Basics of a CTD station

A large portion of our duties during this cruise concerns the gathering of data during a "CTD station".  In fact, the CTD (conductivity-temperature-depth recorder) is but one of the measurement devices attached to the Rosette wheel.  The Rosette wheel holds the CTD; two Chi-Pods for measuring temperature variance (an instrument from an Oregon State collaborator Jonathan Nash); two lowered ADCPs (acoustic Doppler current profilers to measure the ocean current velocity, one looking up and one looking down); some thermistors (temperature sensors with more precision than the CTD), and six Niskin bottles (to take water samples for salinity calibration). There are many details that need to be checked before releasing the Rosette to the crew for deployment into the ocean, with steps logged by hand on a sheet specific to each "cast".  

Each step of the process is not complex, but there are many. Fortunately, Carson is a patient teacher. Carson is an engineer tasked with running and fixing electronics related to the scientific instrumentation.  He is in charge of the Rosette and how its instruments take and store data.  We also have Adam and Andy, the two ship IT gurus who help set up the network of computers onboard, which are essential for hosting the data as it is gathered and then analyzed. Everyone has a role, with constant collaboration and interaction necessary to make the whole process work.

Crew handling a Rosette wheel as it is raised onto the deck at the end of a CTD cast. 

 

 

Downward cast of the Rosette

The process of placing the Rosette into the water involves some rather tricky work from the ship's winch operators Cliff and Craig.  Imagine a pendulum with a 500kg bob on a steel cable hanging from a crane on a rocking ship. This work requires extreme precision and care not to hurt anyone or destroy the gear (about two or three hundred thousand UK pounds worth of gear!).  We only have one experienced winch operator on the cruise (Cliff), with another (Craig) being trained on the job to allow more hours of CTD casts.

The CTD winch operators Craig and Cliff in their work station in the UIC.  Note the great view out the windows behind them. Their minds are very focused on the task at hand when the CTD is in the water or on deck. 

The Rosette is lowered into the ocean at about 60 metre/minute.  Inside the Unified Instrument Centre (UIC), the scientists (with Carson guiding us, especially during our training periods) monitor ocean properties measured from the Rosette, such as temperature, salinity, pressure, velocity, and oxygen. The data is saved in computer files for each particular cast, to be later analyzed.

We aim to bring the Rosette to within 10 metres of the bottom, without hitting the bottom!  Recall, the bottom is around 4000m-5000m below the ship, with the ship moving up and down with the surface waves.  If the Rosette hits the ocean bottom, it can damage instruments and create lots of head-aches (costing much money and very unfortunate downtime).  To avoid this sort of damage, we monitor acoustic signals that send information back to the ship telling us the distance of the Rosette from the bottom.  We also have alarms to help ensure we are properly monitoring things (important especially for night watches). It is a rather delicate process requiring coordination between the winch operator and the scientists monitoring the bottom sensors.  Both scientists and winch operator are seated nearby in the UIC, making communication very efficient (e.g., "OK, please stop now").

Christian Buckingham (my fearless shift leader) and me (right) looking at some read-out from the CTD in the UIC (unified instrumentation centre), showing us vertical profiles for temperature, salinity, oxygen, radiance, and fluorescence. We are possibly discussing what depths to fire the Niskin bottles as the Rosette is raised toward the surface.

The upward cast of the Rosette

Upon reaching the bottom, we log details such as metres of cable out, depth of the Rosette (not the same given swell and currents), time (GMT), and position (latitude/longitude).  We then "fire" one of the Niskin bottles (i.e., close the bottle top and bottom) to take a sample of seawater at the deepest depth.  "Firing" a bottle requires a signal to be sent to the Rosette to release a spring to close the top and bottom of the bottles.  Before closing, the bottles are fully open to water flowing through them during the cast.  The goal is to have each bottle sample be a pure part of the ocean at that depth, without contamination from water at other depths.  So it is important for the bottles to shut tightly and precisely when told to do so.

After firing the bottom Niskin bottle, the winch operator raises the Rosette wheel towards the surface, again at about 60 metre/minute.  At five more pre-selected depths during the rise, we stop to fire more Niskin bottles to capture further water samples.  Depths for these samples are chosen based on "interesting" features seen in the vertical salinity profile viewed on the downward portion of the cast.

On many cruises, there can be far more bottle samples, determined by any number of pre-selected depth levels, thus making each cast very time consuming indeed.  Our cruise is a process physics cruise, so the high sampling needed for hydrography, chemistry, biology are not taken.  Rather, we are mostly taking water samples to calibrate salinity measurements taken from the CTD (seawater conductivity is a function of salinity and temperature).  The reason for the salinity calibration is that conductivity measurements on the CTD can drift, making it necessary to calibrate the CTD by taking selected water samples.  These samples are later analyzed onboard using the ship's salinometre.

On the deck again

When the Rosette reaches the surface, it is brought back into a covered location off the side of the starboard deck (like a small garage). Various scientists then go downstairs to check their favorite instruments, fill sampling bottles for running through the salinometre (again, needed for calibrating the CTD), and to give the full suite of instrumentation a check to ensure that everything survived the 5000m down/up through the ocean column.  We then re-charge the bottles (snap open the top and bottom latches), hook up the LADCP batteries to the chargers, wipe down other instruments, all in preparation for the next station cast.  The full process of lowering and raising the Rosette takes about three hours depending on depth and number of bottles.

