Daily Archives: September 14, 2017

Rip currents @Guadalupe

Graduate students Alex and Spencer have been capturing drone footage of rip currents along the beach by our radar site and comparing optical versus radar observations. Above left is a snapshot of one. Rip neck and rip head are evident by their entrained sediment. Rip head is particularly interesting  with its intermediate scale wave breaking and smaller scale instabilities along the outer edge. Sped up version of the movie (8x) can be viewed here. Above right, the GEarth radar overlay on the right pins the drone location just off the beach from our radar site. The green dots note the southeastern corner of the OSU Oceano array. The scale of these rips may seem a bit small (200-300 m cross-shore) compared to the cross-shelf distances you all have been covering with your ship ops, but don’t count them out yet!

You can check out a radar movie of the rip here, or on YouTube here. In the movie you can see at least a half dozen rip currents, even cooler, at 1541PDT on Sept. 11 you can clearly see a set of solitons crashing into the surf zone and into the rip currents. Afterwords there are interesting cross-shore dark features that we suspect are slicks of some sort.

  

Fighter pilots at the beach.

Sally Ride soliton measurements

While this experiment highlights the integration of measurements made across our small armada, the scientists on each ship also integrate data from several instrument platforms. On Sally Ride, we are chasing solitons like those seen in the aerial photos from a previous post.

We are using a profiling instrument (affectionately known as the Very Merry Profiler) cast over the stern of the ship

as well as a daisy chain of thermistors hanging from the bow.

In the figure, the bottom panel displays temperature in the top 20-m as measured by the bow chain. Black dots show the turbulent dissipation rate, indicating the strength of mixing, overlaid on the temperature measurements. The areas where we see large dissipation correspond to areas of large temperature gradients, indicating high mixing where solitons depress the thermocline dramatically changing the temperature.

 

 

Tidal variability of headland eddy shedding

 

The Sally Ride and Oceanus are both wrapping up long (36-48 hour) repeat sections down here at Pt. Sal before heading back to Oceano. Following up on the Sounder post below, we have also been intrigued by the strongly time-dependent nature of the wake eddies being shed south of Pt. Sal.

The figure above shows 15 passes around our “L” shaped Sally Ride pattern, with local time given at the top. Color is SST and arrows are depth-averaged currents.

During and following periods of strongly southward (tidal+mean) flow, e.g. the second and third panels, an energetic recirculation eddy results on the onshore end, consistent with small boat measurements reported below. Related (somehow!) to these recirculations, there is a frequently occurring clear front (red to blue colors).  At times it seems clear that the sharpness of this front is related to convergences between separated southward flow and recirculation eddies, for example in panel 13. At other times, the structure and location of both the density fronts and the current patterns seem a tad mysterious.

We can only imagine that once we collectively integrate measurements from all seven ships! (3 big, 3 small) plus drifters!  into this pattern, the nature of the flow will either be even more mind-blowingily confusing, or magically crystal clear. So much fun analysis to look forward to.

 

 

 

Oceanographers use this one weird trick to see internal waves!

Click-bait headline aside, we did see some fairly astounding internal wave characteristics in combing through the airborne IR data today. Specifically, Melissa and our pilot Dave flew a final transect along a bore front about 5 miles offshore Pt Sal and saw this:

The view shows an IR snapshot of a portion of an internal wave packet, looking perpendicular to the wave (offshore).  Relative brightness temperature (deg C) is indicated by the colorbar (warmer features are brighter).  The horizontal scale of the image is about 500m along the bottom edge and 1000m along the top edge, so the billow (instability?) structures with cool centers are about 200-300m in scale!  Here’s a link to a movie (37 MB) that shows about a minute of realtime flyby of this bore (?) front.  Lot’s to think about!

APL flight highlights

The APL plane flew for about 5 hours today in the afternoon and early evening. Lots to see! See below for a few highlight photos. Also see Chris’s post from earlier today for even more good stuff!

Sally Ride awaiting the mother of all internal wave packets, and another packet in the distance.

 

Signatures of long internal wave

 

Sounder and Kalipi ready for alongshore surveys starting at Mussel Pt!

 

Internal wave and bloom material?

 

scripps oceanography uc san diego