Ocean research cruise blog of Jonathan Sharples
Work at the shelf edge has started well. One big difference
between the work here and the work that we did at the first station on
the cruise is that we have no moored instruments here. The shelf edge is
the most heavily fished part of the seas around NW Europe, so long-term
deployments of moored instruments tend to be unsuccessful as the chance
of moorings being snagged by fishing gear is very high. For the
duration of our work at this site we instead hang a chain of instruments
from the ship. Jo Hopkins and Chris Balfour, from the National
Oceanography Centre in Liverpool, spent the previous day setting up
about 40 temperature, salt and chlorophyll loggers so that their clocks
were all synchronised and they all take measurements at the same rate
(once per minute). The instruments were then clamped every 2.5 metres on
a 200 metre wire lowered over the stern, with a 300 kg ball of lead on
the end of the wire keeping it vertical in the water.
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| Jo with chain instruments |
We are using this chain of instruments to track a particular feature
of the shelf edge. As the tide moves onto and off the shelf, the steep
slope in the seabed causes the tide to push the thermocline up (tide
flowing onto the shelf) and down (tide flowing off the shelf). This up
and down motion generates waves on the thermocline that move away from
the shelf edge, both onto the shelf and away into the deep ocean. These
underwater waves can be very large, 100 metres from peak to trough and
15 km long. They are important because they result in a lot of mixing at
the shelf edge, bringing nutrients from the deeper water up towards the
surface. Our chain of instruments will track this up and down motion of
the thermocline wave, so we have a picture of how rapidly the physics
of the water below us is changing as we collect all of the biological
and chemical samples
Original post
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| t-chain deployment |
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