CTDs, VMPs, whales and birds

Progress on the CTD stations

As of 31 March, we have completed 34 CTD stations. We were greatly helped in these activities when Craig was given permission to work nights driving the winch, to thus complement Cliff's work during the day. We now have 24 hours a day for CTD stations (Craig and Cliff each do a 12 hour shift). The night shift was quite happy when Craig was given the green-light, as they were otherwise unable to do much work at all.

During recent days, each shift has developed a reasonably efficient routine with their CTD work. As mentioned in an earlier post, measurements at a CTD station include LACDP (ocean currents), CTD (salinity, temperature, and pressure), ChiPod (temperature variance), oxygen concentration, and penetrative radiation (light). Unfortunately, we have not obtained turbulent shear information given the loss of one VMP (stuck at the ocean bottom; see below), and poor performance of sensors on the second VMP.

After completing the most recent CTD section on 31 March, we returned in earnest to the question of what to do about the stuck VMP and the poorly performing VMP. A decision was made to focus efforts during 1 April on recovering the stuck VMP. But before that story, we briefly discuss the basics of what a VMP provides to oceanographers.

Basics of the Vertical Microstructure Profiler (VMP)

The VMP is used to measure very fine scale (order centimetre) fluctuations in the ocean currents. These measurements are used to diagnose properties of ocean turbulence at the very small scales, just above the molecular. The instruments are very sensitive, so much so that they need to be set to fall at a precise rate so as to not corrupt their measurements with flow initiated by the apparatus that holds the profiler instruments. Additionally, the ends where the profilers are located are somewhat pointy, so that they can act like a spear moving through biology (e.g., jellyfish). If they spear biology, that too can corrupt the measurements.

Alberto (white helmet looking left) explaining elements of the VMP (yellow instrument) to Nikki (white helmet with back to camera). The pointy end of the VMP is on the right.  It is the pointy end, with very fine sensors, where all the important measurements take place. The remainder of the VMP houses the electronics and offers the necessary buoyancy for the instrument as it goes down (pointy end down) into the ocean abyss.  When it reaches a specified depth near to the bottom, it drops its weights and returns to the surface.

After a good deal of data processing to remove instrument noise, VMPs offer us information about levels of turbulent dissipation of kinetic energy, as well as the variance of temperature. This data helps oceanographers to understand how the ocean mixes properties such as heat, salt, carbon, nutrients, etc. Think of how we mix milk in coffee. Without stirring to bring the milk into fine stringy filaments, it would take days to homogenize the milk and coffee. Likewise, without turbulent processes in the ocean, watermasses created near Antarctica would never mix with other water as it moves northward. Understanding the physics of such ''watermass transformations'' occupies a great deal of physical oceanographic research today.  One reason is that we wish to understand the causes of such transformations, some of which are natural and others of which are due to anthropogenic (i.e., human induced) warming. 

On this cruise, Kurt, Alberto, and Eleanor are experts at interpreting turbulence measurements from VMPs in the context of the ocean's general circulation.  A key aspect of this cruise concerns such measurements, so it was with great disappointment when on 27 March we had a bit of bad luck.

Preparing to launch a VMP.  In contrast to most other ships, oceanographic research vessels are built in a way that facilitates placing  expensive equipment overboard and then recovering the equipment when the job is done. Doing so is not so easy as it may sound. Difficulties arise particularly when the seas are rough.  For example, the stern region, shown here, can move up/down many metres in a swell.  If one were to drop a VMP overboard in those seas, it may get crushed if it floated under the ship. Recovering instruments in rough seas is very difficult, both in finding the instruments and in getting them safely onboard again. The skills and experience of the crew are critical for sucessful seagoing oceanography. Most scientists get out of the way during deployment and recovery of instruments in order for the crew to do their job safely and efficiently. Note Billy Platt in the shorts on the left.  It is, in fact, quite chilly out there!

VMP temporarily stuck at the ocean bottom

Some bad luck hit early on the morning of 27 March: one of the two VMPs hit the ocean bottom at around 3600m. Unfortunately, it remains stuck there. We used an incorrect depth reading when setting the depth of the VMP dive. This event is not uncommon, but is quite inconvenient. Will it surface under its own ''volition''? Will we find it before the forecast stormy weather makes logistics dangerous? If it does not dislodge, should we drag for it with a hook on a cable some 4000m long? Will hooking the VMP damage the instrumentation beyond repair? 

