D334: Monitoring ocean circulation in the Atlantic


 D334



Cruise diary


3 November 2008

Craig writes...

Work is finally all done on the Eastern Boundary after a busy few days recovering, servicing and then re-deploying our two large moorings in this station – ‘EB1’ and its partner ‘EB2’. They provide continuous information, via the microCat sensors, on the density of the sea-water in the eastern part of the Atlantic at different depths in the ocean. We can then compare this information with readings from moorings on the other side of the Atlantic and calculate how fast the circulation is moving in between. The whole basis for this approach makes use of the fact that water, like air, moves from areas of high pressure to areas of lower pressure. And in the sea, the pressure at a certain depth relies on the density of the water lying above!

Recovering ‘tall’ moorings (i.e. ones that measure almost the whole depth of the ocean), such as EB1 and EB2 is a major operation and sees the ship’s crew and the technical team in full swing.  The process begins by sending a high-energy sound signal from the ship to the base of the mooring when the ship enters the vicinity of the mooring (see photo, right). When this is received, the code causes a metal hook at the base of the mooring to release from the anchor, and the flotation devices attached to the mooring wire (see diagram below) brings the complete unit to the surface.  Once the top end of the wire has been hooked and brought on board the aft (rear!) deck of the ship, it is attached to a mechanical winch which begins to reel in the wire so the sensors can be recovered (see photo, below).


Darren fires the release code from the ship to the base of the
mooring freeing the wire, floats and sensors from the anchor

The recovery processes is meticulously logged so that the time each sensor begins to travel to the surface is known and can be taken into account when the data are analysed.  The recovery process is also long – taking around four hours from start to finish.  Once all of the instruments are successfully recovered from the mooring wire the data are downloaded to laptops and the science team can get to work processing the information.

There are three more tall moorings waiting for us at the Mid-Atlantic Ridge, each carefully positioned to assess how the sea-floor mountains of the ridge might interfere with the circulation. The ship has been heading west, full steam, for the past 48 hours seeing us move ship-time back by two hours in order to keep in sync with the rising and setting of the sun. We hope to be on station early tomorrow morning when the recovery process will begin again!

Left: A typical ‘tall’ mooring, like EB1 and EB2, designed to build an ocean profile, from top to bottom, of the sea-water density.
Above left: The hydraulic winch reels the wire onboard stopping at regular intervals so the microCats can be taken off the wire and the information downloaded to laptop computers.
Above right: location of the RRS Discovery, Monday 3rd November, steaming west to the Mid-Atlantic Ridge.



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