From right to left Bonnie, Emma and Ines

By Bonnie Teece, Macquarie University; Emma Gale, Charles Darwin University; Ines Richter, University of New South Wales

The RV Investigator, in port at Freemantle, Western Australia

Greetings from the inaugural CAPSTAN (Collaborative Alliance Postgraduate Sea Training Alliance Network) voyage, where we are travelling aboard the RV Investigator across the Great Australian Bight. After spending the first few days hugging the coast of the southern parts of Western Australia it is now time to let go of the protection of the land and venture further south. The first feature we’re expecting to cross is the subtropical front, a front that brings together the cold dense water from the Southern Ocean with warmer waters from the coastal Australian and water and the Leeuwin and Flinders current.

Bonnie: BA/BSc; MSc (Res); Organic Geochemist and Palaeobiologist; Ship Specialty area: Sedimentology

An underway map of the RV Investigator’s progress across the Great Australian Bight

Crossing the front was not a particularly defining moment in the sedimentology laboratory – we had already taken all our samples. However, I was on the overnight shift (1.30am -7am) down in Observations, and was digitally (and manually) logging our latitude, longitude, depth, air and sea surface temperature, ship heading and speed, wind speed and direction, and gravity every 15 minutes.

I knew I would have to wake two of the trainers if we the sea surface temperature decreased below 16°C. And, at 19.45 UTC time (~4.45am local ship time) we crossed below 16°C – a full degree of longitude earlier than expected (35°11’45’’ S) – and I woke two sleepy academics who organised an early morning CTD, and Plankton Haul, on the cool side of the front, and then we reversed track an hour to take replicate within the warm side. We soon discovered that we were in eddy territory, and would exit and enter the front a few more times before exiting it completely.

A thermosalinograph taken by the RV Investigator between 14:00 11/19/2017 UTC and 14:00 UTC 11/20/2017

Sea Surface Temperatures shown to the south west of Australia. MNF represents the RV Investigator ship track (Source: IMOS 2017, SST Maps, http://oceancurrent.imos.org.au/sst.php, accessed 22/11/2017)

Emma: B.Sc. (Hons), PhD (Env. Eng); Coastal Scientist; Ship Specialty: Hydrochemistry

I awoke in the morning, expecting us to cross the Sub-Tropical Front at some stage during that day, only to be told that we crossed it earlier than expected, during the night. The Sub-Tropical Front is a water mass boundary in the Southern Ocean that roughly separates the more saline sub-tropical waters (on the northern side) from the fresher sub-Antarctic waters (on the southern side) and is identified by a sharp temperature gradient over a short spatial scale. For our voyage this sharp gradient occurred around the 15-16 C mark (Fig.4) and was followed by an increase in dissolved oxygen and fluorescence (Fig.5). These were hopeful figures given the lower levels of fluorescence observed on the northern side of the front. So now we look forward to investigating the data from the CTDs and plankton haul nets, either side of the front, to hopefully confirm the premise that the sub Antarctic waters are more nutrient rich.

Fluorescence and dissolved oxygen from seawater under the ship

Ines: BSc Marine Science, currently honours in marine microbial ecology, ship speciality: plankton

Signing up to a voyage that crosses the Southern Ocean, by many I was called brave before boarding the ship. I’ve once experienced Southern Ocean swell south of Tasmania and crossed my fingers the RV Investigator was a very stable ship. To our surprise leaving the coast didn’t bring the big swells we expected and so far it has been smooth sailing. Thanks to our trainer in all things physical oceanography, Professor Matthias Tomczak, we learn, that the high pressure system that is currently sitting right across the Great Australian Bight is actually a summer pattern for this area. Typically, during winter, and also through this time of year, strong storms sweep through this part of the world, bringing with them big swells, in fact one of the biggest around the world. Lucky for us this is not the case and we can smoothly sail into the next sunrise towards Hobart.

Flat oceans make for smooth sailing

High-pressure cell hovering over the Australian Bight Source: BOM

Want to hear more about the CAPSTAN voyage or the CAPSTAN initiative?
CAPSTAN website: https://goto.mq/6g
CAPSTAN voyage blog: https://voyage9181.wordpress.com
CAPSTAN director’s blog: https://aprilabbott.wordpress.com