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Quantitative Description of Particle Dispersal over Irregular Coastlines

Quantitative Description of Particle Dispersal over Irregular Coastlines. Tim Chaffey, Satoshi Mitarai, Dave Siegel. BACKGROUND. Topographic eddies may be important in determining habitat connectivity Eddies can retain larvae for time scales comparable with their PLD

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Quantitative Description of Particle Dispersal over Irregular Coastlines

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  1. Quantitative Description of Particle Dispersal over Irregular Coastlines Tim Chaffey, Satoshi Mitarai, Dave Siegel

  2. BACKGROUND • Topographic eddies may be important in determining habitat connectivity • Eddies can retain larvae for time scales comparable with their PLD • High local recruitment is observed in island wake eddies (e.g., Swearer et al, 1999) • But, such clear pattern will not be observed in coastal eddies where currents are less persistent in direction (Graham & Largier, 1997) • Filament formation may be important in offshore transport of larvae (Haidvogel, 1991) • Offshore filaments present an obstacle to nearshore settlement in PLD. • Few notable studies (Largier, 2003) • Geostrophic size & flow time scale considered

  3. GOAL OF THIS STUDY • Use coastal 3-D physical model to investigate the wind driven circulation around idealized irregularities in the coastline. • Estimate the role of coastal headland eddies on larval dispersal usingidealized ROMS simulations • Do headlands create a consistent connectivity or a stochastic connectivity? • Are the spatial scales of settlement similar to the straight coastline case (50 km)? • If there is a consistent connectivity, does it remain constant under different wind regimes? • Is there a critical headland amplitude/width? • How important is headland spacing? • Describe the physics of filament formation and eddy recirculation around headlands? • Can we develop theoretical relationships between headland size, wind forcing, bathymetry and release-settlement relationship? • If we can develop theory, we can use this theory to predict release-settlement relationships over realistic coastlines given wind, headland size, and bathymetry.

  4. How this all comes together 1 - Collect CalCOFI data (SSH, temperature), bathymetry data, wind data from buoys. 2 - Initialized three-dimensional coastal circulation model (ROMS) with above data. 3 - Use 3D flow, 3D temperature, and SSH from model to compute flow statistics offline (Matlab). 4 - Use 3D flow from model to track particles offline (Matlab). We have ultimate flexibility in how we manipulate particles.

  5. Model Setup • Wind field from buoy measurements • Wind field sum of a mean and perturbation component • July wind field (upwelling period) • Pressure gradient quantified using dynamic height data from CalCOFI July ship track survey. • Pressure gradient rotated to along the coast line inside the 500 m isobath Domain: 288 km Cross-shore, 256 km Alongshore • Bathymetry - 0 m to 500 m offshore • Irregular coastline (headland) created using a Gaussian function

  6. HEADLAND DESIGN • Gaussian-shape headland in idealized simulations • Three parameters 3. Domain size (d) = distance between headlands 1. Amplitude (a) 2. Width (w) (twice the std of Gaussian function) a = 20 km w = 20 km d = 256 km

  7. Variable Model Parameters • Wind Field • Uniform - Wind field principal axis has uniform (N-S) direction over domain • Alongshore - Wind field principal axis rotated to follow land inside 500 m isobath • Pressure Gradient • Alongshore - Pressure gradient principal axis rotated to follow land inside 500 m isobath • Bathymetry • Compression - Distance from land to 500 m isobath compresses near headland • Alongshore- Distance from land to 500 m isobath same at all points.

