Saturday, April 6, 2013

Endeavor Update 13-04-06

From Chief Scientist Ray Schmitt,

Blog Entry: The mother of all tangles...

One of our missions on EN-522 was to rescue and re-anchor one of the NOAA "Prawler" moorings that had come adrift. The Prawler is a unique new type of mooring that has a surface float with a jacketed wire rope suspended beneath it upon which a CTD can travel.  The CTD is part of a package that uses a clever ratcheting mechanism to crawl up the wire as the wire heaves up and down with the surface waves.  Once it gets near the surface the ratchet is released and the CTD package drops and collects a profile of temperature and salinity, transmitting the data by inductive coupling through the wire to the surface pod, where it is radioed via satellite to the scientists back home.

The wire extends to 700 m depth, where it is connected to synthetic line.  There are two types of line used, the first after the wire is heavier than water, then that transitions to line that is lighter than water.  Finally, there are glass balls for floatation, an acoustic release, and anchor.  The arrangement of heavy and light synthetic causes the line to take on a sideways "S" shape in the water, since much more line is used than the depth of the water.  This makes for a sizable "watch circle" that the buoy will occupy at the ocean surface, but an "S-tether" mooring means much less stress on all the line than a taut mooring would have, and the line does not get wrapped around the anchor.

When we recovered the top portion of the mooring from its drift 100km to the south, it was still working and collecting data and we had no problem locating it and pulling in the wire.  We found no synthetic at all at the end of the wire, just an empty thimble in the shackle. It looked like the eye-splice had failed.

Several days later we sent the the acoustic signal for the release to let go of its hold on the anchor.  This allows the very buoyant hollow glass balls to bring the lower portion of the mooring to the surface.  Waiting for the balls to reach the surface is always a bit of a tense time in a mooring recovery, balls might have collapsed and there might not be sufficient buoyancy to bring everything to the surface.  We could tell by acoustic ranging on the release that it was coming up, but very slowly.  It ended up taking three hours, but the bright yellow hardhats were easy to spot a few hundred meters off the port quarter when they did surface.  We grabbed the balls and started reeling in the many kilometers of line.  The first section of buoyant line came in very nicely, but we quickly ran into trouble with the heavy line.  A lot of it was muddy, so it had been lying on the seafloor in a big tangle.  And what we brought up was a series of large tangled knots, as you can see in the photos.  It looked to be a hopeless tangle, and prospects did not look good for redeploying the mooring.

We were particularly interested in finding the bitter end of the line, to see how it had failed.  But because of the large tangle and the necessity to make several cuts juts to get it aboard, we never found the expected shorter transitional lines that the design called for.  WHOI mooring maestro Jeff Lord and crewman Paul Rousell worked diligently over the next few days to untangle the mess.  Jeff went over every inch of the line and re-spliced the larger pieces and re-spooled them on to the winch.  However, since we could not find the expected termination of the line, we did not know how much of the line we had left.  How a line could part in two places is a mystery, but the most likely scenario has the line parting somewhere mid-length, then a short length still attached to the wire but with no tension, was cyclically whipped around by the vertically heaving wire rope till the eye-splice parted.

We decided to go ahead with the redeployment; we had brought another anchor for the purpose.  But since we did not know how much line we had, we had to be flexible in the depth of water we chose for the anchor site.  As the mooring was deployed we had three different people count the revolutions of the drum and use the minimum and maximum radii to estimate the line length.  The estimates came to about 2600m.  It was deployed with 3400m of the heavy line so we were missing some 800m of line!  These moorings were designed with quite a bit of scope but clearly we had to choose the shallowest spot available.  This we could do by choosing where to release the anchor, as we trailed over 6 kilometers of line behind us.  Fortunately we had a very good SeaBeam survey of the bottom in the area that we had done last September from the Knorr.  Chris Duncombe-Rea searched for the minimum depth in the data set and found a nice rise in the seafloor that would still give a reasonable shape to the array of three moorings. We watched the echosounder anxiously as we steamed for the spot, and the bottom did come up smoothly.  In fact we got exactly the same depth at the same spot that the Knorr had measured 6 months previously!  We steamed another 300 meters to allow for anchor fall back and let it go.  We surveyed the anchor by triangulating on three acoustic distances from nearby sites and hoped for the best.

Fortunately, a few hours later we got word from the folks at NOAA that the Prawler was working nicely.  Its very gratifying when such a big mess can be straightened out by smart, hardworking people, the numbers fall into place nicely and a novel contribution to our ocean observing tools can be put right again!

