Antarctic voyage: Sections through the ocean

By Guest Author 08/03/2013

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.

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.