Rosette parked in its garage.  The CTD is the gold cylinder on the bottom center. One of the Niskin bottles is the vertical gray PVC cylinder on the right.  Two ChiPods are on the left. One of the two yellow ADCPs (the downward facing ADCP) is barely visible behind the CTD. Other detectors are spread around.  We only have six Niskin bottles.  On some cruises, the whole Rosette wheel is full of bottles.

What data do we get from a CTD cast?

The CTD cast provides measurements of ocean properties sampled throughout the water column, from near the bottom to the surface.  First and foremost, there is the temperature, salinity, and pressure from the CTD itself.  We can then compute the speed of sound in the seawater from knowledge of the temperature, salinity, and pressure.  Knowledge of the sound speed, and the navigation position, allows one to convert the acoustic information from the upward and downward facing LADCPs into horizontal velocity vector of the ocean current.  Other monitors on the Rosette offer refined information about temperature (i.e., from the very precise thermistors), as well as the ChiPods that measure turbulent properties of the fluid as manifest in the temperature field.  This data for one CTD station will be combined with other stations to offer us a measure of the ocean properties during the days of the cruise.

As these, and other, stations are "reoccupied" over many years, we are able to build up a map of ocean properties from year to year and decade to decade.  Such information then allows us to understand the "climate" (long-term statistical averages) for this region of the ocean.  Variations in the long-term averages then indicate how properties may be changing over decades. For the most part, the dominant long term signal seen throughout the ocean is related to anthropogenic climate warming.  In addition to this long term warming trend, there are many interesting "wiggles" that reflect dynamical processes that oceanographers also like to understand.  

Christian working the CTD instruments, while Alberto (principle scientific officer), Eleanor (his colleague at Southampton University), and Sonya (my colleague at Princeton University) enjoy the view out the window of the UIC. In the background are Peter, Carson, and Andy. 

A few words about watermasses

Much of the art and science of oceanography is related to understanding the dynamics of ocean "watermasses".  Watermasses are largely defined by their temperature and salinity properties, as well as properties such as oxygen and nutrient content, or dynamical properties such as potential vorticity.  As water encounters the surface in the high latitudes, such as near the Antarctic continental shelves, the waters are exposed to powerful winds (such as Katabatic winds blowing from the Antarctic continent), as well as heat, fresh water, and salt fluxes from the atmosphere, rivers, ice shelves, and sea ice.  These property fluxes imprint themselves on the seawater (the water has a memory).  Indeed, the seawater can maintain elements of these properties as they sink into the ocean abyss and travel for thousands of kilometres throughout the ocean interior.  Antarctic  Bottom Water (AABW) is the canonical example for the southern hemisphere.  Indeed, AABW is the densest water on the planet, filling the bottom of many of the ocean basins around the world. Understanding the history and dynamics of such watermasses becomes a compelling aspiration when watching the computer screen register information from the Rosette such as the temperature, salinity, and oxygen.

In the Weddell Sea, one of the first identifiable water masses we encounter is the very cold water (less than 0C) that originates from near Antarctica, and which lives in the upper 100-200m of the column. This water originates from the previous winter, and is known as the "Antarctic Winter Water" layer.

Just below the Winter Water lives a huge plug of water that last felt the atmosphere somewhere in the North Atlantic about one or two centuries ago, and has been moving southward ever since.  When it reaches the Southern Ocean, this water is known as the "Circumpolar Deep Water" (CDW).  CDW is one or two degrees Celsius warmer than Winter Water, and it is generally above 0C.

Schematic figure illustrating some of the many processes and pathways for water moving around the Southern Ocean. Understanding these processes is a central focus of Southern Ocean physical oceanography, and the role it plays in the global climate system.  This image is taken from a Physics Today paper by Morrison, Frolicher, and Sarmiento (2015), written for an audience of physicists with little knowledge of oceanography.

One reason oceanographers are interested in CDW is that it can impress itself against portions of the Antarctic shelf.  The presence of this relatively warm CDW layer next to the coast allows it to exchange its heat with ice shelves that rim the Antarctic continental shelf.  As with ice in a cup of water, heat from the liquid water readily melts the ice shelves.

Through various features of anthropogenic climate change, scientists have determined that CDW is a major player in the dramatic loss of ice shelf mass seen around much of Antarctica during recent decades.  This loss of ice on the ice shelves then acts like the removal of a cork stopper from a bottle, thus allowing for ice from the continental ice sheets to move downstream towards the ocean.  This process then contributes to sea level rise, accounting for a growing fraction of the sea level rise measured around the planet.  Another source for sea level rise is the warming seen in the AABW (formed on the continental shelves).  There are many ideas for why the AABW is warming. One leading idea is related to changes in Southern Hemisphere winds moving waters around the Weddell Sea, with this idea one of the motivations of this cruise.  

In general, it is vitally important for scientists to determine the potential for sea level rise, whatever the reason. Consequently, it forms a key mandate for ongoing research into fundamental ocean processes such as those in the Southern Ocean being investigated on this cruise.