OSCAR Documentation

	Data Product Abstract Investigator's Name and Credit Data Description - Filenaming Convention - File Format - Temporal Coverage & Resolution - Spatial Coverage & Resolution - Data Parameters & Format - Sample Data Record - Global Attributes - Sample Global Attributes	Available Read Software Processing Methods Filtering Data Calibration information Sources of Error Known Problems		References Acronyms Contact Information Document Information



Data Product Abstract This product contains global near-surface current estimages, derived using quasi-linear and steady flow momentum equations. The horizontal velocity is directly estimated from sea surface height, surface vector wind and sea surface temperature, these data were collected from various satellites and in situ instruments (see processing methods). The model formulation combines geostrophic, Ekman and Stommel shear dynamics, and a complementary term from the surface buoyancy gradient [1]. The ocean surface velocity files are in netCDF format which contain zonal (u) and meridional (v) velocities.
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Investigator's Name and Credit The OSCAR product was developed by Gary Lagerloef and Fabrice Bonjean from Earth and Space Research (ESR). If you use OSCAR data in publications, please include the following citation: The OSCAR data were obtained from JPL Physical Oceanography DAAC and developed by ESR. Also, ESR would appreciate receiving a preprint and/or reprint of publications utilizing these data for inclusion in the OSCAR bibliography. These publications should be sent to: OSCAR Project Office Earth and Space Research 2101 Fourth Avenue, Suite 1310 Seattle, WA 98121
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Data Description Each file contains one year with a 5 day resolution.
	Filenaming Convention The file names have a naming convention of world_oscar_vel_5dYYYY.nc world_oscar - name of product vel - product was made with a merged altimetry product composing of TOPEX/Poseidon, Jason-1, ERS 1-2, GFO, and ENVISAT altimeter data. 5d - 5 day resolution YYYY - year of the data
	File Format The data are in NetCDF files and have a maximum size of 32 MB when zipped.
	Temporal Coverage and Resolution One file consists of an entire year's worth of data, except for the present year. The time resolution within each file is 5 days, or half a Jason-1 or TOPEX/Poseidon cycle. For the present year, data are added as it comes in.
	Spatial Coverage and Resolution The data covers -69.5° to 69.5° latitude and 20.5° to 379.5° longitude. This range for longitude was chosen so that the Pacific and Atlantic Ocean would be continuous when plotted. The resolution is 1° in each direction. A 1/3° resolution dataset will soon be released (Spring 2009).
	Data Parameters and Format Latitude Definition: Latitude Dimension: 140 Unit: degrees North Element type: float Longitude Definition: Longitude Dimension: 360 Unit: degrees East Element type: float Time Definition: Time Dimension: 72 Unit: days since 1992-10-05 00:00:00 Element type: integer Date Definition: Date Dimension: 72 Unit: date in integer format Element type: integer Depth Definition: Depth Dimension: 1 Unit: meters Element type: float Mask Definition: Mask Dimension: latitude, longitude Unit: - Element type: integer Comment: This parameter is not needed since there are nan's in the velocity data and therefore should be ignored.		U Definition: Zonal Velocity Dimension: time, depth, latitude, longitude Unit: m/s Element type: float Missing value: nan V Definition: Meridional Velocity Dimension: time, depth, latitude, longitude Unit: m/s Element type: float Missing value: nan Uf Definition: Filtered Zonal Velocity Dimension: time, depth, latitude, longitude Unit: m/s Element type: float Missing value: nan Comment: The "filtered" data set is best used for a description of large scale and low frequency variations of surface flow in the tropical regions (Lx>10,000km, Ly>2,000km, T>40days). It is provided as an example of what can be done with this data set and it may not suit all research needs. It is suggested that you use the unfiltered data and create your own filters for your specific needs. Vf Definition: Filtered Meridional Velocit Dimension: time, depth, latitude, longitude Unit: m/s Element type: float Missing value: nan Comment: The "filtered" data set is best used for a description of large scale and low frequency variations of surface flow in the tropical regions (Lx>10,000km, Ly>2,000km, T>40days). It is provided as an example of what can be done with this data set and it may not suit all research needs. It is suggested that you use the unfiltered data and create your own filters for your specific needs.

