Headland Eddy Formation and Dissipation
at Three Tree Point, Washington
Funded by the National Science Foundation (PI's: P. MacCready, G. Pawlak)
The objective of the study is to observe 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. The selected site for the observations is Three Tree Point in Puget Sound, WA. The first set of observations took place from July 31 through Aug. 7, 2001 using the UW/APL R/V Miller.
Our primary goals for the first 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 include 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) which is a goal for a future cruise (2002). In addition, the cruise is intended to shed light on the longevity of eddies as well as on their baroclinic structure.
Tidal currents and cruise schedule
pdf format
CTD and ADCP Scheme
Green dots are CTD stations;
White lines outline ADCP spatial survey sites;
white pentagram shows location of bottom mounted ADCP
pdf format
(click on image for a .avi sequence - 1.5 MB)
Drifter observations from the first set of observations at Three Tree Point show that the flow is very repeatable over distinct tidal cycles. The plot below shows 5 days of drifter tracks (with 3 hour tails) at the end of the flood tide. The structure of the separation eddy is clearly visible in the lee (south) of the headland. The sequence (click on image) shows the coherence of the structure over the distinct cycles and follows it's evolution over 24 hours. Also apparent is a stagnation point south of the point, below which flow continues to move southward through the ebb tide.
(click on image for a .avi sequence -1.5 MB)
Bowtie ADCP surveys were carried out over a complete flood tide over two circuits during the August cruise. The figure above shows velocity vectors along with the corresponding vorticity field for five separate depths near the headland. Velocities represent a least-squares fit of the M2 and M4 tidal frequencies to the ADCP data. Vorticity is calculated at each point within the circuits using data points within a 600 m diameter annulus with a width of 50 m (illustrated in the upper right of each panel).
Personnel for the Aug. 2001 Three Tree Cruise:
G. Pawlak (UH Ocean Engineering)
P. MacCready (UW Oceanography)
K. Edwards (UW Oceanography)
R. McCabe (UW Oceanography)
W. Martin (UW Oceanography)
E. Boget (UW APL)
Return to Three Tree Point page
for more info contact:
Geno Pawlak or Parker MacCready
last modified: 02/26/2004