SciBlogs

Posts Tagged oceanography

Antarctic voyage: Sections through the ocean Guest Work Mar 08

No Comments

Written by NIWA oceanographer and voyage leader Dr Mike Williams.

Date: 7/3/2013
Location: 58.600213°S, 158.239401°E
Weather: Cloudy, rain, 20-30 knots wind
Sea state: 2-3 m swell

Both on our transit to Antarctica, and on our way home, we have repeated lines of CTD stations know as sections.  On our way south we occupied the southern part of a section known as SR3 (from Tasmania to Antarctica along 139°51S), and on the way north a section called 150°E (unsurprisingly along longitude 150°E).

This is not the first time either of these sections has been measured. SR3 was first occupied in 1991, and has seen 9 full repeats (all the stations along the section between Tasmania and Antarctica) and 6 partial repeats.  150°E has only been measured a couple of times.

As for the section names, they are historical. In the late 1980s and throughout the 1990s a bold attempt was made to understand the ocean circulation around the whole globe. Named WOCE (World Ocean Circulation Experiment), it consisted of a set of oceanographic sections along which standard measurements would be made. There were too many sections to do all at once, so they were undertaken over a 10 year period. To understand if there were any changes some sections were repeated. This is what gives SR3 its name – S for Southern Ocean, R for repeat, and 3 for the third section in the Southern Ocean.

For oceanographers these sections provide us with a way to understand the ocean. Some of these sections form the side of a box in the ocean, and we are able to compare the sides and learn something about the changes within the box. By repeating the sections we can also monitor changes in the ocean over time.

Our two sections have been chosen as they lie to the east and west of the Mertz Polynya region.  Between them we are looking for changes in the deepest waters in the ocean, a water mass called Antarctic bottom water (see blog post 21: The formation of the Antarctic bottom water). Water masses form at the surface of the ocean where interaction with the atmosphere sets their temperature, salinity, and other chemical properties. These properties allow the water mass to be tracked through the ocean.

On 150°E we expect to see bottom water that has formed in the Ross Sea. While along SR3 we should see a combination of Ross Sea water and bottom water formed in the Mertz Polynya Region.  These can be identified by their subtle differences in salinity and temperature, as well as dissolved oxygen and CFCs.

sr3_ptso_4helen

Temperature, salinity and oxygen data from the SR3 section. [Beatriz Pena-Molino, ACE CRC]

A few years ago these repeats of SR3 and 150°E found that the properties of the Antarctic bottom water mass, the coldest, densest water in the deep ocean, are changing. It is not as dense, or salty, as it was 10 years earlier, and is much fresher than it was in the 1970s.  This suggests there have been major changes around Antarctica. Whether this is less sea ice, or more melt water is hard to tell, and the challenge that we need to tease apart.

Antarctic Voyage: The CTD Guest Work Mar 05

No Comments

Written by NIWA oceanographer and voyage leader Dr Mike Williams.

Date: 4/03/2013
Location: 63.394131°S, 149.553421°E
Weather: Cloudy, snow and rain, 25 knots of wind
Sea state: 3-4 m swell

CTD about to be deployed. [Jill Scott]

With 10 days left of the voyage we are now heading north, starting to make our way back to Wellington. As with our trip south, we are undertaking some sampling with the CTD (see previous blog posts) along the way.

The CTD is the primary research tool for oceanographers. At the heart of it are the probes which measure pressure, temperature, conductivity, and dissolved oxygen. From temperature and conductivity, we calculate salinity (i.e. how salty the ocean is). Around the CTD sits the ‘Rosette’, with up to 24×10 litre water sampling bottles. On this trip, however, we have removed 2 of these bottles so that we can mount a lowered Acoustic Doppler Current Profiler (LADCP) and Tiny oceanographic gyroscope system (TOGS).

The LADCP sends out a high frequency sound which bounces off particles in the ocean and is reflected back. It uses the return signal to determine the speed of the water going past, and so it gives us measurements of the ocean currents.

TOGS is a gyrocompass that tells us direction, without requiring a magnetic field to determine compass direction. This is particularly important this close to the south magnetic pole (see blog post 16: The South Pole). Unfortunately, this particular instrument is only rated to 3000m water depth, so it cannot be used on the really deep stations (3000 to 4500m).

The 10 litre sample bottles are open at both ends and can be triggered remotely to close at different depths. Once released the ends close, trapping the water inside the bottle so it can be brought to surface for sampling.

As the ship nears each CTD station we turn all the instruments on and reset all the bottles on the CTD, and it is moved into the cutaway – an area of the ship which has a low side for launching and retrieving equipment. After it is launched by the crew, it will take about three hours to go down to the seafloor and back again for a station of 3000 m water depth, as it takes roughly one hour for each 1000 m of water.

While the CTD is in the water several of the scientists will watch the numbers coming back from the CTD sensors, mainly to check it’s working but also to communicate with the crew, who drive the winch, about where we need to stop. The first stop is at about 15m to wait for the pumps on the CTD to turn on. The pumps make sure we have an even flow of water over the sensors. Once the pumps are on, it’s back to surface, then straight down to the bottom.