  8. Wind Field Rotation

  9. Bathymetry - Compression of Isobaths at Headland

  10. Bathymetry - Non-Compression of Isobaths at Headland

  11. Wind Alongshore Uniform Compression BC - WA BC - WU Bathymetry Alongshore BA - WA BA - WU Notation for Cases Tested

  12. Particle Tracking • 90,000 particles randomly released 10 km from land and in the upper 10 m, but with a uniform release distribution over 90 days. • Particles locations updated every three hours. • Particles settle when within competency window and 10 km of land. • Particles are treated as packets that can settle multiple times. • Particle bouncing schemes vary • Reflecting - particles are reflected off boundary • Non-reflecting - particles entering land are returned to prior time step over water

  13. Wind Alongshore Uniform Compression BC - WA BC - WU Bathymetry Alongshore BA - WA BA - WU Model Parameters

  14. BA-WA(1) BA-WA(2) BA-WA(3) Mean Connectivity, Reflecting Condition (∆t = 3 hr)

  15. BA-WA(1) BA-WA(2) BA-WA(3) Arrival Diagram - Reflecting Condition

  16. BA-WA(1) BA-WA(2) BA-WA(3) Mean Settler Dispersal Kernel - Reflecting Condition

  17. BA-WA(1) BA-WA(2) BA-WA(3) Mean Connectivity, Non - Reflecting Condition (∆t = 3 hr)

  18. BA-WA(1) BA-WA(2) BA-WA(3) Arrival Diagram - Non -Reflecting Condition

  19. BA-WA(1) BA-WA(2) BA-WA(3) Mean Settler Dispersal Kernel - Non - Reflecting Condition

  20. Nearshore Flow Structure BA-WA

  21. Wind Alongshore Uniform Compression BC - WA BC - WU Bathymetry Alongshore BA - WA BA - WU Model Parameters

  22. BC - WU(1) BC - WU (2) Connectivity, Reflecting Condition (∆t = 3 hr)

  23. BC-WU (1) BC-WU (2) Arrival Diagram - Reflecting Condition

  24. BC - WU(1) BC - WU (2) Settler Dispersal Kernel - Reflecting Condition

  25. BC - WU(1) BC - WU (2) Connectivity, Non-Reflecting Condition (∆t = 3 hr)

  26. BC-WU (1) BC-WU (2) Arrival Diagram - Non -Reflecting Condition

  27. Settler Dispersal Kernel - Non-Reflecting Condition BC - WU(1) BC - WU (2)

  28. Nearshore Flow Structure BC-WU

  29. Summary • Dispersal for the alongshore wind cases are realistic while uniform wind cases are artificial. • For uniform wind cases, nearshore irregularities in the flow create artificial particle accumulation on the wayward side of the headland • A semi-annual pattern of dispersal is present in the alongshore wind case for both bathymetrys and both bouncing schemes. • For alongshore wind cases, particle dispersal over 1.5 years has a uniform distribution.

  30. Future Work • Experiment with alternative bouncing scheme to remove artificial accumulation of particles • Compare time integrated temperature fields to CALCOFI data to validate simulations • Investigate vertical velocity at fixed depth and mixed layer depth to theoretical predications to validate simulations • Collaborate with Bernardo Broitman (NCEAS) to assess the variation of isobathic distances with coastline • Bridge the connections between headland size, flow structure, and dispersal. Time scales will be important!

  31. Wind Alongshore Uniform Compression BC - WA BC - WU Bathymetry Alongshore BA - WA BA - WU Model Parameters

  32. BC-WA(1) BC-WA(2) BC-WA(3) Mean Connectivity, Reflecting Condition (∆t = 3 hr)

  33. BC-WA(1) BC-WA(2) BC-WA(3) Arrival Diagram - Reflecting Condition

  34. BC-WA(1) BC-WA(2) BC-WA(3) Mean Settler Dispersal Kernel - Reflecting Condition

  35. BC-WA(1) BC-WA(2) BC-WA(3) Mean Connectivity, Non-Reflecting Condition (∆t = 3 hr)

  36. BC-WA(1) BC-WA(2) BC-WA(3) Arrival Diagram - Non - Reflecting Condition

  37. BC-WA(1) BC-WA(2) BC-WA(3) Mean Settler Dispersal Kernel - Non - Reflecting Condition

  38. Nearshore Flow Structure

  39. Mean Dispersal Statistics • Mean alongshore distance - 35.9 km • Standard deviation of alongshore - 56.1 km • 11.5 % particles settle at least once • Mean PLD of settling particles 25.2 days (competency window = 20 - 40 days)

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