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OSCAR Surface Current Velocity

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Data access via file system: /data/icdc/ocean/oscar_surface_current_velocity/

 

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Description

The subsequent data set contains global near-surface current estimates, derived using quasi-linear and steady flow momentum equations. The horizontal velocity (zonal: u- and meridional: v-components) is a vertical average over a surface layer thickness of 30 m and is directly estimated from sea surface height, near surface vector wind and sea surface temperature data as described in brief in the Users Guide.

The data used were collected from various satellites and in-situ instruments. These comprise

  • gridded SSH anomalies (AVISO)
  • an absolute SSH product based on 7 years of GRACE mission data, altimetry and hydrologic and drifter data
  • Near surface wind velocity (SSM/I until 07/1999, QuikSCAT (and presumably ASCAT) from 08/1999 until present
  • Reynolds SST at weekly temporal resolution

The model formulation used combines geostrophic, Ekman and Stommel shear dynamics, and a complementary term from the surface buoyancy gradient [Bonjean and Lagerloef, 2002]. More details about the method are also given on the OSCAR webpage.

For both grid resolutions u- and v-components are given. For the 1 degree product additionally filtered velocities are included; see the OSCAR webpage for more information. For the 1/3 degree product additionally so-called "Maximum mask velocities" are included; these are basically the geostrophic component at every grid point plus any concurrent Ekman and buoyancy components. Inclusion of this latter product is driven by the different coverage of the gridded input data sets.

Last update of this data set at ICDC: October 25, 2019.

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Parameters

Name Unit 1 degree 1/3 degree
Zonal surface velocity (u-component) m/s X X
Meridional surface velocity (v-component) m/s X X
Filtered u-component m/s X  
Filtered v-component m/s X  
Maximum mask zonal surface velocity (u-component) m/s X
Maximum mask meridional surface velocity (v-component) m/s X

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Coverage, temporal and spatial resolution

Period and temporal resolution: 

  • 1992-10-05 to 2019-10-10
  • about 5 days (or 1/72 year)

Coverage and spatial resolution: 

  • Global
  • Spatial resolution: 1 degree x 1 degree and 1/3 degree x 1/3 degree; cartesian Grid
  • Geographic latitude: 1 degree data: -69.5°N to 69.5°N; 1/3 degree data: -80.0°N to 80.0°N
  • Geographic longitude: 1degree data: 20.5°E to 379.5°E (data are shifted East by 20°); 1/3 degree data: 20.0°E to 420.0°E (data are shifted East by 20° and are repeated for another 40°)
  • Dimension: 1 degree data: 360 columns x 140 rows; 1/3 degree data: 1201 columns x 481 rows
  • Altitude: 0.0 m

Format:

  • NetCDF

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Data quality

The data set does not contain estimates of the uncertainty.

The authors of the data set note that the gridded data sets used to generate the OSCAR products (see description) create errors in the product due to, e.g. clouds, high wind velocity and rain in case of the near surface wind velocity and SST, residual orbit errors and long wavelength errors in case of the SSH products, and formal mapping and sampling errors.

The OSCAR product accuracy and representativity is limited by the usage of a simplified diagnostic model of the surface circulation. Effects on the near surface currents due to local acceleration and non-linearities are not taken into account. This limits the applicability of the OSCAR product to rather larger spatial scales (> 5° in longitude) and low frequency variations (> 20 days).

Information about validation and inter-comparison studies is given at the respective webpage:  oscarweb.esr.org and in various publications (see references).

The authors of the data set note that the Johnson et al. (2007) paper is a bit outdated because in the current version (2009f for the 1/3 degree data set) an updated version of the AVISO SSH anomaly is used which reduced uncertainties.

For differences in the methodology used between the 1 degree and the 1/3 degree product we refer to the Users Guide.

We note that both the land mask and the sea ice mask change kind of irregularly in the two products and a not consistently used - which might prevent usage of the product too close to the coast and in certain basins.

1 degree data set:

  • Until Oct 26 2000: Relatively coarse landmask, some basins missing; seasonally static sea ice mask
  • Since Nov 1 2000: Finer resolved landmask, particularly in Caribbean and Indonesia; seasonally static sea ice mask
  • Since Sep 6 2014: Further refinement of landmask, now Mediterranean and Red Sea included; seasonally static sea ice mask

1/3 degree data set:

  • Until Oct 1 1999: Relatively coarse landmask, some basins missing, seasonally static sea ice mask; dat cut off at 60 degrees N or S
  • Since Oct. 6 1999: Much finer resolved landmask, but once in a while it switches back to coarser resolution; seasonal sea ice mask
  • Since April 7 2014: Further refinement of landmask, now Baltic Sea, Mediterranean and Red Sea included; seasonal sea ice mask
  • Since July 2 2015: Further small refinement of landmask, but this one is not stable over time; seasonal sea ice mask

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Contact

Kathleen Dohan
Earth & Space Research (ESR), Seattle, WA
email: kdohan (at) esr.org

John T. Gunn
Earth & Space Research (ESR), Seattle, WA
email: gunn (at) esr.org

Jessica Hausman
Physical Oceanography DAAC
email: podaac (at) podaac.jpl.nasa.gov

Stefan Kern
ICDC / CEN / University of Hamburg
email: stefan.kern (at) uni-hamburg.de

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Data citation

When using the data please cite:

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, 2938-2954.

and for the 1/3 degree data:

ESR. 2009. OSCAR third degree resolution ocean surface currents. Ver. 1. PO.DAAC, CA, USA. Dataset accessed [2019-10-15] at http://dx.doi.org/10.5067/OSCAR-03D01.

while for the 1 degree data use:

ESR. 2007. OSCAR 1 degree ocean surface currents. Ver. 1. PO.DAAC, CA, USA. Dataset accessed [2019-10-15] at http://dx.doi.org/10.5067/OSCAR-10D01.

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