Research visit to the Woods Hole Oceanographic Institute

Cape Cod, Massachusetts, USA

Investigator voyage to address puzzle of Southern Ocean current

Southern Ocean, 55°S 152°E

This is actually the first blog I have ever written and with the increasing social media pressure it probably won’t be my last, so bear with me.

Exactly 31 days from today, we started this adventure from Hobart, Tasmania to explore some unknown territory at the end of the world. We steamed from 43 to 56 degrees latitude down south, where we found the current that we were looking for, the Polar Front. The Polar Front is part of a much larger global scale current that encircles Antarctica. In this area, we will be looking at how the current passes over rough topography and creates a meander similar to a meandering river.

After 3 days of steaming, we arrived at the mooring location (see Figure 1 below), where we deployed a 4000 meters depth mooring. Along the mooring line from the bottom to the surface a spread of current meters, temperature and conductivity sensors (from which we derive the salt content in the water) will measure how the temperature, salinity and water velocities change over depth and over the lifetime of the mooring (approximately 2 years).

Figure 1: Voyage plan with the positions of the instrument deployments, the color contours show the topography of the seafloor and the black thin lines show the stream lines of the flow.

Two days after deploying the mooring we started with a series of transects crossing the Polar front current from south to north just downstream from where the current flows over an underwater mountain ridge. Each transect consisted of multiple way points where an instrument, called a CTD was dropped down to just above the sea floor. By crossing the current and making several transects downstream, we can estimate how the temperature and salt content, which determine the density of the water varies across and along the meander. This is important for estimating how warm water can be transported across the front, and potentially melt the Antarctic ice sheet.

Every CTD cast takes about 3 hours, roughly 1 hour to reach the bottom and then 2 hours to come up again, like a yo-yo. We watch the data come in on several computer screens, and look for any misbehaving sensors. On the way up, we have to close 36 bottles that take water samples at different depths. These are attached to the CTD frame. Once the CTD is on board, we take these water samples from the bottles to determine oxygen, salinity and nutrient levels. The sampling undergoes a thorough procedure of cleaning and making sure that the water does not get contaminated.

After each CTD, an instrument called a VMP then goes into the water. This instrument is supposed to free-fall in the water and makes measurements of turbulence, temperature and salinity, but on a much finer scale than the CTD. During the first transect we also deployed 6 EM-APEX floats, these floats go down to 1600 meters depth while flowing with the current. From each dive we can derive temperature, salinity and velocities at different depth levels. When reaching the final depth an internal bladder is inflated, which increases their buoyancy bringing them back to the surface. When they reach the surface, the collected data is transmitted via satellite communication to shore. While the CTD transects give us high resolution observations in the cross-stream direction, the EM-APEX floats give us high resolution observations in the along stream direction. Ideally, we acquire a high resolution field in both directions. Because these floats are often deployed in rough seas sometimes instruments get lost. And so, for 2 of the 6 floats we deployed, we unfortunately lost communication just after deployment.

You probably think by now, what else can you throw in the water. Well, we have used another instrument called the TRIAXUS, which we tow behind the ship. Although it has three degrees of freedom, rolling, yawing and pitching, we only used the pitching part, which let the TRIAXUS go from the surface to a depth of about 300 meters in a sawing motion. Between the CTD transects we used the TRIAXUS to make diagonal transects and obtain very high resolution observations of temperature and salinity measurements of the upper water layer.

As a cherry on the cake, attached to the ship there are a few more instruments, such as the Multibeam and SADCP, which give us a detailed map of the seafloor depth and velocities up to about 600 meters in depth. However, after 6000km of surveying, together with 77 CTDs, 56 VMPs and 8 TRIAXUS transects, it is time to finish this story and go home.

Unfortunately, aside from the 2 EM-APEX floats we will also not bring home our beloved VMP, since the VMP was lost in challenging conditions during one of its last planned deployments, but as they say: “if you are not prepared to lose an instrument, then do not put it in the ocean”.

It is a rough and risky environment and from this voyage I learned how valuable ocean observations are and how much effort and precision is needed to acquire them. The next two years I will continue working on this dataset as part of my PhD, and I can only hope for many more voyages to come, but for now I say from the research vessel Investigator, OVER and OUT!