

Products

http://www.cpc.ncep.noaa.gov/products/GODAS

Data

http://cfs.ncep.noaa.gov/cfs/godas/monthly

http://cfs.ncep.noaa.gov/cfs/godas/pentad

http://www.cgd.ucar.edu/cas/catalog/ocean/godas_variable.shtml

• Background

Air-sea coupled modes such as ENSO, IOD, TAV, PDO and AMO have robust impacts on climate.

The coupled modes vary on interannual to decadal time scales due to both natural variability and anthropogenic forcings. It is imperative to monitor and assess those modes in real time to support operational climate predictions at CPC.

A real-time assessment of CFS’s performance in forecasting those coupled modes is desired.

A real-time assessment of GODAS biases and their potential impacts on the seasonal forecasts of CFS is needed.

• The briefing

Conference call on the 6th-9th day of every month.

PowerPoint Presentation and Ocean Briefing web page

Outside colleagues are welcome to contribute to and participate in the Ocean Briefing.

Monthly Ocean BriefingMonthly Ocean Briefing - A New CPC ProductA New CPC Product

A new version of the CFS, operational by 2010.

Fully global with atmosphere, ocean, land and ice components

ODAS based on MOMv4 / 3DVAR (NCEP)

ODAS based on MOMv4 / KF (JPL)

ADAS based on GFS / GSI

Reanalysis (1980-present), 1st guess for each component from the CFS

Retrospective forecasts (1980-present)

A stand alone operational version of GODAS.

Frees ocean analysis from long update cycle of the CFS and thus

Serves as a useful platform for introducing new methods and data.

Serves as a tool for CPC “state of the ocean” products and discussions.

Future PlansFuture Plans