	Sample Data Record These data are from world_oscar_vel_5d2005.nc latitude = 69.5, 68.5, 67.5, 66.5, 65.5, 64.5, 63.5, 62.5, 61.5, 60.5, 59.5 longitude = 20.5, 21.5, 22.5, 23.5, 24.5, 25.5, 26.5, 27.5, 28.5, 29.5 time = 4471, 4476, 4481, 4486, 4491, 4496, 4501, 4506, 4511, 4516, 4521 date = 20050101, 20050106, 20050111, 20050116, 20050121, 20050126, 20050131 depth = 15 mask = 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, u = nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan v = nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, na uf = nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan vf = nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan, nan Global Attributes Year Definition: The year that the data represents Description Definition: What the data product is of Version Definition: Version of software that created the product Source Definition: Creators of the product Contact Definition: The contacts at ESR Company Definition: Company where the product was made Reference Definition: Reference to find out more details on the product that are not covered in this document. Date Definition: Date that the data set was created Sample Global Attributes These global attributes come from world_oscar_vel_5d2005.nc. :year = "2005" ; :description = "Sea Surface Velocity" ; :version = 2006.f ; :source = "Gary Lagerloef, ESR (lager@esr.org) and Fabrice Bonjean (bonjean@esr.org)" :contact = "Fabrice Bonjean (bonjean@esr.org) or John T. Gunn (gunn@esr.org)" ; :company = "Earth & Space Research, Seattle, WA" ; :reference1 = "Bonjean F. and G.S.E. Lagerloef, 2002 ,\"Diagnostic model and analysis of the surface currents in the tropical Pacific ocean\", J. Phys. Oceanogr., 32, 2,938-2,954" ; :date = "27-Apr-2007" ;
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Available Read Software Read software are available from PO.DAAC's anonymous ftp site for IDL and Matlab at http://podaac.jpl.nasa.gov/pub/ocean_currents/OSCAR/software. Unidata also has software available to read NetCDF files with the ncdump command at http://www.unidata.ucar.edu/downloads/netcdf/index.jsp
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Processing Methods The near-surface velocity is directly derived from sea surface height (ssh), wind stress ( ) and sea surface temperature (sst). It is the sum of the geostrophic, Ekman-Stommel and thermal wind currents, where

f - Coriolis parameter - total velocity g - gravity acceleration - sea surface displacement h - depth - buoyancy force - wind stress A - eddy viscosity U' - vertical shear. The total velocity is the vertical average over a surface layer thickness of 30 m [2]. The data used to calculate the velocity are: - TOPEX/POSEIDON, Jason-1, ERS1-2, GFO and ENVISAT ssh anomalies, merged into a gridded product (AVISO). An absolute ssh product is obtained by adding to the anomalies a mean dynamic topography which is a combined product recovering 7 years(1993-1999) based on GRACE mission, altimetry and in situ data (hydrologic and drifters data)[2], [3] - wind velocity from SSM/I [4] October 1992 to July 1999 and QScat [5] August 1999 to present. - sst from in situ and AVHRR measurements [6]. The altimeter data are initially gridded on a "Mercator" grid (variable, from 1/3°x1/3° at the equator to higher latitude resolution poleward), and horizontal gradients are calculated onto a 1°x1° grid with a temporal sampling of 5 days. Winds and sst are on the same temporal and 1°x1° grid as described above. For more detailed information on the processing methods refer to Bonjean and Lagerloef 2002 [1] and http://www.oscar.noaa.gov/methodology.html.
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Filtering Data The filter procedure is twofold: 1) spatial filter using a best linear unbiased estimate (BLUE) procedure with an exponential decorrelation function applied with 10°, 2° (lon, lat) decorrelation scales; these are appropriate decorrelation scales in the tropics; 2) temporal filter (Butterworth) with 40-day cut-off period. This filtered field is primarily used for describing large scale features and low frequency variability. For quantitative analyses users are encouraged to download the unfiltered dataset (direct version of the OSCAR calculations) and use their own filtering techniques.
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Caibration Information Validation information on this product can be found in Johnson et al. 2007 [7]. Note: since the Johnson et al. (2007) article was published, a new OSCAR dataset corresponding to the one described on this web page has been released with significant improvements, notably the implementation of the new AVISO merged altimetry product (2006) into the OSCAR processing. Updated validation results can be found at http://www.esr.org/~bonjean/oscar/global_validation/.
