Time Varying OSCAR Surface Currents in the
Garbage Patch Regions
Kathleen Dohan
Earth and Space Research
Seattle, WA
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
OSCAR surface currents
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OSCAR surface currents (Ocean Surface Current Analyses- Realtime) are global
ocean surface velocities calculated from satellite-sensed SSH, wind, and SST (SSS).
The dataset is produced on a 5-day timebase, on a 1/3 degree regular grid.
Here:
• OSCAR currents in the convergent zones
• How best to describe the convergent zones?
• Eulerian vs Lagrangian views
• Compare with drifters
• Effects of vertical variation
• Discussion
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
OSCAR Surface currents from satellite fields
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Ocean Surface Currents Analyses-Realtime processing system (OSCAR) is a satellitederived surface current database provided in near-real time based on a
combination of quasi-steady geostrophic and locally wind-driven dynamics
(Bonjean and Lagerloef, 2002).
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geostrophic term is computed from the gradient of ocean surface topography
fields (AVISO/CLS)
wind-driven velocity components are computed from an Ekman/Stommel
formulation with variable eddy viscosity using QuikSCAT vector winds
(FSU/COAPS) and NCEP winds
thermal wind adjustment using Reynolds OI SST data.
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Data is available at http://podaac.jpl.nasa.gov and http://www.oscar.noaa.gov.
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Areas of development:
• time-dependent wind-driven dynamics
• Turbulent mixing scheme
• Vertical variation
• Coastal altimetry
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
OSCAR Movie
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Validation against drifting buoy velocities
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OSCAR surface velocities are
interpolated onto drifter
locations (which have been
averaged over 1 day). Zonal and
meridional currents vs drifter
velocities are plotted on the
scatter plot.
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Drifter data distributed by
NOAA/AOML
www.aoml.noaa.gov/phod/dac/g
dp.html
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
OSCAR in the Pacific Vortex Region
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OSCAR currents in the Pacific
• Snapshot in time
• Relatively quiet region
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
OSCAR animation
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Variability
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Region of low EKE
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Speed
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Low mean speed
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Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Convergent Zones
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Simple calculation of w from du/dx and dv/dy
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Convergent Zones Year Average
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Simple calculation of w from du/dx and dv/dy
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Convergent Zones Year Average
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Transport into the sides of a box around the Pacific patch
Need concentrations of pollutants to connect to garbage patch
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Lagrangian approach: Particles in OSCAR
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Simple advection of initial seeding of “drifters” by interpolated currents
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Particles in OSCAR
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Final positions after running from 2000 to end of 2009
Stop when drifters approach coasts (NaN velocities)
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Global Impact of Eddies on Inertial Oscillations of the Mixed Layer Planning Meeting Feb 22-24, 2011
Global Impact of Eddies on Inertial Oscillations of the Mixed Layer Planning Meeting Feb 22-24, 2011
Particles in OSCAR
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Drifter paths, color coded by initial position
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Particles in OSCAR
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Westward and eastward tracks
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Compare with AOML drifters
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Initial position of subset of total drifter array
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Compare with AOML drifters
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Final position of subset of total drifter array
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Compare with AOML drifters
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Drifter paths (some)
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Compare with AOML drifters
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Drifter “end” points are related to concentration of initial deployments
Area binned final locations divided by initial area binning
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Compare with AOML drifters
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Amount of time drifter tracks are in bins
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
OSCAR
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Same plot for OSCAR drifters
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Vertical Variation
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Vertical variation is implicit in the OSCAR analytical model. OSCAR currents are
averaged over the top 30m of the model.
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The wind-driven component in OSCAR uses an eddy viscosity formulation (at the
moment), dependent on the wind, with
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Optimal choice for a in OSCAR blends from 8 x 10-5 m2s-1, b = 2.2 at the equator as in Santiago-Mandujano & Firing (JPO
1990), to 2.85 x 10-4 m2s-1, b = 2 for the global value.
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Following Cronin and Kessler (JPO 2009), we’ve used a vertically varying eddy
viscosity which decays with depth (exponential profile), so that stress is zero at
depth H.
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Vertical Variation
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Green and cyan (OSCAR) show the vertical variation implicit in OSCAR. Blue
compares with using a vertically varying eddy viscosity.
Timeseries of averaged 30m currents are compared with TAO mooring data. Less
significant for surface measurements, but significant once you are considering
transport at varying levels.
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Vertical Variation
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Extreme test case of linearly decaying u with depth to base of mixed layer
• Using Holte mixed layer climatology from Argo http://mixedlayer.ucsd.edu
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Holte, J., J. Gilson, L. Talley and D. Roemmich, 2010: Argo Mixed Layers, Scripps Institution of Oceanography/UCSD.
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Linearly decaying currents to depth of mixed layer
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Lose the convergent zones (albeit unrealistic)
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Vertical Variation
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Second test case of surface wind-driven u scaled by depth of mixed layer
• U wind = U wind * 30m/MLD
Idea of momentum being distributed over well-mixed layer
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Questions
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Are coarse velocity fields enough to measure Lagrangian pathways?
• Coasts: we aren’t getting the offshore transport correct (I think)
How reliant are any calculations of transport on the vertical profile?
How deep do particles reside?
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How do we quantifiably describe the convergent/divergent zones?
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Need concentrations of particles to connect with transport
Turbulent small-scale processes
Vertical processes
Coastal processes
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Combine the Lagrangian with Eulerian
• Drifters give locations of origin
• Transform concentration along coasts into concentrations within
convergent regions
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
Global Impact of Eddies on Inertial Oscillations of the Mixed Layer Planning Meeting Feb 22-24, 2011
Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI
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Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI