Satellite Derived Tropical Ocean Surface Currents



Principal Investigators

Fabrice Bonjean
Gary Lagerloef

Others:

John Gunn
Eric Johnson

OSCAR - Ocean Surface Current Analyses - Real time



Introduction

ESR, in cooperation with other researchers, has been developing and improving methodologies to compute and display tropical surface currents since 1996. This analysis is based on geostrophic, Stommel/Ekman and thermal wind dynamics, using satellite-based, altimeter derived sea level data, scatterometer derived wind data, and SST data.


The above graphic links provide access to displays of the calculated surface current throughout the global tropical ocean. The original diagnostic model dealt with the Tropical Pacific and eventually matured into the product and services available on the OSCAR web site (www.oscar.noaa.gov) described in further detail below. This model has been extended, as a part of continuing research at ESR, into the other ocean basins, as shown in the above links. Future efforts will extend this analysis to higher latitude regions.


The methodology for computing surface currents is described in Bonjean and Lagerloef (2002). Briefly, the model formulation combines geostrophic, Ekman and Stommel shear dynamics, and a complementary term from the surface buoyancy gradient. The field compares well with velocity observations from 15-m depth buoy drifters and equatorial Tropical Ocean-Atmosphere mooring data. Geostrophic velocities are computed with sea level gradients derived from sea surface height analysis of TOPEX/Poseidon (1992-2002) and Jason (2002-present) (Lagerloef, Mitchum, Lukas and Niiler, 1999). Surface wind pseudostresses are obtained from satellite scatterometers (SSM/I (1992-2000) and QuikSCAT (2000-present)). A 7-year mean surface height field was subtracted and replaced by a mean dynamic topography derived from altimetry, in-situ measurements and the CHAMP geoid model (Rio and Hernandez, 2004, JGR, 109).



Applications and NOAA Web Site

The present implementation of this diagnostic model of the surface circulation in the tropical Pacific is presented on the NOPP project website "Ocean Surface Current Analysis-Real-time". OSCAR is a pilot processing system and data center delivering tropical Pacific Ocean surface velocity fields via the Internet (www.oscar.noaa.gov). Surface currents are computed from satellite altimeter and vector wind data using methods developed during the Topex/Poseidon altimeter research mission. OSCAR is a transition to operational oceanographic applications using Jason-1 altimeter data. The various uses (actual or potential) include large scale climate diagnostics and prediction, oceanographic education and research, fisheries management, monitoring debris drift, larvae drift, oil spills, fronts and eddies, search and rescue, naval and maritime operations, and idle curiosity. OSCAR provides velocity maps updated on a 1-week basis within 10 days of Jason altimeter measurements. This is made possible by (1) the rapid availability of Jason-1 Interim Geophysical Data Records (IGDRs) within 2-3 days delay from the time of satellite measurement, and (2) the fact that Jason-1 IGDR data approach the accuracy of scientific Topex/Poseidon and Jason-1 altimeter Geophysical Data Records (GDRs) which are generally delayed 1-2 months. The velocity fields are published in the monthly NOAA Climate Diagnostics Bulletin and are particularly useful in monitoring the recent evolution of ENSO conditions.

The OSCAR web server allows user-selected geographic regions, display resolution and scaling of image display maps, as well as access to the entire Topex/Poseidon/Jason-derived field from 1993-present.

The OSCAR website (http://www.oscar.noaa.gov/) allows downloading of surface velocity data in NetCDF format, or through the OpenDAP/DODS network. The original source of the NetCDF data package is:

http://my.unidata.ucar.edu/content/software/netcdf/netcdf-4/newdocs/netcdf/Summary.html

But a PC interface edition is maintained at:

http://mexcdf.sourceforge.net/

Alternatively, there is an archived version of similar software at:

http://woodshole.er.usgs.gov/staffpages/cdenham/public_html/MexCDF/nc4ml5.html

Although we prefer you access data through the OSCAR web page, qualified researchers with unique research needs may discuss specific research data sets via email to John Gunn at ESR (gunn@esr.org).



Acknowledgements

This work is a product of the Topex/Poseidon/Jason altimeters and the NSCAT/SeaWinds mission science team studies involving collaborations with G. Mitchum (USF), A. Busalacchi (NASA/GSFC), R.Lukas(U.H.), T. Liu (JPL), and P. Niiler (SIO). Gary Lagerloef (lager@esr.org ) and Fabrice Bonjean (bonjean@esr.org) direct the ESR activities of these projects. John Gunn (gunn@esr.org) assists with the model implementation and data processing, and Susan Howard (howard@esr.org) manages the web site.



Web Links

The official OSCAR web site is: www.oscar.noaa.gov

The most recent EOF map of the surface velocity and SST anomalies can be found for the Pacific Ocean here.




Publications

Lagerloef, G.S.E., R. Lukas, F. Bonjean, J.T. Gunn, G.T. Mitchum, M. Bourassa, A.J. Busalacchi, 2003: El Nino Tropical Pacific Ocean surface current and temperature evolution in 2002 and outlook for early 2003, Geophys. Res. Lett., Vol. 30, No. 10, 1514, doi:10.1029/2003GL017096, 2003.

Bonjean, Fabrice, Gary S. E. Lagerloef, 2002: Diagnostic Model and Analysis of the Surface Currents in the Tropical Pacific Ocean. J. Physical Oceano. Vol. 32, No. 10, pp. 2938-2954.

Lagerloef,G.S.E., G.Mitchum, R.Lukas and P.Niiler, 1999: Tropical Pacific near-surface currents estimated from altimeter, wind and drifter data, J. Geophys. Res., 104, pp. 23,313-23,326.



Presentations

OSCAR: Ocean Surface Current Analysis - Real time, (poster) Jason/Topex/Poseidon Science Working Team Meeting, October 2002, New Orleans, LA.

Tropical Ocean Currents Inferred From Satellite, (poster) "En Route to GODAE" International Symposium, June 2002, Biarritz, France.

Seasonal to Interannual Variations of the Surface Currents in the Equatorial Oceans, (Oral presentation) Ocean Sciences 2002, Honolulu, Hawaii.

Seasonal to Interannual Variations of the Surface Currents in the Tropical Atlantic Ocean during 1993-2000, (Oral presentation) U.S. CLIVAR Atlantic Conference 12-14 June 2001, Boulder, CO.