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 Ekman and geostrophic dynamics, using satellite-based, altimeter derived sea level data and scatterometer derived wind data.
The methodology for computing surface currents is described in Lagerloef, Mitchum, Lukas and Niiler (1999) with further developments and refinements presented 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 current meters. Geostrophic velocities are computed with sea level gradients derived from TOPEX/Poseidon (1992-2002) and Jason (2002-present) sea surface height analyses. Surface wind pseudostresses are obtained from satellite scatterometers (SSM/I (1992-2000) and QuikSCAT (2000-present). A 6-year mean surface height field was subtracted and replaced by the mean annual 0-1000 dbar dynamic height derived from the World Ocean Atlas (Levitus).
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 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 pilot 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 will eventually allow downloading of user selectable data sets. In the interim, qualified researchers with unique research needs may request 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) directs the ESR activities of these projects. Fabrice Bonjean(bonjean@esr.org) conducts model development and implementation, John Gunn (gunn@esr.org) assists with the data and image processing, and Susan Howard (howard@esr.org) manages the web site.
Web Links
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., accepted.
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.