Today, 1 April, we have returned to the location of the stuck VMP. The plan is to send out a cable with some weights and an acoustic beacon to dislodge the VMP, hopeflly allowing it to then float to the surface. The only way we can "see" the wire and the VMP is through acoustic beacons. There is no video camera and spotlight to offer a more detailed view. Hence, this exercise really needs some good luck. The weather is cooperating thus far, with light snow falling yet only a modest wind sea.

The whole process of dragging for a stuck VMP is not without risks, which in turn lends to slightly different perspectives on appropriate strategies. One risk is that we miss the VMP altogether; another is that we hit the VMP and damage it; another is that we damage the beacon on the cable, which can be a problem for other activities planned for the cruise.

Given the multiple risks, we still must try. The VMP is a very important instrument, particularly for the upcoming portion of the cruise. It is for that portion that we really need good turbulence measurements to test a central portion of the scientific hypothesis for the cruise. Namely, that bottom mixing in the Orkney Passage outflow region contributes to the transformation (i.e., modification) of deep Weddell Sea deep water as it flows north into the Atlantic. There is no other working hypothesis for this transformation, and it must be tested to know for sure.

I will report back on my next posting with results from today's VMP recovery work.

Whales (our friendly charismatic mega fauna)

During recent days, as the CTD work shifted into high gear and as angst grew regarding the stuck VMP and its poorly performing sibling, we were blessed by a number of southern right whales visiting us as they fed on krill. On many occasions, they came to the side of the ship, often quite near to the CTD wire. The winch operator, Cliff, guessed that the whales must be attracted to something related to the CTD: the vibrations perhaps, or the electrical signals? Or were they using the CTD wire and ship to help organize their krill feeds? 

A Southern Right Whale's fluke and a seabird. When the fluke is shown, it generally means the whale is going down for a deep dive and will not surface again for a few minutes.

Regardless the reason, the sight of "our whales" led to great fun during the otherwise routine CTD casts. Everyone on board took great joy in seeing the whales follow us to the next CTD station, blow air from their holes, share with us a view of their curved mouths and huge eyes, and say farewell with their fluke as they dove into the depths. 

Here is a southern right whale feeding near to the ship. Note the blue-green waters, which generally appear when they blow bubbles to herd the krill for feeding. The following information is from a WWF website: ''Southern right whales are medium to large baleen whales, distinguished by the white/grey growths on their head, known as callosities. The shape and arrangement of callosities – above the eyes and top lip, and along the lower jaw – is different for every whale and commonly used as a means of identification. Like all baleen whales, right whales have two blowholes to make breathing at the ocean surface easier. Their jaws are highly curved to allow some 250 baleen plates to hang down from either side of the top jaw like a giant curtain. The plates are up to two metres long and trap some two tonnes of krill and other small crustaceans a day. Made of keratin, the baleen plates continue to grow and fray throughout the whale's busy life.''

Southern right whales were given their name by whalers of 100+ years ago due to their relatively slow swimming speed, good oil and baleen content, and their tendency to float after being harpooned. These characteristics made them the ''right'' whale for harvesting. Fortunately for us and for the whales, we are not here to harpoon them. However, their name still seems appropriate to us on this cruise. That is, their presence was often just at the "right" time to give us a reprieve from our head-games, to thus open the heart for a few moments and bring us back to some of the deeper reasons for being here.

More whale feeding action near to the ship.

Birds and Whales

Sea birds are often found in tandem with the whales, perhaps since the whales stir up food for the birds to eat. I found it fascinating how the birds deftly and nonchalantly position themselves so near to the lumbering whales. Indeed, sea birds are an amazing example of how evolution has positioned a species into a viable niche, even in the presence of stark and stunning environmental difficulties. I was reminded of this point while watching a bird one day during a storm. This bird floated straight into the wind with sea spray pounding its face. It blinked a bit, shook its head a bit, but otherwise seemed unfazed, as if it did not matter whether the day was calm or raging.

One of the many seabirds attracted to the ship, and to the whales.