Headland Eddy Formation and Dissipation 
at Three Tree Point, Washington

Funded by the National Science Foundation (PI's: P. MacCready, G. Pawlak)

Coastal stirring, caused by the interaction of tidal currents with complex bathymetry, is important to the dispersal of plankton, larvae, and pollutants. It may also exert a large form drag on alongshore currents.  Our recent numerical and theoretical work on stratified flow past a headland on a slope allows us to predict whether a given headland will be more likely to produce internal waves or horizontal eddies. In either case the pressure drag on the headland is substantial. Our laboratory studies (Pawlak and MacCready, 2002) of homogenous tidal flow past a headland indicate that if headland eddies persist longer than a tidal cycle the resulting eddy interactions may give rise to a residual flow from offshore toward the headland, opposite to the result for tidal headland flow with high dissipation.

 

This project, funded by NSF, uses field observations and numerical modeling to examine the formation and dissipation of a headland eddy in deep water.  Included in this is examination of the vertical structure of vorticity within an eddy and of the baroclinic response of the flow around the sloping headland.  Field observations involve the use of subsurface drogued drifters to follow individual eddies and examine the horizontal dispersion pattern, along with boat-mounted ADCP and CTD surveys which measure the 3-D flow structure over the tidal period.  

Cruises

Publications

Cruises

The selected site for the observations is Three Tree Point in Puget Sound, WA.  Four sets of observations have been carried out at Three Tree Point:

March, 2001: In collaboration with the Ocean Mixing Group at OSU:

August, 2001: R/V Miller (Cruise Page)

Our primary goals for the this cruise were to verify that flow structure (i.e., a headland eddy) exists and to characterize the velocity and density field within that structure.   Secondary goals included diagnosing the variability of the flow field and determining the viability of the use of phase-averaged spatial sampling (i.e. Geyer and Signell, 1990).  In addition, the cruise intended to shed light on the longevity of eddies as well as on their baroclinic structure. 

June, 2002:  R/V Miller and R/V Barnes (Cruise Page)

Sept. 2002:  R/V Barnes (Cruise page)

Publications:

K. Edwards, P. MacCready, J. Moum, G. Pawlak, J. Klymak and A. Perlin, " Form Drag and Mixing due to Tidal Flow past a Sharp Point", Journal of Physical Oceanography,  in press, 2004 (download pdf, 1.9MB)

Pawlak, G., P. MacCready, K. Edwards and R. McCabe, 2003, "Observations on the Evolution of Tidal Vorticity at a Stratified Deepwater Headland", Geophysical Research Letters, Vol. 30, No. 24, p. 2234. (download pdf, 1.0MB)

Conference Proceedings:

G. Pawlak, P. MacCready and R. McCabe, “Evolution of vortical flow structure in ocean boundary processes”, ‘Aha Huliko’a Winter Workshop: Near Boundary Processes and Their Parameterization, Honolulu, Hawaii, Jan. 2003 (download proceedings pdf, 0.7 MB )

P. MacCready, G. Pawlak, R. McCabe and K. Edwards, "Observations and Modeling of Form Drag on Rough Coastal Topography”, Aha Huliko’a Winter Workshop: Near Boundary Processes and Their Parameterization, Honolulu, Hawaii, Jan. 2003 (download proceedings pdf, 1.1 MB)

G. Pawlak, P. MacCready, K. Edwards,  and R. McCabe, "Evolution of Tidal Vorticity in Stratified Coastal Flow", Proceedings of OMAE’02 21st International Conference on Offshore Mechanics and Arctic Engineering, Oslo, Norway, 2002, (download proceedings pdf, 1.3MB)

MacCready, P., and G. Pawlak 2001: Tidal Eddy Generation at Three Tree Point: Theory and Numerical Modeling. Proceedings of the Puget Sound Research Conference 2001, 6 pp.

Conference Presentations:

G. Pawlak, P. MacCready, R. McCabe, "Evolution and Dissipation of Tidal Vorticity at Stratified Deepwater Headland”, American Geophysical Union, Ocean Sciences Meeting, Portland, Oregon,  Jan. 2004

P. MacCready, G. Pawlak, R. McCabe and K. Edwards, "Form Drag on Flow Over Rough Coastal Topography: Problems for Parameterization”, American Geophysical Union, Ocean Sciences Meeting, Portland, Oregon,  Jan. 2004

P. MacCready, R. McCabe, G. Pawlak, and K. Edwards, "Form Drag on Flow Across a Sloping Ridge”, American Geophysical Union, Fall Meeting, San Francisco, California,  Dec. 2002

R. McCabe, P. MacCready, and G. Pawlak, "Estimates of Form Drag from Drifter Tracks, American Geophysical Union, Fall Meeting, San Francisco, California, Dec. 2002

G. Pawlak, P. MacCready, K. Edwards,  and R. McCabe, "Evolution of Tidal Vorticity in Stratified Coastal Flow", American Geophysical Union, Ocean Sciences Meeting, Honolulu, Hawaii,  Feb.. 2002

K. Edwards, P. MacCready,  G. Pawlak,  J. Moum,  J. Klymak, A. Perlin, and R. Dewey, "Flow Features at a Sharp Coastal Point", American Geophysical Union, Ocean Sciences Meeting, Honolulu, Hawaii,   Feb. 200

Flow Dynamics at Three Tree Point

Numerical Modeling
(click on image for a larger version)

 

  Volume visualization of relative vorticity in a numerical simulation of stratified, tidal flow at Three Tree Point in the Main Basin of Puget Sound, WA.  Inside the red volume is water with relative vorticity greater than 3x10-4 s-1.  Inside the blue volume is water with relative vorticity less than -3x10-4 s-1.  This snapshot is taken at the end of flood tide, where the flood direction is toward the lower right.  The main feature is a large red blob of positive vorticity, which was created by flow separation from the Point during this flood tide.  The current reverses early at the Point, and this is evident in blue (negative) vorticity on the slope behind the big red blob.  At the right edge of the red blob is a small blue blob, which is the remnant of the vortex created during the preceding ebb tide.  It has been carried back to the south (lower right direction) by the flood tide.  Vorticity of this strength takes 11 hours to complete a full circle. 

    It is apparent that the vorticity signature of the eddy (especially the new red eddy) extends throughout the water column, and is fairly distorted by horizontal flows near the surface.  In addition the eddy from the previous ebb (the small blue blob) shows that the eddy vorticity only survives at this level near the surface, and is apparently destroyed in some way deeper down.

7/17/2001  Parker MacCready

for more info contact:

Geno Pawlak or Parker MacCready

last modified: 02/26/2004