It’s official, today I became an Oceanographer! I threw stuff into the ocean today “in the name of Science”! And yes, channelling Bono, I (almost) sang it aloud as I threw it. Let’s start at the beginning, we were up at 6 for a ‘toolbox’ on the bridge. The bridge has these cool Hunt for Red October stairs, one feels very important ascending them…
I have since realised the red stairs are actually a warning to the ascendor, they are saying at your own risk! As the bridge is higher on the ship it is more prone to the pig-like roll the Southern Surveyor is susceptible to, my stomach was quick to alert me to this. By sheer force of will (think of the antiemetics!), I did manage to keep my breakfast. We digress.
The safety briefing was for the upcoming test-run CTD (Conductivity, Temperature, Depth) profile and to talk through the accompanying risks. At 8.30, Sue Reynolds taught Sjoerd Groeskamp and I how to prep’ a CTD, and most importantly, how to not drop it on our feet, it weighs an actual ton when filled with water! CDTs are the observational oceanographer’s workhorse.
A CTD is made up of a set of probes attached to a large metal rosette wheel, it’s basically a fancy bucket. Whilst different CTD set-ups are used for different scientific interests our CTD has the following probes attached to it: Niskin bottles that collect water samples at different depths for measuring sea water properties, two Lowered Acoustic Doppler Current Profilers (LADCP) that measure the horizontal velocity, an altimeter to stop us hitting the bottom and oxygen sensors that measure the dissolved oxygen content of the water.
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The rosette is lowered on a cable down to the seafloor, and we can observe the water properties in real time via a conducting cable connecting the CTD to a computer on the ship. A remotely operated device allows the water bottles to be closed selectively as the instrument ascends allowing us to collect water samples at heights of interest. The samples can then be analysed in the lab. For our test run this morning we were particularly interested in comparing our dissolved oxygen sensor with lab measurements. Down in the operations room of the ship we get a continuous time series of depth, salinity, and temperature, from which measurements of density can be derived. Depth measurements are derived from measurement of pressure, and salinity is measured from electrical conductivity.
And now for the exciting part where Chris got to throw an instrument into the ocean! The instrument was an XBT or expendable bathythermograph, essentially it is a probe that is dropped from a ship and measures the temperature as it falls through the water. A very thin wire transmits the temperature data to the ship where it is recorded for later analysis. Rebecca Cowley and Alicia Navidad showed Sjoerd Groeskamp and I how to prepare an XBT for launch, they then walked us through the software setup and then came the fun part… [gallery type=”rectangular” ids=”360,359,358,357,356,442”] The probe is designed to fall at a known rate, so that the depth of the probe can be inferred from the time since it was launched. Its selling points are that it can be dropped from a moving ship (e.g. freight ships and the like), relatively cheap (~$70) and require very little training to operate (CSIRO runs a volunteer program). The downsides are that they don’t go all the way to the ocean floor and the fall rate can depend on the batch of XBTs. One of our scientific staff, Rebecca Cowley wants to release 72 XBTs to calibrate the fall rate from a particular batch of XBTs, it turns out the “fall[ing] at a known rate” isn’t as reliable as we’d like (Cowley et al. 2013). At twelve of our CDT sites we will drop six XBTs.
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In the right figure, we can see how the XBT is doing as compared to the CSIRO Atlas of Regional Seas, the software picks out the date of year and location (and no CARS doesn’t include XBT data). The red line is just where the connection to the XBT was lost.
Launching XBTs on the Southern Surveyor normally has another usage, namely, calibrating the Swath mapper. This part of the ocean has already been mapped so we are not using the mapper on this trip. As an example application of XBT data, our chief scientist, Ken Ridgway has used (Ridgway et al. 2008) 15 years of high density XBTs to produce a time series of the East Australia Current’s (extension) net baroclinic and mean transport through the Tasman Sea. In this study XBT data was combined with satellite altimetry data and 42 CTDs.
Having now completed all the XBT releases, Rebecca Cowley showed me the temperature profile comparison of the XBTs vs CDTs, you can see that the XBTs do capture the main features with an offset that suggests the XBT is falling slower than the fall rate we use. Their agreement (offset aside) is really impressive, don’t you think?
Cowley, Rebecca, Susan Wijffels, Lijing Cheng, Tim Boyer, and Shoichi Kizu. 2013. “Biases in Expendable Bathythermograph Data : A New View Based on Historical Side-by-Side Comparisons.” Journal of Atmospheric and Oceanic Technology 30.
Ridgway, K. R., R. C. Coleman, R. J. Bailey, and P. Sutton. 2008. “Decadal Variability of East Australian Current Transport Inferred from Repeated High-density XBT Transects, a CTD Survey and Satellite Altimetry.” Journal of Geophysical Research 113(C8): C08039.In category: Southern_Surveyor