Craig Macaulay, Latest Southern Ocean research shows continuing deep ocean change, CSIRO [Commonwealth Scientific and Industrial Organization] (18 May 2012)
Steve Rintoul, Ship to shore: scientists return from Southern Ocean, CSIRO (18 May 2012) (MP3 podcast) (access via the above URL, the podcast link is on the right side of the page, under “Downloads”)
Dr. Rintoul’s podcast is significantly more informative than CSIRO’s press release.
How this study was done
Australian and American scientists (apparently) collected temperature and salinity samples from deep Southern Ocean water at 77 spots on a transit between Commonwealth Bay, Antarctica and Fremantle, Australia — in addition to taking samples along the Antarctic coast from Commonwealth Bay west.
What Dr. Rintoul’s team found
The Commonwealth Scientific and Industrial Organization reported that:
“The amount of dense Antarctic Bottom Water has contracted each time we’ve measured it since the 1970s,” said Dr. Steve Rintoul, of CSIRO and the Antarctic Climate and Ecosystems CRC.
“There is now only about 40 per cent as much dense water present as observed in 1970.”
“When we speak of global warming, we really mean ocean warming: more than 90 per cent of the extra heat energy stored by the earth over the last 50 years has gone into warming up the ocean.”
© 2012 Craig Macaulay, Latest Southern Ocean research shows continuing deep ocean change, CSIRO [Commonwealth Scientific and Industrial Organization] (18 May 2012)
From the podcast
Dr. Rintoul’s podcast explains that the expedition combined its Antarctic bottom water mission with Australia’s centennial commemoration of Douglas Mawson’s hair-raising 1912 Antarctic expedition.
Mawson’s oceanographic measurements, when compared to those which Rintoul’s team took this year, indicate that ocean currents in the region have changed a lot.
Apparently among these changes is a significant thinning of Antarctica’s densest, most saline bottom water layer. In 1970, it was 1000 meters thick. Today, Rintoul estimates it to be at least 60 percent thinner.
This matters, of course, because the Antarctic’s bottom water affects global water and atmospheric currents and climate.
Mechanisms for this change in bottom water density?
CSIRO indicates that the causes of this are unknown.
However, I can hypothesize four obviously possible mechanisms:
(i) ice melt — contributing fresh water to salty ocean water
(ii) increased sub-glacier water flow — as a result of being released from glacial ice dams’ previous blocking effect
(iii) increased amounts of open water mixing — as ice cover vanishes and winds have water currents have freer reign
(iv) the turbulence-enhancing mixing effect of increased heat in ocean water and atmosphere
Ice dynamics — hypotheses (i) through (iii)
In regard to ice dynamics, Rintoul noticed that freshwater contributions to the Southern Ocean play a dramatic role in reducing water density.
In February 2010, for example, an 80 by 30 kilometer chunk of ice broke off the Mertz Glacier (on the George V Coast of east Antarctica). The next summer, the team noticed that the deep water in the area was significantly less dense than it had been before the ice chunk broke off.
Perhaps surprisingly, according to Rintoul, the ice chunk’s freshening effect in only one year (2010-2011) equaled the total freshening that occurred from 1950 to February 2010.
Extrapolating from these facts, we can infer that some of this ice chunk melted. Some of its damming effect also vanished, thereby releasing more under-glacier water directly into the ocean. And the ice’s disappearance may also have had a redistributive effect on the pattern of wind and water currents.
Heat dynamics — hypothesis (iv)
In regard to heat content, if 90 percent of the planet’s increased heat is going into warming its waters, then these waters are experiencing a noticeable increase in energy.
One can, at least initially, assume that much of this manifests in increased water mixing, as molecular motion speeds up.
Even leaving occasionally more turbulent atmospheric conditions out of our thinking, we can hypothesize that such a boost in water energy would cause increased mixing across (at least) the edges of density-layer boundaries.
The moral? — Seeing all this play out will be interesting
The Australian CSIRO team will return to Antarctica next year. Dr. Rintoul, however, is going to stay home for a change.