--Ray Schmitt  4/06/13

Tuesday, April 2, 2013

Endeavor Update 13-04-02

From Chief Scientist Ray Schmitt,

Cruise Narrative, EN-522

EN-522 got underway at about 10:30 on Friday March 15 from the URI pier in Narragansett.  Seas were calm and winds were light at first, but picked up as the day wore on and we got further offshore.  Overnight we had winds in excess of 30 knots and the ship slowed and altered course to cope with the seas.  Conditions had improved by Saturday evening and we were able to make 10 knots again as we got offshore and entered the Gulf Stream.

The Thermosalinographs were running and a sampling schedule was set up for calibration bottle samples.  An additional TSG was set up and fed with a special pump and debubbling system utilizing a long hose dragging across the water.  The hose was held about 20' off the starboard side of the ship well forward of the CTD deployment area.  When the ship is going faster than a few knots the hose is sampling the upper 10 cm of the water. This system allowed us to sample any near-surface changes in salinity due to diurnal warming or rainfall.  It was set up by Julian Schanze and tuned during the transit.

After eight and a half days of steaming we arrived at the SPURS site under much improved conditions.  Winds were light and the sun was mostly bright for much of our time on site.

Sunday 3/24
The first order of business was to reset the WHOI mooring.  The releases were fired and the balls and line retrieved Sunday morning, 3/24.  We did not have to recover the whole mooring, just the synthetic line at depth.  There was a concern for chaffing at the eyesplice termination that caused us to do this line change, and indeed we did observe some chaffing.  The whole mooring operation proceeded without incident under fine weather conditions.  Completing the mooring operation allowed us to clear the decks somewhat and move the small boat to the main deck for easier deployment.

Monday 3/25
We used the small boat to launch two micro-gliders.  Seagliders were deployed from the ship.  Two WaveGliders were recovered from their holding sites near the WHOI mooring, They were snagged from the small boat then lifted aboard with the knuckle crane.  We then started steaming south to retrieve a Seaglider, the Mixed Layer Float and the drifting NOAA mooring (Pico- North).

Tuesday 3/26
This was a day of securing lost assets.  Seaglider 189 had run low on battery power and had been parked at the surface for some time.  The Mixed Layer Float deployed in September had some ballasting issues but fortunately had not drifted too far afield.  The NOAA Pico ?North mooring had broken free about a month earlier and we hoped to collect it and redeploy if possible.  All three of these were found and retrieved on Tuesday.  The small boat was used each time, in the case of the MLF and the NOAA mooring, it was to secure a means of lifting the gear aboard the ship or bring the gear to the ship, SG189 was retrieved in the small boat. That night we did a south to north U/W CTD section back toward the mooring array.

Wednesday 3/27
We performed microstructure profiling with the VMP near to ASIP and the gliders.  Several of us went over the Sarmiento for "International Salinity Summit Talks" and dinner while the Endeavor chased down the remaining WaveGlider.  This WaveGlider was steaming its programmed mission but had lost communications, so could not be commanded to its holding site.  However, it was easy to find by following its assigned track.  Endeavor returned to Sarmiento before sunset to retrieve the three scientists via small boat.  Overnight we did CTDs at the corners of the old mooring ?control volume? and UnderWay CTDs between.

Thursday 3/28
At 0800 GMT we fired the release for the NOAA Pico-North mooring to recover the release, glass balls and line in order to be able to redeploy it.  It took three hours for the balls to surface, and the initial (buoyant) line was reeled in easily.  However the polyester (heavy) line came up all snarled in large tangles.  It had clearly been sitting on the bottom, some of it was muddy.  We worked hard to untangle it but found that we had to make numerous cuts in the line to get it all aboard.  It looks like we would have to re-evaluate the plan to redeploy the mooring, though Jeff may be able to untangle and splice the line back to a usable length.  This recovery took much of the day.  That evening we deployed a Seaglider and did a CTD calibration cast near it when it dove.  We then started a CTD section along 38 W to the south with 1000m stations every 5 nm.   The Sarmiento had found a patch of fresh water intruding from the south in its SeaSoar surveys and we were planning ASIP, Glider and VMP work in a suitable fresh intrusion site so this section served in the search for the fresh intrusion.

Friday 3/29
After completing the CTD section we returned to the northwest to retrieve Seagliders 190 and 191 with the small boat.  We then steamed toward ASIP for VMP profiling.  By this time the  micro-gliders were not close to ASIP.  We worked on improving VMP profiling by adjusting winch settings and boat speed.  VMP?s continued till midnight.

