Secrets of Southern Ocean probed
The (expanse of water) … absorbs about half of the man-made carbon dioxide taken up by the oceans
THEY can withstand severe storms, freezing temperatures and waves 10m tall. SA’s intrepid sea robots have since last month been hard at work in the Southern Ocean collecting and relaying data to scientists snug and warm in their offices on land.
The sea robots are part of an ambitious year-long experiment that scientists hope will demonstrate the importance of myriad small yet powerful events taking place in this vast and hostile tract of water.
The Southern Ocean, which surrounds Antarctica, plays a disproportionately important role in the earth system. It connects the world’s oceans with currents that transfer heat and carbon dioxide from the atmosphere to the deep ocean, and supports the oceans’ productivity with the nutrients it supplies.
A big question confronting scientists is how climate change will affect the Southern Ocean’s capacity to fulfil these crucial tasks, and whether these changes will result in feedback that accelerates or dampens the pace of climate change.
Human activity puts about 10 gigatons of carbon dioxide into the atmosphere each year, about half of which is absorbed by terrestrial and ocean reservoirs. The Southern Ocean is one of the world’s most important carbon sinks, as it absorbs about half of the man-made carbon dioxide taken up by the oceans.
Scientists have historically studied the Southern Ocean with intermittent measurements taken on board “ships of opportunity” travelling to and from Antarctica mainly in the summer months.
But these snapshots have captured a mere fraction of the activity in its icy waters, says Council for Scientific and Industrial Research (CSIR) oceanographer Sandy Thomalla.
SCIENTISTS are now turning their attention to the detailed dynamics at play, scrutinising changes taking place over short time spans — within a few days — and over areas less than 30km wide in the hope of developing a better understanding of the Southern Ocean.
“Only once you have accurately characterised the system can you predict how it will respond to… climate change,” says Dr Thomalla, whose team is trying to get a better grasp of the Southern Ocean’s year-long seasonal cycle and its sensitivity to parameters such as temperature change and storm frequency. Anticipating this will enable them to better predict changes over a much longer time.
Last month the polar research vessel SA Agulhas II delivered two pairs of sea robots to sites at 42 and 45 degrees south, in the heart of the tempestuous “roaring forties”, which interests scientists because it has a particularly high uptake of carbon dioxide.
The midwinter delivery of the sea robots marked the start of the CSIR’s year-long Southern Ocean Seasonal Cycle Experiment III. This dedicated science voyage was funded by the Department of Science and Technology and the National Research Foundation.
Each pair of robots includes a yellow surfboard-like wave glider that moves in a 25km² octagon, taking continuous measurements of the weather and the flux of ocean-atomosphere carbon dioxide; and a buoyancy glider that travels up and down the water column to 1,000m below the surface four to six times a day collecting data on water density, temperature and phytoplankton.
The phytoplankton is not measured directly, but calculated from measurements of fluorescence (which indicates how much chlorophyll is present) and the water’s light-scattering properties which shows how much particulate carbon is present.
Each glider relays the data it collects via satellite to CSIR scientists in Cape Town.
Scientists think the seasonal cycle of the Southern Ocean is important, because it links physical adjustments associated with climate change to ecosystem responses in productivity, plant diversity and ultimately to carbon export, says Dr Thomalla. She is interested in the spring bloom of phytoplankton, when microscopic plants at the base of the Antarctic food web start to grow.
THESE plants play a crucial role in the Earth’s carbon cycle, because they remove carbon dioxide from seawater when they photosynthesise, enabling the ocean to absorb additional carbon dioxide from the atmosphere. When these tiny plants die, they sink with their carbon stores to the ocean floor, where they remain for centuries.
“We’re trying to work out how the bloom sustains itself for so long into the summer. We think storms and eddies play a critical role in supplying nutrients to the surface waters late in the season,” Dr Thomalla says, explaining that scientists have observed a host of miniblooms in the eddies whirling off the major currents, in addition to the main spring bloom. “This is why we think its so important to resolve everything on a fine scale.”
When SA Aghulas II makes her summer voyage to SA’s Antarctic base in November, she will stop at both sites to retrieve the robots. The wave gliders will be refurbished and the buoyancy gliders replaced so they can continue collecting data until the vessel returns to Cape Town in late February. On her voyage south, a set of bio-optic floats will be deployed at each site, and left to float with the Antarctic circumpolar current: they will take measurements of carbon, chlorophyll, temperature, salinity, and dissolved oxygen until autumn, possibly longer — it all depends on whether they are floating near the vessels’ route when she travels to Marion Island in April, says Dr Thomalla.