To make sure we don’t actually hit the seafloor with the CTD, it has an altimeter. This tells us how far we are off the bottom once we are less than 50m from the seafloor. We aim to sit the CTD between 5 and 10m off the bottom. If we hit the bottom we could stir up mud that might block the sensors, or if the bottom is rocky and we crash into it, we might damage the expensive instruments.

On the way back up to the surface we trigger the bottles on the rosette, allowing them to spring closed. We sample at the most interesting parts of the temperature, salinity and dissolved oxygen profiles that we have measured on the way down. The water samples are important because they give us the water we can analyse either in the laboratories on the ship, or back on shore. Although we measure dissolved oxygen and salinity electronically, the sensors are often not accurate enough. So some of the water is used to measure salinity and dissolved oxygen to check the sensors. The rest of the water is used to undertake measurements that we don’t have sensors.

The weight of the water at 3000m is 3000 times the weight of the water at 1m depth and the pressure is 300 times greater. So if we sampled surface water on the way down and took it down to the bottom of the ocean the extreme pressure would crush the full sample bottle. Empty ones don’t get crushed as the pressure inside the open bottle is the same as on the outside. The bottles are made strong enough to cope with 10 litres of water under pressure coming up, but can’t go the other way.

Antoine Martin sampling the CTD. [Aitana Forcen]

Once all the bottles have been fired, we bring the CTD out of the water and collect samples from each of the 10 litre bottles for all the different analyses. There is a strict order in which water samples are taken. Dissolved gases are taken first as they can escape into the atmosphere once the bottles are opened. So we have been taking dissolved oxygen, total carbon, alkalinity, salinity, nutrients (phosphate, nitrate and silicate) and then carbon and oxygen isotopes.

It takes about an hour to sample for all these different analyses from the bottles on the rosette, and download the data from the LADCP, TOGS and camera. While we are doing this the ship moves on to the next station. Often there is just enough time to sample before we are on the next station, then we start all over again…unless the geologists are deploying a core, in which case we might have time for a cup of tea.

 

Antarctic Voyage: The oceanography team Guest Work Feb 05

No Comments

Dr Helen Bostock, marine geologist at NIWA, writes

Date: 4/2/2013
Position: 45.66345˚S, 171.2626˚E
Weather: Cloudy and raining
Sea state: Calm

While we are transiting to our first station over the next few days, I will introduce the members of the science teams and the main instruments they will be using during the voyage.

CTD hanging off the side of the Tangaroa (from TAN0803 voyage, credit: NIWA)

There are scientists from three different disciplines on board: oceanographers, chemists and geologists (the latter are also doing a little biology).

Some other statistics: although we are from institutions in 3 different countries, there are actually 8 different nationalities on board – New Zealanders, Australians, English, Americans, Canadians, French, Spanish, and the ice pilot is Danish. Interestingly, there are more female scientists (14) than male (8) scientists participating on this voyage!

The oceanography team is made up of Mike Williams (the voyage leader), Fiona Elliot, Matt Walkington (NIWA), Bea Molino, Mark Rosenberg (ACE CRC), Emmanuelle Sultan, Marie Noelle Houssais, Hervé Le Goff (L’ocean) and three PhD students – Aitana Forcen (NIWA), Eva Cougnon (ACE CRC), and Antoine Martin (L’ocean).

Together this team is responsible for running the CTD (conductivity, temperature, depth profiler – pronounced seeteedee), which measures the salinity, temperature, pressure, and oxygen in the water as we lower it the sea floor. The team also collect water samples at different depths on the way back up to the surface, to calibrate the sensors on the CTD, and for the chemists to analyse. Similar sensors on the ship’s ‘underway’ system also collect continuous surface water data along the ship’s track.

Aitana Forcen sampling the CTD during TAN1106 voyage (credit: Bruce Hayward, Geomarine Research)

Attached to the CTD frame is an ADCP (Acoustic Doppler Current Profiler) which measures flow speeds at different depths in order to get an idea of the currents. There is another ADCP attached to the bottom of the ship that constantly measures the currents in the upper few hundred metres. The ADCP works by sending out pulses of sound that bounce off objects in the water; then they pick up the change in frequency of the reflected sound pulse to determine the speed of the objects in the water, which we take to be is the same as the water speed.

The oceanography team is also responsible for the moorings. These are a series of instruments measuring temperature, salinity, and currents, attached to a long cable. The cable is held down by a weight which sits on the seafloor, and then held upright in the water with a series of floats. While a CTD takes a snapshot of what is going on in the water column, moorings are usually deployed for a few months or a couple of years to monitor seasonal or interannual changes.

Mooring hanging above the water (credit: Mark Rosenberg, ACE CRC)

One of the main objectives of this voyage is to retrieve some moorings that the Australians and French teams put out into the ocean over a year ago to measure changes in the water column on the shelf near the Mertz Glacier. As the moorings sit below the surface of the ocean, the instruments should have measured the water column even in the middle of winter when the sea ice freezes over this region and there is no way a ship can get in.

Together the physical and chemical properties of the water can be used to trace different water masses in the ocean. When the data is compared to previous voyages to this region, it also tells us how the water mass properties are changing over time.

 

Network-wide options by YD - Freelance Wordpress Developer