Words and photography: Thomas Moore
Latitude – 44.5 degrees south
Longitude – 152.3 degrees west
Ocean Depth – 4768m
The TTIDE team is back aboard the R/V Revelle and we are again plying the fickle waters of the Tasman Sea. This is the final cruise in our latest venture to better understand ocean mixing and how internal waves can influence both climate and the ecosystems that support marine life and our communities and industries back at home.
On the maiden cruise our team, a capable mix of veterans and first time volunteers, deployed fifteen moorings, towering strings of scientific instruments thousands of metres tall supported by giant floats and anchored to the seabed by hefty weights. Like 15 giant balloons tethered sky-high in a watery atmosphere, our moorings have swayed in the ocean currents for over a month recording characteristics like temperature, salinity, pressure, and the speed and direction of ocean currents. The TTIDE moorings have been listening carefully to the ocean for a long time but all that they have learned will stay a secret unless we can get them back safely aboard the Revelle and in one piece.
Hello, are you still there?
On our first day the Revelle steamed from Hobart, Tasmania roughly one third of the way to New Zealand to arrive at location “A1”, the tallest and most complicated of all our moorings at 4727 metres worth of wire, engineering, and technology. Mooring A1 is designed specifically to measure the energy in the beam of internal waves rolling towards Tasmania before that beam can be influenced by the bottom when it eventually travels into the shallower waters of the slope and shelf. Here the water is almost 5km deep and therein lies our first challenge. To get the mooring to rise to the surface we must “talk” across this underwater distance with sound waves, literally commanding it to return from the bottom. What if we speak but nobody is listening? It’s always the first unknown in a mooring recovery operation and on “A1” the TTIDE team had some anxious moments from the very start.
We carry aboard Revelle three different options for acoustically communicating with the mooring releases, the mechanical “hands” that grasp the bottom anchors on the sea floor. Each of these three communication units sends acoustic messages from the surface and receive back responses from the release units, or at least that is what is meant to happen. For much of the first operational hour of our twelve day cruise the science team sat huddled around the acoustic unit at first expecting, later hoping, and finally praying that we’d get a response. “Hello, are you still there”?
Mooring release failure is one of the worst situations a sea-going oceanographer can encounter. After sometimes years of proposals, funding, planning, construction, and deployment you now have a precious cargo of pricey scientific instruments and priceless oceanographic data that sits silent on the ocean floor – unresponsive, unmoving, stuck out of reach. If you talk to the deep but the deep is not listening you have but one extreme measure left to consider – dragging. Nobody wants to drag for their mooring.
Dragging for a mooring involves streaming many kilometres of heavy cables, weights, and hooks behind the ship in blind hope that you can literary rake the mooring off the seafloor and force it to the surface. It’s an arbitrary and highly destructive option reserved for the most desperate of situations. Luckily for the TTIDE team, after the failure of our first two acoustic units, we were all very happy to get a response from the mooring release on our third and final backup unit. A few minutes later there was immense relief when the top mooring float finally broke the surface about 300 metres away from our position. Now the team could get cracking bringing in over 4700 metres of wire and some sixty odd scientific instruments.
A day on the wire
Getting a 4700 metre mooring to the surface is just the start of the job and it took all the daylight hours we had on hand to get all the bits of wire rope, floats, and scientific instruments back aboard the Revelle.
The first day of mooring recovery on the back deck of any research ship is always somewhat unpredictable as a new group of veterans and volunteers gets into the rhythm of wire and winch. For our first day we were blessed with great weather, relatively calm seas, and a team that immediately hit their stride. Effective teamwork is the most vital component for success on a cruise with such an ambitious agenda of recovering 15 moorings in a short period of time. The operational tempo for TTIDE leg 3 over the next 11 days or so will necessarily remain extremely high for the whole crew and science team.
It was wonderful for us all to get mooring A1 back from the depths and onboard today. The instruments are cleaned and loaded into the labs aboard Revelle and in a few days all the data should be downloaded.
The deep does sometimes talk back when called upon.
TTIDE leg 3 team, aboard the R/V Revelle over and out for now.
– Thomas Moore, for the TTIDE team