Saturday 3/30
We continued the CTD section along 38 W further to the south overnight.  We then steamed to the NOAA Pico-East mooring, launched a small boat to secure a lifting tether and hoisted the buoy up at the stern A-frame using the mooring winch.  The old Prawler unit was swapped out for a new unit.  Our NOAA colleagues confirmed that it worked later in the day.  We then steamed to the WHOI Mooring and Jeff Lord visited the buoy to check meteorological equipment, one system on the buoy seems to be down.  We will likely revisit the Buoy later in the cruise once the engineers have diagnosed the problems. We then steamed to recover the two microgliders, Helo and Saul, securing them just before sunset.  Plans were made to re-deploy with ASIP on Sunday at a site to the south-south west so a north to south CTD section was done overnight along 38 25' W.

Sunday 3/31/
We stopped the section at 23 50' N and steamed a few miles east to retrieve one of last year?s ARGO floats that had developed a small leak.  We then steamed to the ASIP position and deployed Helo and Saul quite near it.  We then deployed the VMP first steaming north away from the cluster of instrument then approaching again from the north along a parallel section about 1 km west of their position.  This was continued till 2100 (local) then we restarted the CTD section further south along 38 25'W.

Monday 4/1
The CTD section along 38 25'W was continued, reaching 23 10'N.  We then steamed north back to the ASIP/Glider site for further VMP profiling.  After several hours the Sarmiento steamed into site and the chief scientists discussed plans over VHF.  The Gliders were recovered at about 16:30, while Sarmiento recovered ASIP.  We then began steaming to the north for a CTD section north along 38 W starting near the NOAA Pico-North mooring site.  The NOAA mooring redeploy is planned for tomorrow morning.

Monday, April 1, 2013

Science Update 01-april-2013

Three nights ago our SeaSoar CTD failed. It was probably a flooding, the second on this cruise. This is as unfortunate as it is unusual, and affected two units that had been recently calibrated . We were on our way towards the southeast corner of the initial SeaSoar survey to confirm the location of the low salinity feature we encountered there a couple of days ago. To our surprise the conditions at the surface had changed quite a bit. TSG and ADCP showed a very different picture than only 4 days ago. What seemed like a broad low salinity “blob” now was only a thin elongated filament.
We spent the night trying to figure out where this feature had gone. It was, after all close to 100 m deep, thus represents quite an amount of freshwater. The rapid changes are once again confirming the high variability in this ‘quiescent blob’ (as one might call it looking at mean climatologies), affirming the need to study it in detail and on various time and space scales.
The experience from that night led to the decision to deploy all our drifter assets now, to give time for a repair of the SeaSoar and furthermore to monitor the rapid changes within the SPURS box. As an additional bonus we are able to repeat most of the SeaSoar radiator survey with the TSG while we deploy the drifters and ASIP.
It turned out that a low salinity feature is now seen in the central eastern part of the SPURS box. Good drifter coverage by the end of today will enable us to plan where we should focus on a freshwater experiment, concentrating ASIP and additional drifters from the ENDEAVOR in a fresh patch to contrast the experiments within the saltiest patches. During that experiment in the last days of the cruise, the Spanish vessel will conduct another SeaSoar survey to provide the large scale context and might be able to examine the evolution of such an intrusion by following it from the ‘fresh’ frontal zone towards the origin, which might be to the south.
TSG surface salinity with SADCP vectors averaged over the upper 100m before (left) and after (right) march 28th 2013

Sarmiento Update 13-03-31

From Chief Scientist Jordi Font,

Sunday report:

- New ASIP deployment at 23 52.5', 38 24.6'. Will be recovered after 34-35 h
- Continued drifters deployment. All Pacificgyre released, now releasing ICM units
- Picked up three ICM+LOCEAN prototype drifters released the previous day

Plan for Monday:

- Finish drifters deployment in the 75 nm x 75 nm central box
- 7 CTDs along 38 41.4' spaced 15 nm
- Recovery of ASIP and Apex
- Decide new SeaSoar + floats sampling strategy. Probably targeting again the SE low salinity area

We placed one drifter (#114994) in the same point where we had deployed the first one on the 27th (#114956) and that apparently has never emitted.

Concerning multiple deployments, we released a triplet near to WHOI mooring, another one outside (SE) from the central box where we had observed the maximum S gradient, and a third one in the NE corner of the box.

Now we have a certain shortage of drifters (CTDs and drogues being used in experimental short-term deployments) and will just fill the 36 points in the box without more triplets. We still will have four drifters (one with CTD at 5, 10 and 50 cm) for work in the low salinity area we are targeting for the last part of the cruise.