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Sources of Error This section only concerns the source datasets used to estimate surface velocity. High winds and rain can cause anomalous readings in SSM/I and QScat. Errors in the satellite altimetry product can come from residual orbit errors and long wavelength errors. All gridded products are also subject to formal mapping errors.
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Known Problems These near-surface currents are estimated through a simplified diagnostic model of the surface circulation. Notably, local acceleration and non-linearities are not represented. As shown in [7], the present velocity field is best used for description of large scale and low frequency variations of surface flow (T>=20 days, L>=5° longitude). ESR currently provides the scientific community with an unfiltered velocity field (no post-processing), on a 1° X 1° grid with a 5 day resolution. Smoothing may have been induced only by the processing of the source data, that is ssh [2], W [4,5] and sst [6]; also some smoothing inherent to the method itself was caused by calculation of spatial gradient (for geostrophic and thermal-wind currents only). When comparing OSCAR to the drifter data, smoothing can also just be implied by the interpolation from the 1° X 1°/5-day grid to the drifter locations and times. This sort of indirect smoothing will be much removed when the new high-resolution OSCAR product is released (2009). ESR encourages researchers to make comparisons between this velocity field and in-situ observations on meso to short scales. As this surface current estimation is a work in progress, we are interested in any result involving the present velocity field, and we remain available for any help and discussion.
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References 1. Bonjean, F., and G. S. E. Lagerloef, 2002. Diagnostic model and analysis of the surface currents in the tropical Pacific Ocean. J. Phys. Oceanogr., vol. 32, pg. 2938-2954. http://www.esr.org/documents/bonjean/bl2002/bl2002.pdf 2. SALP-MU-P-EA-21065-CLS, Edition 1.9, June 2008 Ssalto/Duacs User Handbook : (M)SLA and (M)ADT Near-Real Time and Delayed Time Products (http://www.aviso.oceanobs.com/fileadmin/documents/data/tools/hdbk_duacs.pdf ) . 3. Rio, M. H. and F. Hernandez, 2004. A mean dynamic topography computed over the world ocean from altimetry, in situ measurements, and a geoid model. J. Geophys. Res., 109, C12032, doi:10.1029/2003JC002226. 4. Atlas, R., R. Hoffman, S. Bloom, J. Jusem, and J. Ardizzone, 1996. A multi- year global surface wind velocity dataset using SSM/I wind observations. Bull. Amer. Meteor. Soc., vol. 77, pg. 869-882. 5. Pegion, P. J., M. A. Bourassa, D. M. Legler, and J. J. O'Brien, 2000: Objectively-derived daily "winds" from satellite scatterometer data. Mon. Wea. Rev., 128, 3150-3168. http://www.coaps.fsu.edu/scatterometry/Qscat/gcv_glob_L2B_1x1.html ) 6. Reynolds, R. W. and T. M. Smith, 1994. Improved global sea surface temperature analyses using optimum interpolation. J. Clim., vol. 20, pg. 1576-1582. http://ingrid.ldeo.columbia.edu/SOURCES/.IGOSS/ ) 7. Johnson, E. S., F. Bonjean, G. S. E. Lagerloef, and J. T. Gunn, 2007. Validation and error analysis of OSCAR sea surface currents. J. Atm. Ocean. Tech., vol. 24, pg. 688-701.
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Acronyms AVHRR - Advance Very High Resolution Radiometer AVISO - Archiving, Validation and Interpretation of Satellite Oceanographic data CHAMP - Challenging Mini-Satellite Payload for Geophysical Research and Application Dh - Dynamic height ESR - Earth and Space Research GDR - Geophysical Data Record GRACE - Gravity Recovery and Climate Experiment JPL - Jet Propulsion Laboratory NetCDF - Network Common Data Form OSCAR - Ocean Surface Current Analysis PO.DAAC - Physical Oceanography Distributed Active Archive Center QScat - QuikSCAT- Quick Scatterometer SSH - Sea Surface Height SSM/I - Special Sensor Microwave Imager SST - Sea Surface Temperature WOA - World Ocean Atlas
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Contact Information Questions or comments about this data product should be directed via email to the Physical Oceanography DAAC: podaac@podaac.jpl.nasa.gov.
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Document Information Jessica Hausman, PO.DAAC Fabrice Bonjean, ESR August 18, 2008
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