A team of British and Australian scientists has discovered how carbon is drawn down from the surface of the Southern Ocean to the deep waters beneath.
British Antarctic Survey (and former CSIRO) scientist, Dr Jean-Baptiste Sallée says we need a better understanding of the pathways by which oceans capture and store carbon.
Credit:
CSIRO
The Southern Ocean is an important carbon sink in the world – around 40 per cent of the annual global CO2 emissions absorbed by the world’s oceans enters through this region.
Reporting in the journal Nature Geoscience, scientists from British Antarctic Survey (BAS), CSIRO and the Antarctic Climate & Ecosystem Cooperative Research Centre (ACE CRC) reveal that rather than carbon being absorbed into the deep ocean in vast uniform areas, it is drawn down and locked away from the atmosphere by intense, plunging currents up to a thousand kilometres wide. Winds, currents and massive whirlpools that carry warm and cold water around the ocean – known as eddies – create these regional pathways or funnels, through which carbon to be stored.
‘The Southern Ocean is a large window by which the atmosphere connects to the interior of the ocean below,’ says the paper’s lead author, Dr Jean-Baptiste Sallée from BAS.
‘Until now we didn’t know exactly how carbon ends up being stored deep in the ocean. It’s the combination of winds, currents and eddies that create these vast carbon-capturing pathways drawing waters down into the deep ocean from the ocean surface.
‘If climate change alters these pathways, it will be harder for carbon to penetrate deep down into the ocean, causing an additional warming effect. Now that we have a better understanding of the mechanisms for carbon draw-down we are better placed to understand the effects of changing climate and future carbon absorption by the ocean.’
CSIRO co-author, Dr Richard Matear, says the rate-limiting step in the anthropogenic carbon uptake by the ocean is the physical transport from the surface into the ocean interior. ‘Our study identifies these pathways and this matches well with observationally derived estimates of carbon storage in the ocean interior.’
Due to the size and remoteness of the Southern Ocean, scientists have only recently been able to explore its depths with the help of small robotic probes known as Argo floats. In 2002, 80 floats were deployed to collect information on the temperature and salinity. This unique set of observations spanning over 10 years has enabled scientists to discover how carbon is taken to the depths.
The team also analysed temperature, salinity and pressure data collected from ship-based observations since the 1990s.
Source: CSIRO & BAS
Published: 25 November 2014
Things warm up as the East Australian Current heads south
JaciBrown
Occasional erratic bursts southward of the East Australian Current (EAC) are thought to have moderated the weather of south-east Australia this autumn and winter and they continue to introduce tropical and sub-tropical marine species to Tasmanian waters.
Tasmania’s east coast: tropical and sub-tropical marine species normally found off NSW are finding their way further south, thanks to changes in the East Australian Current.
Ocean monitoring by Australia’s Integrated Marine Observing System is providing scientists with significant new insights into the changing structure of the EAC. Over the past 50 years sporadic warm bursts have become more common as the EAC moves further south. With global warming, the warm burst we’ve seen this year may also become the norm.
Had our little friend Nemo the clownfish been riding the EAC this year he might have found himself holidaying in Tasmania rather than admiring the Sydney Opera House. He wouldn’t have been on the trip alone, though. Sea nettles (Chrysaora spp.) have headed from their usual home in Sydney to be found for the first time ever in Tasmania and the Gippsland Lakes.
Chrysaora woodbridge, or sea nettle, was found in surprising numbers in Tasmania this year.
Credit: copyright Lisa-ann Gershwin
Waters in the EAC travel southward along the east coast of Australia, with most of it splitting from the coast near Sydney and heading for New Zealand. A small part of the current, known as the EAC Extension, works its way southward past Victoria and Tasmania.
A typical signature in this region are the large eddies, around 200 kilometres across and hundreds of metres deep. Some of the warm water is trapped here along with marine life.
The EAC starts at the Great Barrier Reef and travels south to Sydney before turning eastward to New Zealand. Some of the water can still push southward via a series of strong eddies.
Credit: Eric Oliver
This year a larger proportion of the EAC was sent southward instead of breaking away to the east. Winter ocean temperatures off Bass Strait were around 19°C, an increase of 4°C. This impacted local fishing, beach conditions and the weather.
In the video (above) the animation on the left shows the actual sea surface temperature and speed of the ocean currents. The animation on the right shows the difference in the temperature from average conditions.
Through autumn and winter, you can see two interesting changes occur. A strong warm current heads down the coast from Sydney to the coast of Victoria. At the same time, warm water peels off from the EAC and swirls around in large eddies as it meanders toward Tasmania.
An unusual catch down south
One advantage of warm eddies is the refuge they provide for tuna. They congregate in the centre of the eddy where the waters are warm and dine at the nutrient-rich edges.
Local fishers in north-east Tasmania report a remarkable year that allowed them to fish longer than usual, providing game fishers with more opportunities to catch tuna.
Last summer’s (2013–2014) warmth provided an abundance of skipjack and striped marlin, while winter brought a run of bluefin tuna.
Redmap is a website where locals can report sightings of marine species that are unusual for a given area.
Last summer a manta ray, a tropical cartilaginous fish (in a group including rays and skates), was sighted off the north-eastern coast of Tasmania. Previously the southern-most sighting of a manta ray was just south of Sydney.
Manta birostris spotted off north-east Tasmania on Australia Day 2014.
It’s not just new species visiting Tassie either. Local jellyfish such as the Lion’s Mane (Cyanea) – more commonly known as ‘snotty’ – are usually quite elusive, but turned up in unprecedented numbers last summer in Tasmania.
But there’s a catch
This movement south of the EAC may have an impact on other systems, including our health. We rely on fish such as those from the Tasman Sea as a source of omega-3 fatty acids for our brain health. But the concentration of omega-3 fatty acids in the fish is likely to decrease with global warming.
Algae are the original source of fatty acids. As our waters warm, we will see more of the algae from the tropics take up residence in the south-east.
But the algae from the tropics are much smaller, which means more steps in the food chain from the algae to the fish we eat. The more steps in the food chain, the more the omega-3 fatty acids in the fish are replaced by fatty acids that are less favourable to brain health.
The warmer coastal waters also contributed to the balmy autumn and winter in south-eastern Australia this year. Afternoon sea breezes cool coastal temperatures by drawing cool oceanic air onto the coast.
Sydney’s heat wave in May this year had 19 consecutive days of 22°C or more – this is partly due to the sea breezes failing to bring in the usual cooling air.
What’s causing the EAC to move south?
Over the past 50 years the EAC Extension has stretched about 350 km further south. This extension doesn’t happen smoothly but in erratic bursts.
The southward extent of the EAC is controlled by the collective behaviour of the winds between Australia and South America. Over that same 50-year period these winds changed their pattern due to a strengthening of a climate system known as the Southern Annular Mode.
The changes to this mode have been attributed to a combination of ozone depletion and increasing atmospheric CO2.
One of the most robust and consistent responses of the climate system to increasing CO2 is a further strengthening of the Southern Annular Mode.
So the result will likely be a further enhancement of the EAC extension southward and even warmer waters in the Tasman Sea.
Dr Jaci Brown is a senior research scientist with the Centre for Australian Weather and Climate Research (CAWCR), a partnership between CSIRO and the Bureau of Meteorology. Her research focuses on the El Nino Southern Oscillation (ENSO) and climate change. This article was originally published on The Conversation. Read the original article.
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