Doing research from a ship poses a number of logistical
challenges that must be taken into consideration along with the science goals
of the mission. We are out here to look for and study a particular phenomenon,
but this means not only finding it in the first place (as discussed in earlier
posts), but also having acceptable conditions in which to operate. In the case
of this project, since we are hoping to look at restratification processes
(several days of de-mixing) following a strong mixing event (i.e. a storm) at a
front (a fairly narrow physical feature), we need to be in the right place at
the right time without putting the ship in danger. Ideally, we’d like to time
it so that the equipment is already in the water when the storm hits, because
if things get too exciting, we either can’t deploy our instruments or we need
to vacate the region, neither of which is good for the science. Fortunately, we
have a very experienced ship crew on the Sikuliaq that is both very helpful in
making sure the science happens but also conscientious about making sure that
no unnecessary risks are taken.
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| Consulting about the optimal measurement spot on the bridge. From left: Chief Scientist James Girton, Captain of the Sikuliaq Diego Mello, Co-PI Eric Kunze, Co-PI John Mickett, and Second Mate John Hamill. (Photo credit: RE) |
We are currently surveying the region of our final
deployment with SWIMS to determine where exactly we want to place the buoy and
the floats for the next round of observations. The first step in selecting this
location was looking at the most recent data sent back from Argo[1]
floats in the region to see if we could find places with a deeper mixed layer
that would also better coincide with the storm track in the northern Pacific
Ocean. We also looked at satellite altimetry (ocean height) and sea-surface
temperature measurements to confirm that the deeper mixed layers could be found
near a front. This allowed us to define a general box of interest, with the
intent of doing on-site verification of the required features as we approached.
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| Ryan Newell, Eric Kunze, and Chris Siani work on reconnecting the winch cable to SWIMS after repairing the electrical connection, which is essential for transmitting data back to the ship as the instrument profiles. (Photo credit: RE) |
Because the ship has to go so much slower when we are towing
SWIMS, once we arrived at the beginning of our large spatial box, we used a
different instrument to get a rough picture of the mixed layer, known as an
“expendable bathythermograph” (XBT). This instrument is standard fare in the
world of oceanography, and is frequently used on ships of opportunity because
they are so simple (and inexpensive) to use. The XBT is a small torpedo shaped
instrument with a lead weight at one end and a thermistor that measures
temperature. Depth is calculated from a simple speed-of-fall estimation, and
the data is transmitted to the ship in real time as a thin copper wire unspools
rapidly. All three deployments, spaced roughly one degree of latitude apart,
showed a mixed layer closer to 70 or 80 meters, rather than the 30 meters we
observed at our last site, which is great news.
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| Marine Tech Bern explains how the XBT works to volunteer Ethan Brush. The tip of the actual instrument is just visible inside the black plastic cylinder it is housed in until deployment. (Photo credit: RE) |
As mentioned earlier, however, picking a site is not as simple
as finding a place in the ocean that has the right properties and going to
work. Since we are now closer to the storm track, it is also important that we
are not putting the ship at risk when doing the science. The box we had
initially outlined for this portion of the cruise included some regions that
were considerably farther north. After consultation between the chief
scientists, the captain, and the mates, it was determined that if we could
achieve the same scientific goals without entering that region of the ocean,
both the ship and our instruments would probably be better off. As a result, we
are about to embark on round three of buoy and float deployment for the home
stretch of the project.
[1]
The Argo program is a worldwide consortium of researchers using floats very
similar to our EM-APEX floats that do a 2000 meter temperature-salinity-depth
profile once every 10 days, often for 5 or more years. (Some floats are now
equipped with additional sensors, such as dissolved oxygen and nitrogen
sensors). One huge asset to this program is that the floats are distributed
throughout the world ocean and profile year-round, often in places that are not
readily accessible to ships all the time. You can find out more about the program and the data at argo.ucsd.edu.
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