Latest Southern Ocean research shows continuing deep ocean change
New research by teams of Australian and US scientists has found there has been a massive reduction in the amount of Antarctic Bottom Water found off the coast of Antarctica.
Comparing detailed measurements taken during the Australian Antarctic program’s 2012 Southern Ocean marine science voyage to historical data dating back to 1970, scientists estimate there has been as much as a 60% reduction in the volume of Antarctic Bottom Water, the cold dense water that drives global ocean currents.
In an intensive and arduous 25-day observing program, temperature and salinity samples were collected at 77 sites between Antarctica and Fremantle. Such ship transects provide the only means to detect changes in the deep ocean.
The new measurements, which have not yet been published, suggest the densest waters in the world ocean are gradually disappearing and being replaced by less dense waters.
“The amount of dense Antarctic Bottom Water has contracted each time we’ve measured it since the 1970s,” said Dr Steve Rintoul, the voyage leader and oceanographer with the CSIRO and the Antarctic Climate and Ecosystems CRC. “There is now only about 40% as much dense water present as observed in 1970.”
The ocean profiles also show that the dense water formed around Antarctica has become less saline since 1970.
“It’s a clear signal to us that the oceans are responding rapidly to variations in climate in polar regions. The sinking of dense water around Antarctica is part of a global pattern of ocean currents that has a strong influence on climate, so evidence that these waters are changing is important,” Dr Rintoul said.
The research was carried out by more than 50 scientists on the Australian Antarctic Division’s research and resupply vessel Aurora Australis, which sailed to Commonwealth Bay, west along the Antarctic coast, and returned into Fremantle.
The Australian Antarctic Division’s Chief Scientist, Dr Nick Gales, said the findings of the oceanographic study are profoundly important.
“Not only will this research improve our understanding of ocean currents, but will also feed into our knowledge of how the Southern Ocean and the Antarctic continent drives the world’s climate processes,” Dr Gales said.
Dr Rintoul was Chief Scientist on the recent voyage and has made a dozen voyages to the Southern Ocean.
“When we speak of global warming, we really mean ocean warming: more than 90% of the extra heat energy stored by the earth over the last 50 years has gone into warming up the ocean. The Southern Ocean is particularly important because it stores more heat and carbon dioxide released by human activities than any other region, and so helps to slow the rate of climate change” Dr Rintoul said. “A key goal of our work is to determine if the Southern Ocean will continue to play this role in the future.”
The causes of the observed changes in the Southern Ocean are not yet fully understood. Changes in winds, sea ice, precipitation, or melt of floating glacial ice around the edge of Antarctica may be responsible. Data collected on the latest voyage will help unravel this mystery.
A major challenge is the lack of observations at high latitude, where much of the ocean is covered by sea ice in winter. During the voyage scientists deployed nine drifting profilers, called Argo floats, which will transmit profiles of temperature and salinity every 10 days for the next five years.
“The Argo floats have revolutionised our ability to measure the ocean, particularly in winter when ship observations are very rare,” said Dr Rintoul. “On this voyage, we deployed a new kind of float designed to survive encounters with the sea ice. These floats will allow us to see how dense water forms in winter for the first time.”
The Aurora Australis visited Commonwealth Bay as part of a celebration of the centenary of Sir Douglas Mawson’s Australian Antarctic Expedition. Dr Rintoul’s team had the opportunity to repeat oceanographic measurements made by Mawson’s team 100 years ago, obtaining one of the few century-long records obtained anywhere in the ocean.
“Our measurements collected in 2012 are quite different to those collected by Mawson in 1912,” Dr Rintoul said. “This is an indication of a change in the ocean currents that may be related to a reduction in the amount of dense water formed near Antarctica.”
“Mawson’s expedition really marked the transition from the “Heroic Age” of Antarctic exploration to a period where science was the primary motivation for Antarctic expeditions. I think he would have gotten a real kick out of the idea that measurements made by his team a century ago are still useful and that Australian scientists are continuing his legacy by studying Antarctica and its connection to the rest of the globe.”
Latest Southern Ocean research shows continuing deep ocean change – Steve Rintoul
Video transcriptDr. Steve Rintoul, Southern Ocean science
So the ocean affects climate by storing and transporting huge amounts of heat and carbon dioxide. And so if we want to understand how climate's going to evolve in the future, we need to know what's happening in the oceans. About ninety percent of the extra heat energy that's been stored by the earth's system over the last fifty years is in the ocean. So when we talk about global warming, we're really talking about ocean warming in a real sense. What we're here to do in this voyage is to, there are three or four main parts to the voyage, but basically we're looking at how the southern ocean is behaving today. What the temperature, salinity, oxygen, carbon dioxide levels, what organisms are living in the sea and where, and relating those distributions to the ocean currents and also comparing what we measured today to measurements we've made in previous decades. Work we've done in the past has shown one of those sinking motions around Antarctica where water sinks down to great depth. Down to four or five kilometres below the sea surface. That seems to be slowing down with time. The water that sinks is becoming fresher. That is, less salty and less dense. We know that in previous parts of the previous periods of the Earth's history the changes in those sinking motions in the polar regions have been linked to changes in climate. We haven't seen changes in climate result yet from the changes in southern ocean currents but it's moving in the direction that would drive similar sorts of changes to those we've seen in the past. The evidence that we have so far is that the southern ocean is changing. It's warming at a greater depth and at a faster rate than the global ocean average. And it's also changing its salinity. The salinity changes are important for two reasons. One is that part of the salinity changes are being driven by more rainfall and snowfall than happened in the past. That's exactly the sort of signal that we expect to see as a result of global warming. That is, the areas that are dry, where there's more evaporation than precipitation, will get drier and saltier in the ocean. Places where it's, there's more rainfall than evaporation, will tend to get wetter or fresher in the ocean. We also see those changes in salinity down deep, right along the sea floor and that's giving us a hint about how the ocean is interacting with the ice sheet on Antarctica. As the ice spreads off the Antarctic continent into the sea, it starts to float and forms glacier tongues or ice shelves. Where that interacts with the ocean, the ocean can either melt (if it's warm enough), it can melt the bottom of the ice, or it can actually freeze to the bottom of the ice. The evidence that we found for freshening around the edge of Antarctica is an indication that that melting process is happening more rapidly than it did in the past and that's a result of the warming of the ocean, we believe, and part of what we're doing on this trip is trying to confirm that picture. In terms of the science, in terms of sorting out what's happening with the the climate system and what the impacts of change are going to be. We've made really dramatic progress, really, in the past five or ten years and so that's a positive. It means that at least we know what we're doing and we know what the impacts of that are going to be so we know what, we have a better idea of what to expect if we do, or don't, act.