Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI Time Varying OSCAR Surface Currents in the Garbage Patch Regions Kathleen Dohan Earth

Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI

Time Varying OSCAR Surface Currents in the Garbage Patch Regions

Kathleen Dohan

Earth and Space ResearchSeattle, WA

OSCAR surface currents


• 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

OSCAR Surface currents from satellite fields


• Ocean Surface Currents Analyses-Realtime processing system (OSCAR) is a satellite-derived 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).

• 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.

• Data is available at http://podaac.jpl.nasa.gov and http://www.oscar.noaa.gov.

• Areas of development:• time-dependent wind-driven dynamics• Turbulent mixing scheme• Vertical variation• Coastal altimetry

OSCAR Movie


Validation against drifting buoy velocities


• 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.

• Drifter data distributed by NOAA/AOML www.aoml.noaa.gov/phod/dac/gdp.html

OSCAR in the Pacific Vortex Region


• OSCAR currents in the Pacific• Snapshot in time• Relatively quiet region

OSCAR animation


Variability


• Region of low EKE

Speed


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Convergent Zones


• Simple calculation of w from du/dx and dv/dy

Convergent Zones Year Average


• Simple calculation of w from du/dx and dv/dy

Convergent Zones Year Average


• Transport into the sides of a box around the Pacific patch• Need concentrations of pollutants to connect to garbage patch

Lagrangian approach: Particles in OSCAR


• Simple advection of initial seeding of “drifters” by interpolated currents

Particles in OSCAR


• Final positions after running from 2000 to end of 2009• Stop when drifters approach coasts (NaN velocities)

Global Impact of Eddies on Inertial Oscillations of the Mixed Layer Planning Meeting Feb 22-24, 2011


Particles in OSCAR


• Drifter paths, color coded by initial position

Particles in OSCAR


• Westward and eastward tracks

Compare with AOML drifters


• Initial position of subset of total drifter array



• Final position of subset of total drifter array



• Drifter paths (some)



• Drifter “end” points are related to concentration of initial deployments• Area binned final locations divided by initial area binning



• Amount of time drifter tracks are in bins

OSCAR


• Same plot for OSCAR drifters

Vertical Variation


• Vertical variation is implicit in the OSCAR analytical model. OSCAR currents are averaged over the top 30m of the model.

• The wind-driven component in OSCAR uses an eddy viscosity formulation (at the moment), dependent on the wind, with

• 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.

• 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.

Vertical Variation


• 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.

Vertical Variation


• 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• Holte, J., J. Gilson, L. Talley and D. Roemmich, 2010: Argo Mixed Layers, Scripps Institution of Oceanography/UCSD.

Linearly decaying currents to depth of mixed layer


• Lose the convergent zones (albeit unrealistic)

Vertical Variation


• 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

Questions


• 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?

• How do we quantifiably describe the convergent/divergent zones?

• Need concentrations of particles to connect with transport• Turbulent small-scale processes• Vertical processes• Coastal processes

• Combine the Lagrangian with Eulerian• Drifters give locations of origin• Transform concentration along coasts into concentrations within

convergent regions



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Documents

Hydrodynamics of Marine Debris” Sunday, March 20, 2011, Honolulu, HI Time Varying OSCAR Surface Currents in the Garbage Patch Regions Kathleen Dohan Earth