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Christopher Bull

Aspiring oceanographer. PhD candidate (UNSW). Big data python enthusiast. Outdoor adventurer.

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The short version: 17 CTDs and 5 moorings VS fifteen days, 14 scientists, 17 crew and one 42 year old boat: can they do it?

The back story. In April 2012 five moorings were deployed by IMOS (Integrated Marine Observing System) to monitor the East Australia Current on voyage similar to the one I am on now. This work is unique as it is the first time we have comprehensive direct observationsof the East Australia Current (EAC) using a mooring array. The only other time the EAC has had a mooring based monitoring program is in 1991 as part of the World Ocean Circulation Experiment (WOCE) the outcomes are documented in (Mata et al. 2000), the experiment had two problems:

  1. Even though they had six moorings, the array was 120-km wide and so did not capture the full extent of the EAC. Comparatively, the array we are retrieving is 170-km wide.

  2. The deep moorings, namely 5 and 6 experienced an unexpected northward flow, this caused the moorings to lose their upright position and the top of the mooring dragged down for up to ten days. This data could not be used.

Being further south meant that they were also looking at transport due to eddies, a more difficult monitoring prospect. It is hoped that by placing our monitoring array further north we can capture a more uniform flow of the EAC.

Our voyage in the East Australia Current (EAC) has five objectives (in the order they are written):

  1. Complete ADCP section from inshore to offshore, to inshore again.

  2. Complete 12 CTDs along section from inshore to offshore mooring.

  3. Complete CTD (plus LADCP) station at each of the five mooring locations starting at mooring 1 (the one closest to Brisbane).

  4. Retrieve each of the moorings at deployed locations.

  5. Complete a final ADCP section along the mooring line.

[caption id=”attachment_408” align=”aligncenter” width=”300”]The five moorings monitoring the East Australia Current we will recover. The five moorings monitoring the East Australia Current we will recover.[/caption]

The main aim of this voyage is step 4 above. Namely, to retrieve 5 full-depth current meter/CTD moorings extending from the continental slope to the abyssal waters off Brisbane. We haven’t started doing this yet, in fact so far we have completed steps 1 and 2. We start tomorrow on step 3 and 4. Personally, I find it easier to think about the voyage in terms of a picture, so here it is:

[caption id=”attachment_407” align=”aligncenter” width=”300”]Our game plan Our game plan[/caption]

Let’s take pause on what we have so far and why we are collecting similar information using 3 different tools. Our data collection methods can be broken up into:

  1. CDTs/XBTs
* Pros: CDTs give lab calibrated highly accurate results, can do full height of the ocean with very high (essentially continuous) sampling frequency. Depending on attached instruments CDTs can collect information that other tools can’t, for example we have been collecting dissolved oxygen and nutrients. XBTs can be cast from a moving ship and requires little training to operate (e.g. CSIRO IX28 on the l’Astrolade in the Southern Ocean).

* Cons: XBTs/CDTs are only at a single spatial location for a single point in time. XBTs have limited depth. Since CDTs are taken from a moving ship, the wire has to stay close to vertical, if the line bends more than 22 degrees the Lowered Acoustic Doppler Current Profilers give unusable readings.
  1. Moorings
* Pros: Continuous time series (due to battery life normally eighteen months to two years), can go to full ocean depth. Can include a range of instruments. We have been recording pressure, temperature, depth, conductivity, horizontal _and_ vertical velocity.

* Cons: Need to be built to withstand long periods of being submerged, thus, moorings are vulnerable to long term exposure risks.Need to be retrieved after deployment. Sensors are not accurate enough to be useful for some variables, e.g., dissolved oxygen.
  1. Acoustic Doppler Current Profilers attached to hull of Southern Surveyor
* Pros: Can log data continuously whilst ship is at sea.

* Cons: Has limited depth (~600m). Only gives horizontal velocity.

Let’s also recap on what we have collected so far.

Here are the results from ADCP (step 1 above). In both plots we can easily make out the core of the East Australia Current (heading south).

[gallery type=”rectangular” ids=”410,409”]

Here are the results from the 12 CDTs (step 2 above). [gallery type=”square” ids=”423,422,421,420,419,418,417,416,415,414,413,412”]

Note: these plots are not yet corrected for salinity/dissolved oxygen calibrations. Images courtesy of Hugh Barker.

Mata, Mauricio M., Matthias Tomczak, Susan Wijffels, and John A. Church. 2000. “Australian Current Volume Transports at 30 S: Estimates from the World Ocean Circulation Experiment Hydrographic Sections PR11/P6 and the PCM3 Current Meter.” Journal of Geophysical Research 105(526): 509–28. (July 18, 2013).

In category: Southern_Surveyor