A tale of two drifters

We have recovered all of our (SIO, Feddersen) instruments, and the fun of sorting out and QCing the data has begun, however, I thought I’d share a sweet tale of loss and love. We were fortunate to only leave 2 instruments at Pt Sal, drifter 23 and 24, although I do believe their fate may have always been intertwined. Their tale begins during experiment preparation during final drifter assembly, when drifters were being labeled with a sharpie: 1… 27 – then 23 and 24 were shipped to Pt Sal, right next to each other in a large box. During IOP-1, both 23 and 24 rose to the occasion and provided excellent data from which internal waves, fronts, and surface waves were observed. However, on the final day of IOP-1, Sept. 17, drifter 23 (red in the figure) decided he/she wasn’t long for this world and sacrificed itself (for science, I presume) on the rocks, either that or that the point (0,0) in Pt Sal (x,y) is some sort of worm hole? (actually, we checked some mooring lines, which were kelp free! delaying recovery of 23). Regardless, we had finally lost a drifter, much to everyones dismay (delight?).

Now drifter 24 was sad, however, he/she performed admirably throughout IOP-2 collecting much data. On the final day of IOP-2 (Oct. 14), about 1 month after losing drifter 23, 24 decided enough was enough and attempted to reunite with 23. Upon release at 8:05, 24 went directly shoreward (green in figure), hitting the surfzone at 12:39. Realizing that 23 was 1 km to the south, 24 stumbled down the beach for 4 hours, getting about 200 m from 23’s final resting place. At this point, after stumbling down the beach for 4 hours (a difficult task for an old drifter from the 80s), 24 needed a rest. After a long 24 hour nap, drifter 24 made tje final push south toward 23, coming to rest about 100 m from his partner in science. Although we mourn the loss of these two drifters, we are happy they are once again together enjoying their retirement on a beautiful coastline. (in the figure, contours are at 2.5 m intervals, and the 20 m contour is thick)


Drifter Test Day

Sep 9, 2017

Today we (SIO and APL crews) tested out releasing and recovering drifters. We released 3 clusters of 9 drifters off the Pismo Beach Pier (see Figs below). All drifters were released by approximately 11:00 and drifted for about 3 hrs. The drifters displaced approximately 3 km in these 3 hours, so the surface current magnitude is |u|~25 cm/s. After deployment, ADCP transects were performed (and numerous whales were spotted). It was a pleasant day and everything went as anticipated. We look forward to the NPS crew joining the fun.


Drifter test day, zoomed out angle. Initial release locations are black dots.

Drifter test day tracks, zoomed in. Initial release locations are black dots.


The R/V Sounder made 4 transects running east-west through the drifter clusters, each time observing a similar velocity structure.  By coincidence, the drifter release points bracketed a current constrained laterally within the array and vertically to 10 m depth.  Very cool feature.

Four transects through the drifter array.

The Nortek Signature profiles collected from the SWIFT drifters are consistent with the center of this velocity signal.

SWIFT relative velocity profiles.

2015 Surface Vorticity Movie

Movie of the surface (top ROMs bin) vertical vorticity for the 2015 simulation. The top panels are for the simulations without waves and the bottom panel are the simulations with waves. Leftmost panel is temperature anomaly on the 200m grid w/ 66m grid (black box) overlaid. 2nd column is the 200m vorticity with 66m overlaid. 3rd column is 66m vorticity with 22m grid (black box) vorticity overlaid. Rightmost panel is the tide (green), Hsig (black), mean enstrophy in the 66m grid with waves (red) and without waves (blue). Notice the large effect of wave coupling on the surface vorticity.

Full 2009 Model Simulation FTLE Movie

In planning for the drifter releases, Matt Spydell has put together this movie for the entire 2009 simulation: Aug 15 – Nov 15 using the 200m and 66 m (north and south) grids. The movie is long, you can speed it up 2x using the tools button.  The top panels are finite-time forward Lyupanov exponents for times at 2, 4, 6, and 10 hour trajectories with bathymetry contoured at 25 m intervals until 150 m depth and then 50 m intervals.  The bottom panels are vorticity, divergence, Okubo-Weiss parameter, and perturbation temperature all at the surface (top most ROMs bin).   In all bottom 4 panels the temperature deviation is contoured.   The fronts are really clearly visible.   The thick arrow indicates surface current off Pt Sal and a thin arrow indicates wind velocity (note, they are scaled differently).   We are using this information to plan our drifter releases.

scripps oceanography uc san diego